Albumin in rabbit skeletal muscle. Origin, distribution and regulation by contractile activity

Oxidised Albumin Levels in Plasma and Skeletal Muscle as Biomarkers of Disease Progression and Treatment Efficacy in Dystrophic mdx Mice

Redox modifications to the plasma protein albumin have the potential to be used as biomarkers of disease progression and treatment efficacy in pathologies associated with inflammation and oxidative stress. One such pathology is Duchenne muscular dystrophy (DMD), a fatal childhood disease characterised by severe muscle wasting. We have previously shown in the mdx mouse model of DMD that plasma albumin thiol oxidation is increased; therefore, the first aim of this paper was to establish that albumin thiol oxidation in plasma reflects levels within mdx muscle tissue. We therefore developed a method to measure tissue albumin thiol oxidation. We show that albumin thiol oxidation was increased in both mdx muscle and plasma, with levels correlated with measures of dystropathology. In dystrophic muscle, albumin content was associated with areas of myonecrosis. The second aim was to test the ability of plasma thiol oxidation to track acute changes in dystropathology: we therefore subjected mdx mice to a single treadmill exercise session (known to increase myonecrosis) and took serial blood samples. This acute exercise caused a transient increase in total plasma albumin oxidation and measures of dystropathology. Together, these data support the use of plasma albumin thiol oxidation as a biomarker to track active myonecrosis in DMD.

Preferent Diaphragmatic Involvement in TK2 Deficiency: An Autopsy Case Study

Our goal was to analyze postmortem tissues of an adult patient with late-onset thymidine kinase 2 (TK2) deficiency who died of respiratory failure. Compared with control tissues, we found a low mtDNA content in the patient’s skeletal muscle, liver, kidney, small intestine, and particularly in the diaphragm, whereas heart and brain tissue showed normal mtDNA levels. mtDNA deletions were present in skeletal muscle and diaphragm. All tissues showed a low content of OXPHOS subunits, and this was especially evident in diaphragm, which also exhibited an abnormal protein profile, expression of non-muscular β-actin and loss of GAPDH and α-actin. MALDI-TOF/TOF mass spectrometry analysis demonstrated the loss of the enzyme fructose-bisphosphate aldolase, and enrichment for serum albumin in the patient’s diaphragm tissue. The TK2-deficient patient’s diaphragm showed a more profound loss of OXPHOS proteins, with lower levels of catalase, peroxiredoxin 6, cytosolic superoxide dismutase, p62 and the catalytic subunits of proteasome than diaphragms of ventilated controls. Strong overexpression of TK1 was observed in all tissues of the patient with diaphragm showing the highest levels. TK2 deficiency induces a more profound dysfunction of the diaphragm than of other tissues, which manifests as loss of OXPHOS and glycolytic proteins, sarcomeric components, antioxidants and overactivation of the TK1 salvage pathway that is not attributed to mechanical ventilation.

On the Rate of Synthesis of Individual Proteins within and between Different Striated Muscles of the Rat

The turnover of muscle protein is responsive to different (patho)-physiological conditions but little is known about the rate of synthesis at the level of individual proteins or whether this varies between different muscles. We investigated the synthesis rate of eight proteins (actin, albumin, ATP synthase alpha, beta enolase, creatine kinase, myosin essential light chain, myosin regulatory light chain and tropomyosin) in the extensor digitorum longus, diaphragm, heart and soleus of male Wistar rats (352 ± 30 g body weight). Animals were assigned to four groups (n = 3, in each), including a control and groups that received deuterium oxide (²H₂O) for 4 days, 7 days or 14 days. Deuterium labelling was initiated by an intraperitoneal injection of 10 μL/g body weight of 99.9% ²H₂O-saline, and was maintained by administration of 5% (v/v) ²H₂O in drinking water provided ad libitum. Homogenates of the isolated muscles were analysed by 2-dimensional gel electrophoresis and matrix-assisted laser desorption ionisation time of flight mass spectrometry. Proteins were identified against the SwissProt database using peptide mass fingerprinting. For each of the eight proteins investigated, the molar percent enrichment (MPE) of ²H and rate constant (k) of protein synthesis was calculated from the mass isotopomer distribution of peptides based on the amino acid sequence and predicted number of exchangeable C–H bonds. The average MPE (2.14% ± 0.2%) was as expected and was consistent across muscles harvested at different times (i.e., steady state enrichment was achieved). The synthesis rate of individual proteins differed markedly within each muscle and the rank-order of synthesis rates differed among the muscles studied. After 14 days the fraction of albumin synthesised (23% ± 5%) was significantly (p < 0.05) greater than for other muscle proteins. These data represent the first attempt to study the synthesis rates of individual proteins across a number of different striated muscles.

