The application of 2H2O to measure skeletal muscle protein synthesis

A novel oral tracer procedure for measurement of habitual myofibrillar protein synthesis

RATIONALE

Conventionally, myofibrillar protein synthesis is measured over time periods of hours. In clinical studies, interventions occur over weeks. Functional measures over such periods may be more representative. We aimed to develop a novel method to determine myofibrillar protein fractional synthetic rate (FSR) to estimate habitual rates, while avoiding intravenous tracer infusions.

METHODS

Four healthy males were given 100 g water enriched to 70 Atom % with (2)H2O as a single oral bolus. Vastus-lateralis needle biopsies were performed and plasma samples collected, 3-13 days post-dose. (2)H enrichment in body water was measured in plasma using continuous flow isotope ratio mass spectrometry (IRMS). Myofibrillar protein was isolated from muscle biopsies and acid hydrolysed. (2)H enrichment of protein-bound and plasma-free alanine was measured by gas chromatography (GC)/pyrolysis/IRMS. Myofibrillar protein FSR was calculated (% day(-1)).

RESULTS

The tracer bolus raised the initial enrichment of body water to 1514 ppm (2)H excess. Water elimination followed a simple exponential. The average elimination half-time was 8.3 days. Plasma alanine, labelled during de novo synthesis, followed the same elimination kinetics as water. The weighted average myofibrillar protein FSR from the four subjects was 1.38 % day(-1) (range, 1.0-1.9 % day(-1) ).

CONCLUSIONS

Myofibrillar protein FSR was measured in free-living healthy individuals over 3-13 days. Using a single oral (2)H2O bolus, endogenous labelling of alanine occurred in a predictable manner giving estimates of synthesis comparable with published values. Furthermore, the protocol does not compromise the ability to measure other important metabolic processes such as total energy expenditure.

Focal adhesion kinase is required for IGF-I-mediated growth of skeletal muscle cells via a TSC2/mTOR/S6K1-associated pathway

Focal adhesion kinase (FAK) is an attachment complex protein associated with the regulation of muscle mass through as-of-yet unclear mechanisms. We tested whether FAK is functionally important for muscle hypertrophy, with the hypothesis that FAK knockdown (FAK-KD) would impede cell growth associated with a trophic stimulus. C₂C₁₂ skeletal muscle cells harboring FAK-targeted (FAK-KD) or scrambled (SCR) shRNA were created using lentiviral transfection techniques. Both FAK-KD and SCR myotubes were incubated for 24 h with IGF-I (10 ng/ml), and additional SCR cells (±IGF-1) were incubated with a FAK kinase inhibitor before assay of cell growth. Muscle protein synthesis (MPS) and putative FAK signaling mechanisms (immunoblotting and coimmunoprecipitation) were assessed. IGF-I-induced increases in myotube width (+41 ± 7% vs. non-IGF-I-treated) and total protein (+44 ± 6%) were, after 24 h, attenuated in FAK-KD cells, whereas MPS was suppressed in FAK-KD vs. SCR after 4 h. These blunted responses were associated with attenuated IGF-I-induced FAK Tyr³⁹⁷ phosphorylation and markedly suppressed phosphorylation of tuberous sclerosis complex 2 (TSC2) and critical downstream mTOR signaling (ribosomal S6 kinase, eIF4F assembly) in FAK shRNA cells (all P < 0.05 vs. IGF-I-treated SCR cells). However, binding of FAK to TSC2 or its phosphatase Shp-2 was not affected by IGF-I or cell phenotype. Finally, FAK-KD-mediated suppression of cell growth was recapitulated by direct inhibition of FAK kinase activity in SCR cells. We conclude that FAK is required for IGF-I-induced muscle hypertrophy, signaling through a TSC2/mTOR/S6K1-dependent pathway via means requiring the kinase activity of FAK but not altered FAK-TSC2 or FAK-Shp-2 binding.