Mass spectrometry analysis of complexes formed by myotonic dystrophy protein kinase (DMPK)

Myotonic dystrophy type 1 (DM1) is caused by an expansion of CTG repeats at the 3'-UTR of the serine/threonine protein kinase DMPK. Expanded CTG repeats are toxic since they are transcribed into an RNA molecule which is then sequestered within the nucleus in the form of foci. RNA cytotoxicity is linked to the aberrant splicing of several developmentally regulated genes. DMPK transcripts undergo alternative splicing giving rise to many isoforms but do not seem to be involved in the splicing dysregulation of DM1. However, decreased levels of DMPK in DM1 patients and DMPK involvement in muscle weakness and cardiac dysfunction in animal models have been reported. The variability in phenotypic expression of DMPK together with its differential subcellular targeting, suggests that different splicing isoforms may be involved in different signalling pathways, possibly through DMPK-interacting proteins. To gain better insight into the DMPK function, we used mass spectrometry to identify proteins co-segregating with DMPK in soluble complexes isolated from high-speed supernatant of rat muscles. We carried out experiments with native DMPK to preserve the physiological stoichiometry with potential partners. DMPK-containing complexes were isolated and immuno-detected by non-denaturing electrophoresis, gel filtration, ionic-exchange chromatography and immunoprecipitation. DMPK peptides were identified by high-resolution mass spectrometry together with several putative DMPK-binding proteins, including several heat shock proteins such as HSP20/HSPB6, HSP60/CPN60, HSP70 and HSP90. We also obtained evidence of a direct interaction of DMPK with alphaB-crystallin/HSPB5 and HSP25/HSPB1.

Hypertrophy of mature Xenopus muscle fibres in culture induced by synergy of albumin and insulin

The aim of this study was to investigate effects of albumin and insulin separately as well as in combination on mature muscle fibres during long-term culture. Single muscle fibres were dissected from m. iliofibularis of Xenopus laevis and attached to a force transducer in a culture chamber. Fibres were cultured in a serum-free medium at slack length (mean sarcomere length 2.3 mum) for 8 to 22 days. The medium was supplemented with (final concentrations): (1) bovine insulin (6 nmol/L or 200-600 nmol/L), (2) 0.2% bovine albumin or (3) 0.2% bovine albumin in combination with insulin (120 nmol/L). In culture medium with insulin, 50% of the muscle fibres became in-excitable within 7-12 days, whereas the other 50% were stable. Caffeine contractures of in-excitable muscle fibres produced 80.4 +/- 2.4% of initial peak tetanic force, indicating impaired excitation-contraction (E-C) coupling in in-excitable fibres. In the presence of albumin, all cultured muscle fibres were stable for at least 10 days. Muscle fibres cultured in medium with insulin or albumin exclusively did not hypertrophy or change the number of sarcomeres in series. In contrast, muscle fibres cultured with both albumin and insulin showed an increase in tetanic force and fibre cross-sectional area of 19.6 +/- 2.8% and 32.5 +/- 4.9%, respectively, (means +/- SEM.; P = 0.007) after 16.3 +/- 1.7 days, whereas the number of sarcomeres in series remained unchanged. We conclude that albumin prevents muscle fibre damage and preserves E-C coupling in culture. Furthermore, albumin is important in regulating muscle fibre adaptation by a synergistic action with growth factors like insulin.

Proteomic profiling of chronic low-frequency stimulated fast muscle

Skeletal muscle fibre transitions occur in many biological processes, in response to alterations in neuromuscular activity, in muscular disorders, during age-induced muscle wasting and in myogenesis. It was therefore of interest to perform a comprehensive proteomic profiling of muscle transformation. Chronic low-frequency stimulation of the rabbit tibialis anterior muscle represents an established model system for studying the response of fast fibres to enhanced neuromuscular activity under conditions of maximum activation. We have conducted a DIGE analysis of unstimulated control specimens versus 14- and 60-day conditioned muscles. A differential expression pattern was observed for 41 protein species with 29 increased and 12 decreased muscle proteins. Identified classes of proteins that are changed during the fast-to-slow transition process belong to the contractile machinery, ion homeostasis, excitation-contraction coupling, capillarization, metabolism and stress response. Results from immunoblotting agreed with the conversion of the metabolic, regulatory and contractile molecular apparatus to support muscle fibres with slower twitch characteristics. Besides confirming established muscle elements as reliable transition markers, this proteomics-based study has established the actin-binding protein cofilin-2 and the endothelial marker transgelin as novel biomarkers for evaluating muscle transformation.

Differential expression of the fast skeletal muscle proteome following chronic low-frequency stimulation

Physiological and biochemical responses of skeletal muscle fibres to enhanced neuromuscular activity under conditions of maximum activation can be studied experimentally by chronic low-frequency stimulation of fast muscles. Stimulation-induced changes in the expression pattern of the rabbit fast skeletal muscle proteome were evaluated by two-dimensional gel electrophoresis and compared to the altered isoform expression profile of established transformation markers such as the Ca2+-ATPase, calsequestrin and the myosin heavy chain. Sixteen muscle proteins exhibited a marked change in their expression level. This included albumin with a 4-fold increase in abundance. In contrast, glycolytic enzymes, such as enolase and aldolase, showed a decreased expression. Concomitant changes were observed with marker elements of the contractile apparatus. While the fast isoforms of troponin T and myosin light chain 2 were drastically down-regulated, their slow counterparts exhibited increased expression. Interestingly, mitochondrial creatine kinase expression increased while the cytosolic isoform of this key muscle enzyme decreased. The expression of the small heat shock protein HSP-B5/alphaB-crystallin and the oxygen carrier protein myoglobin were both increased 2-fold following stimulation. The observed changes indicate that the conversion into fatigue-resistant red fibres depends on: (i) the optimum utilization of free fatty acids via albumin transportation, (ii) a rearrangement of the creatine kinase isozyme pattern for enhanced mitochondrial activity, (iii) an increased availability of oxygen for aerobic metabolism via myoglobin transport, (iv) the conversion of the contractile apparatus to isoforms with slower twitch characteristics and (v) the up-regulation of chaperone-like proteins for stabilising myofibrillar components during the fast-to-slow transition process.

Adaptation of muscle size and myofascial force transmission: a review and some new experimental results

This paper considers the literature and some new experimental results important for adaptation of muscle fiber cross-sectional area and serial sarcomere number. Two major points emerge: (1) general rules for the regulation of adaptation (for in vivo immobilization, low gravity conditions, synergist ablation, tenotomy and retinaculum trans-section experiments) cannot be derived. As a consequence, paradoxes are reported in the literature. Some paradoxes are resolved by considering the interaction between different levels of organization (e.g. muscle geometrical effects), but others cannot. (2) An inventory of signal transduction pathways affecting rates of muscle protein synthesis and/or degradation reveals controversy concerning the pathways and their relative contributions. A major explanation for the above is not only the inherently limited control of the experimental conditions in vivo, but also of in situ experiments. Culturing of mature single Xenopus muscle fibers at high and low lengths (allowing longitudinal study of adaptation for periods up to 3 months) did not yield major changes in the fiber cross-sectional area or the serial sarcomere number. This is very different from substantial effects (within days) of immobilization in vivo. It is concluded that overall strain does not uniquely regulate muscle fiber size. Force transmission, via pathways other than the myotendinous junctions, may contribute to the discrepancies reported: because of substantial serial heterogeneity of sarcomere lengths within muscle fibers creating local variations in the mechanical stimuli for adaptation. For the single muscle fiber, mechanical signalling is quite different from the in vivo or in vitro condition. Removal of tensile and shear effects of neighboring tissues (even of antagonistic muscle) modifies or removes mechanical stimuli for adaptation. It is concluded that the study of adaptation of muscle size requires an integrative approach taking into account fundamental mechanisms of adaptation, as well as effects of higher levels of organization. More attention should be paid to adaptation of connective tissues within and surrounding the muscle and their effects on muscular properties.

Alteration in albumin level during modified muscular activity

We have previously reported that albumin protein is increased in the atrophied muscle induced by hindlimb immobilization. The purpose of this study was to evaluate the effects of several disuse models on albumin protein and mRNA levels in mice skeletal muscle and to investigate whether the elevated amount of albumin returns to control level by muscular activity increased by hindlimb remobilization. Western blot analysis revealed that hindlimb immobilization, denervation, and tenotomy, except for hindlimb unloading, significantly increased albumin levels in soleus muscles by 2.1-, 1.9- and 2.0-fold, respectively (P < 0.001). Immunohistochemical analysis showed that albumin protein accumulates in the widened extracellular space. Reverse transcription-polymerase chain reaction (RT-PCR) assay revealed albumin gene expression to be downregulated in all disuse models relative to control level. During hindlimb remobilization, the amounts of albumin protein appeared to remain higher level after 3 and 7 days and had returned to control level after 14 days and muscle mass, the amounts of myosin heavy chain, and actin proteins seemed to restore control levels after 21 days. These results indicate that the amount of interstitial albumin protein may be modulated by muscular activity.