Abnormal protein turnover and anabolic resistance to exercise in sarcopenic obesity

Obesity may impair protein synthesis rates and cause anabolic resistance to growth factors, hormones, and exercise, ultimately affecting skeletal muscle mass and function. To better understand muscle wasting and anabolic resistance with obesity, we assessed protein 24-h fractional synthesis rates (24-h FSRs) in selected hind-limb muscles of sedentary and resistance-exercised lean and obese Zucker rats. Despite atrophied hind-limb muscles (-28% vs. lean rats), 24-h FSRs of mixed proteins were significantly higher in quadriceps (+18%) and red or white gastrocnemius (+22 or +38%, respectively) of obese animals when compared to lean littermates. Basal synthesis rates of myofibrillar (+8%) and mitochondrial proteins (-1%) in quadriceps were not different between phenotypes, while manufacture of cytosolic proteins (+12%) was moderately elevated in obese cohorts. Western blot analyses revealed a robust activation of p70S6k (+178%) and a lower expression of the endogenous mTOR inhibitor DEPTOR (-28%) in obese rats, collectively suggesting that there is an obesity-induced increase in net protein turnover favoring degradation. Lastly, the protein synthetic response to exercise of mixed (-7%), myofibrillar (+6%), and cytosolic (+7%) quadriceps subfractions was blunted compared to the lean phenotype (+34, +40, and +17%, respectively), indicating a muscle- and subfraction-specific desensitization to the anabolic stimulus of exercise in obese animals.

Measuring protein synthesis with SUnSET: a valid alternative to traditional techniques?

Protein synthesis rates commonly are measured using isotopic tracers to quantify the incorporation of a labeled amino acid into muscle proteins. Here we provide evidence supporting our hypothesis that the nonisotopic SUnSET technique is a valid and accurate method for the measurement of in vivo changes in protein synthesis at the whole-muscle and single-muscle fiber levels.

Influence of amino acids, dietary protein, and physical activity on muscle mass development in humans

Ingestion of protein is crucial for maintenance of a variety of body functions and within the scope of this review we will specifically focus on the regulation of skeletal muscle mass. A quantitative limitation exists as to how much muscle protein the body can synthesize in response to protein intake. Ingestion of excess protein exerts an unwanted load to the body and therefore, it is important to find the least amount of protein that provides the maximal hypertrophic stimulus. Hence, research has focused on revealing the relationship between protein intake (dose) and its resulting stimulation of muscle protein synthesis (response). In addition to the protein amount, the protein digestibility and, hence, the availability of its constituent amino acids is decisive for the response. In this regard, recent studies have provided in-depth knowledge about the time-course of the muscle protein synthetic response dependent on the characteristics of the protein ingested. The effect of protein intake on muscle protein accretion can further be stimulated by prior exercise training. In the ageing population, physical training may counteract the development of “anabolic resistance” and restore the beneficial effect of protein feeding. Presently, our knowledge is based on measures obtained in standardized experimental settings or during long-term intervention periods. However, to improve coherence between these types of data and to further improve our knowledge of the effects of protein ingestion, other investigative approaches than those presently used are requested.

Metabolic labeling reveals proteome dynamics of mouse mitochondria

Mitochondrial dysfunction is associated with many human diseases. Mitochondrial damage is exacerbated by inadequate protein quality control and often further contributes to pathogenesis. The maintenance of mitochondrial functions requires a delicate balance of continuous protein synthesis and degradation, i.e. protein turnover. To understand mitochondrial protein dynamics in vivo, we designed a metabolic heavy water ((2)H(2)O) labeling strategy customized to examine individual protein turnover in the mitochondria in a systematic fashion. Mice were fed with (2)H(2)O at a minimal level (<5% body water) without physiological impacts. Mitochondrial proteins were analyzed from 9 mice at each of the 13 time points between 0 and 90 days (d) of labeling. A novel multiparameter fitting approach computationally determined the normalized peak areas of peptide mass isotopomers at initial and steady-state time points and permitted the protein half-life to be determined without plateau-level (2)H incorporation. We characterized the turnover rates of 458 proteins in mouse cardiac and hepatic mitochondria and found median turnover rates of 0.0402 d(-1) and 0.163 d(-1), respectively, corresponding to median half-lives of 17.2 d and 4.26 d. Mitochondria in the heart and those in the liver exhibited distinct turnover kinetics, with limited synchronization within functional clusters. We observed considerable interprotein differences in turnover rates in both organs, with half-lives spanning from hours to months (≈ 60 d). Our proteomics platform demonstrates the first large-scale analysis of mitochondrial protein turnover rates in vivo, with potential applications in translational research.