Induced synthesis of albumin-like protein in damaged rat reticulocytes

In this study, we present evidence that red blood cell (RBC) membrane p68 in the Belgrade (b/b) rat is similar if not identical to rat serum albumin (RSA). Structural homology between RSA and the RBC p68 has been determined by a variety of biochemical and immunological criteria. This albumin-like protein is a normal constituent of rat RBC and it is partially exported by exosomes during erythroid differentiation. The endogenous origin of rat RBC albumin-like protein was demonstrated by monitoring protein synthesis in reticulocytes and by identification of reticulocyte mRNA for albumin. Haemolytic anaemia, either hereditary (b/b rat) or drug-induced (phenylhydrazine-treated normal rat), results in increased accumulation of the albumin-like protein in rat RBCs as a result of its induced synthesis.

Measurement of interstitial albumin in human skeletal muscle and adipose tissue by open-flow microperfusion

The absolute concentration of albumin was measured in the interstitial fluid of subcutaneous adipose tissue and skeletal muscle in six healthy volunteers by combining the method of open-flow microperfusion and the no-net-flux calibration technique. By use of open-flow microperfusion, four macroscopically perforated double lumen catheters were inserted into the tissue regions of interest and constantly perfused. Across the macroscopic perforations of the catheters interstitial fluid was partially recovered in the perfusion fluid. Catheters were perfused with five solutions, each containing different concentrations of albumin. Absolute interstitial albumin concentrations were calculated by applying linear regression analysis to perfusate vs. sampled albumin concentration (no-net-flux calibration technique). Interstitial albumin concentrations were significantly lower (P < 0.0001) in adipose tissue (7.36 g/l; r = 0.99, P < 0.0003; range: 4.3-10.7 g/l) and in skeletal muscle (13.25 g/l; r = 0.99, P < 0.0012; range: 9.7 to 15.7 g/l) compared with the serum concentration (48.9 +/- 0.7 g/l, mean +/- SE, n = 6; range: 46.4-50.4 g/l). Furthermore, interstitial albumin concentrations were significantly higher in skeletal muscle compared with adipose tissue (P < 0.01). The study indicates that open-flow microperfusion allows stable sampling of macromolecules from the interstitial space of peripheral tissue compartments. Moreover, the present data report for the first time in healthy humans in vivo the true albumin concentrations of interstitial fluid of adipose tissue and skeletal muscle.

Chronic resistive loading induces diaphragm injury and ventilatory failure in the hamster

The purpose of this study was to examine the effects of tracheal banding for 30 days on arterial blood gases, and diaphragm structure and function. Hamsters were tracheal banded (TB) or underwent a sham procedure (C) (n = 16 and 18, respectively). After 30 days, arterial blood gases from awake TB hamsters showed hypoxemia and a respiratory acidosis. Histochemical analysis of diaphragm cross-sections showed a five-fold greater area fraction of abnormal muscle; a greater variation in fiber size; and a 3% higher proportion of type 1 fibers in TB than C hamsters. In vitro physiologic studies of costal strips from TB hamsters showed lower stress (45-70% over 10-100 Hz) than C values. Maximal esophageal pressure during occlusion was 45% higher and normalized diaphragm mass was 10% higher in TB hamsters than C hamsters. We conclude that the lower stress in vitro was attributable, at least in part, to diaphragm injury. Hypercapnea was present in spite of the higher diaphragm mass and maximal esophageal pressures in banded hamsters.

What does chronic electrical stimulation teach us about muscle plasticity?

The model of chronic low-frequency stimulation for the study of muscle plasticity was developed over 30 years ago. This protocol leads to a transformation of fast, fatigable muscles toward slower, fatigue-resistant ones. It involves qualitative and quantitative changes of all elements of the muscle fiber studied so far. The multitude of stimulation-induced changes makes it possible to establish the full adaptive potential of skeletal muscle. Both functional and structural alterations are caused by orchestrated exchanges of fast protein isoforms with their slow counterparts, as well as by altered levels of expression. This remodeling of the muscle fiber encompasses the major, myofibrillar proteins, membrane-bound and soluble proteins involved in Ca2+ dynamics, and mitochondrial and cytosolic enzymes of energy metabolism. Most transitions occur in a coordinated, time-dependent manner and result from altered gene expression, including transcriptional and posttranscriptional processes. This review summarizes the advantages of chronic low-frequency stimulation for studying activity-induced changes in phenotype, and its potential for investigating regulatory mechanisms of gene expression. The potential clinical relevance or utility of the technique is also considered.

Serum albumin as a potential carrier for the apocrine secretion of proteins in the rat coagulating gland

A protein of 66k was purified to homogeneity from the total secretion of rat coagulating gland. Its close structural relationship to serum albumin was demonstrated by N-terminal amino acid sequence analysis, proteolytic fingerprinting and Western blotting studies using polyclonal antibodies raised against the 66k protein and rat serum albumin. Immunofluorescence staining showed that the 66k protein was localised in the cytoplasm of coagulating gland epithelial cells from which it is released via apocrine blebs. Performing immunoelectron microscopy, the 66k protein was by no means detectable in the endoplasmic reticulum and the Golgi apparatus. Reverse transcription-PCR, Northern blotting studies and in situ hybridisation experiments demonstrated that mRNA of albumin is not expressed by coagulating gland epithelial cells. Therefore, intravascular albumin should be transferred into the epithelial cells of the rat coagulating gland followed by secretion via aposomes. Furthermore, overlay blots proved that the 66k protein binds to the apocrine proteins carbonic anhydrase II and secretory transglutaminase and vice versa. In contrast, no binding was evident to the merocrine 115k protein and to cytoplasmic resident proteins e.g. lactate dehydrogenase. These findings point to the assumption that serum albumin taken up from extracellular sources could function as a selective carrier for cytoplasmic proteins destined for apocrine secretion.

Sequential increases in capillarization and mitochondrial enzymes in low-frequency-stimulated rabbit muscle

To investigate temporal changes in capillarization and increases in mitochondrial enzyme activity, rabbit tibialis anterior muscles underwent chronic low-frequency stimulation for up to 50 days. Capillary density (CD), capillary-to-fiber ratio (C/F), intercapillary distance (ICD), and mean capillary area (MCA), as well as several other parameters of capillarization, were examined. In addition, tissue levels of mRNA specific to vascular endothelial growth factor (VEGF) were assessed by reverse transcriptase-polymerase chain reaction. Citrate synthase (CS) activity, a marker of aerobic-oxidative metabolic potential, was measured in the same muscles. Significant increases in CD and C/F, respectively, and decreases in ICD and MCA were observed after 2 days. These changes reached stable maxima by 14 days. The increases in capillarization occurred in a fiber-type-specific manner, affecting type IId fibers before types IIda and IIa. VEGF mRNA levels increased in a bimodal time pattern with a first elevation (2.5-fold) after 1 day and a second (9-fold) after 6-8 days. Increases in CS were first noted after 8 days. Obviously, increases in capillarization as induced by enhanced contractile activity precede increases in the aerobic-oxidative potential of energy metabolism.

Metabolite patterns related to exhaustion, recovery and transformation of chronically stimulated rabbit fast-twitch muscle

Rabbit fast-twitch tibialis anterior muscle was subjected to chronic low-frequency stimulation (10 Hz, 24 h/day). Measurements of the time course of changes in the concentration of metabolites of energy metabolism were performed in order to test the hypothesis whether or not alterations in the metabolite profile might represent possible signals for triggering muscle fibre type transformation. Most of the investigated metabolites displayed triphasic changes in response to persistently increased contractile activity. During the first 15 min of stimulation, drastic reductions were observed for adenosine triphosphate (ATP, 56%), phosphocreatine (PCr, 60%) and glycogen (76%), as well as 3- to 4-fold and 10-fold increases for glucose and lactate, respectively. This early metabolic perturbance coincided with a rapid reduction of isometric force. The next phase, extending to 4 days of stimulation, was characterized by a nearly complete recovery of ATP and PCr, and an overshoot in glycogen. The first signs of metabolic recovery were already detectable in 60-min-stimulated muscle when isometric force was still markedly depressed. These results demonstrated an impressive capability of the muscle to recover with ongoing stimulation from an initial, dramatic disturbance in energy metabolism. During the final phase, extending to 50 days, the metabolite profile approached that of a slow-twitch muscle with moderate reductions in total adenine nucleotides, ATP, total creatine, PCr and glycogen. A conspicuous result was the finding that, contrary to the recovery of most metabolites, the ratio of ATP to the product of free adenosine diphosphate and resting free inorganic phosphate was persistently depressed with ongoing stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrostimulation-induced increases in fatty acid-binding protein and myoglobin in rat fast-twitch muscle and comparison with tissue levels in heart

Chronic stimulation of rat fast-twitch muscle increased the content of both fatty acid-binding protein (FABP) and myoglobin. The increases in FABP, which reached values close to that of cardiac muscle, exceeded those in myoglobin and those in citrate synthase and 3-hydroxyacyl-CoA dehydrogenase activities.