Protein turnover, amino acid requirements and recommendations for athletes and active populations

Skeletal muscle is the major deposit of protein molecules. As for any cell or tissue, total muscle protein reflects a dynamic turnover between net protein synthesis and degradation. Noninvasive and invasive techniques have been applied to determine amino acid catabolism and muscle protein building at rest, during exercise and during the recovery period after a single experiment or training sessions. Stable isotopic tracers (¹³C-lysine, ¹⁵N-glycine, ²H₅-phenylalanine) and arteriovenous differences have been used in studies of skeletal muscle and collagen tissues under resting and exercise conditions. There are different fractional synthesis rates in skeletal muscle and tendon tissues, but there is no major difference between collagen and myofibrillar protein synthesis. Strenuous exercise provokes increased proteolysis and decreased protein synthesis, the opposite occurring during the recovery period. Individuals who exercise respond differently when resistance and endurance types of contractions are compared. Endurance exercise induces a greater oxidative capacity (enzymes) compared to resistance exercise, which induces fiber hypertrophy (myofibrils). Nitrogen balance (difference between protein intake and protein degradation) for athletes is usually balanced when the intake of protein reaches 1.2 g·kg⁻¹·day⁻¹ compared to 0.8 g·kg⁻¹·day⁻¹ in resting individuals. Muscular activities promote a cascade of signals leading to the stimulation of eukaryotic initiation of myofibrillar protein synthesis. As suggested in several publications, a bolus of 15-20 g protein (from skimmed milk or whey proteins) and carbohydrate (± 30 g maltodextrine) drinks is needed immediately after stopping exercise to stimulate muscle protein and tendon collagen turnover within 1 h.

A new metabolomics analysis technique: steady-state metabolic network dynamics analysis

With the recent advances in experimental technologies, such as gas chromatography and mass spectrometry, the number of metabolites that can be measured in biofluids of individuals has markedly increased. Given a set of such measurements, a very common task encountered by biologists is to identify the metabolic mechanisms that lead to changes in the concentrations of given metabolites and interpret the metabolic consequences of the observed changes in terms of physiological problems, nutritional deficiencies, or diseases. In this paper, we present the steady-state metabolic network dynamics analysis (SMDA) approach in detail, together with its application in a cystic fibrosis study. We also present a computational performance evaluation of the SMDA tool against a mammalian metabolic network database. The query output space of the SMDA tool is exponentially large in the number of reactions of the network. However, (i) larger numbers of observations exponentially reduce the output size, and (ii) exploratory search and browsing of the query output space is provided to allow users to search for what they are looking for.

Acute resistance exercise augments integrative myofibrillar protein synthesis

The purpose of this study was to determine whether an acute bout of high-intensity resistance exercise (RE) would augment integrative mixed muscle and myofibrillar protein fractional synthesis rates (FSRs) when total energy and macronutrient intake was controlled. Twelve healthy young men were studied over 24 hours and performed an acute bout of exhaustive (5 sets until volitional failure of their 85% 1-repetition maximum) unilateral leg press and knee extension exercise, such that one leg was exercised (EX) and the other served as a control (CON). (2)H(2)O (70%) was provided to measure mixed muscle and myofibrillar FSR, and muscle biopsies (vastus lateralis) were collected from the EX and CON legs 16 hours following the RE session. (2)H-labeling of body water over the course of the experiment was 0.32 ± 0.01 mole percent excess. Interestingly, integrative mixed muscle FSR (percent per hour) was similar between the CON (0.76% ± 0.08%) and EX (0.69% ± 0.06%) legs. In contrast, upon determination of myofibrillar FSR, there was an RE effect (EX, 0.94% ± 0.16% vs CON, 0.75% ± 0.08%; P < .05). High-intensity RE without prior training impacts integrative myofibrillar 24-hour FSR, perhaps without altering total responses.

Measurement of human plasma proteome dynamics with (2)H(2)O and liquid chromatography tandem mass spectrometry

Dysfunction of protein turnover is a feature of many human diseases, and proteins are substrates in important biological processes. Currently, no method exists for the measurement of global protein turnover (i.e., proteome dynamics) that can be applied in humans. Here we describe the use of metabolic labeling with deuterium ((2)H) from (2)H(2)O and liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis of mass isotopomer patterns to measure protein turnover. We show that the positions available for (2)H label incorporation in vivo can be calculated using peptide sequence. The isotopic incorporation values calculated by combinatorial analysis of mass isotopomer patterns in peptides correlate very closely with values established for individual amino acids. Inpatient and outpatient heavy water labeling protocols resulted in (2)H label incorporation sufficient for reproducible quantitation in humans. Replacement rates were similar for peptides deriving from the same protein. Using a kinetic model to account for the time course of each individual's (2)H(2)O enrichment curves, dynamics of approximately 100 proteins with half-lives ranging from 0.4 to 40 days were measured using 8 μl of plasma. The measured rates were consistent with literature values. This method can be used to measure in vivo proteome homeostasis in humans in disease and during therapeutic interventions.

Cumulative responses of muscle protein synthesis are augmented with chronic resistance exercise training

AIM

The purpose of this study was to determine the anabolic response of a single bout of high intensity resistance exercise (RE) following 5 weeks of RE training.

METHODS

To complete these studies, Sprague-Dawley rats were assigned by body mass to RE, exercise control (EC), or sedentary cage control (CC) groups and studied over 36 h after 5 weeks of RE (squat-like) training. Cumulative (final 36 h) fractional rates of muscle protein synthesis (FSR) were determined by ²H₂O, and acute (16 h post-RE) rates of muscle protein synthesis (RPS) were determined by flooding with l-[2,3,4,5,6-³H]phenylalanine. Regulators of peptide-chain initiation, 4E-BP1, eIF4E and the association of the two were determined by Western blotting and immunoprecipitation respectively.

RESULTS

No differences were observed with acute measures of RPS obtained 16 h following the final exercise bout in the plantaris or soleus muscles (P > 0.05). Consistent with this observation, 4E-BP1 was similarly phosphorylated and bound to eIF4E among all groups. However, upon determination of the cumulative response, FSR was significantly increased in the plantaris of RE vs. EC and CC (0.929±0.094, 0.384±0.039, 0.300±0.022% h(-1) respectively; P<0.001), but not the soleus.

CONCLUSION

With the advantage of determining cumulative FSR, the present study demonstrates that anabolic responses to RE are still evident after chronic RE training, primarily in muscle composed of fast-twitch fibres.

Isotopomer enrichment assay for very short chain fatty acids and its metabolic applications

The present work illustrated an accurate GC/MS measurement for the low isotopomer enrichment assay of formic acid, acetic acid, propionic aicd, butyric acid, and pentanoic acid. The pentafluorobenzyl bromide derivatives of these very short chain fatty acids have high sensitivity of isotopoic enrichment due to their low natural isotopomer distribution in negative chemical ionization mass spectrometric mode. Pentafluorobenzyl bromide derivatization reaction was optimized in terms of pH, temperature, reaction time, and the amount of pentafluorobenzyl bromide versus sample. The precision, stability, and accuracy of this method for the isotopomer analysis were validated. This method was applied to measure the enrichments of formic acid, acetic acid, and propionic acid in the perfusate from rat liver exposed to Krebs-Ringer bicarbonate buffer only, 0-1mM [3,4-(13)C(2)]-4-hydroxynonanoate, and 0-2mM [5,6,7-(13)C(3)]heptanoate. The enrichments of acetic acid and propionic acid in the perfusate are comparable to the labeling pattern of acetyl-CoA and propionyl-CoA in the rat liver tissues. The enrichment of the acetic acid assay is much more sensitive and precise than the enrichment of acetyl-CoA by LC-MS/MS. The reversibility of propionyl-CoA from succinyl-CoA was confirmed by the low labeling of M1 and M2 of propionic acid from [5,6,7-(13)C(3)]heptanoate perfusates.