The application of stable-isotope tracers to study human musculoskeletal protein turnover: a tale of bag filling and bag enlargement

Post-prandial tracer studies of protein and amino acid utilisation: what can they tell us about human amino acid and protein requirements?

Nitrogen balance (NB), the principal methodology used to derive recommendations for human protein and amino acid requirements, has been widely criticised, and calls for increased protein and amino acid requirement recommendations have been made, often on the basis of post-prandial amino acid tracer kinetic studies of muscle protein synthesis, or of amino acid oxidation. This narrative review considers our knowledge of the homeostatic regulation of the FFM throughout the diurnal cycle of feeding and fasting and what can and has been learnt from post-prandial amino acid tracer studies, about amino acid and protein requirements. Within the FFM, muscle mass in well fed weight-stable adults with healthy lifestyles appears fixed at a phenotypic level within a wide range of habitual protein intakes. However homoeostatic regulation occurs in response to variation in habitual protein intake, with adaptive changes in amino acid oxidation which influence the magnitude of diurnal losses and gains of body protein. Post-prandial indicator amino acid oxidation (IAAO) studies have been introduced as an alternative to NB and to the logistically complex 24 h [13C-1] amino acid balance studies, for assessment of protein and amino acid requirements. However, a detailed examination of IAAO studies shows both a lack of concern for homeostatic regulation of amino acid oxidation and  major flaws in their design and analytical interpretation, which seriously constrain their ability to provide reliable values. New ideas and a much more critical approach to existing work is needed if real progress is to be made in the area.

Post-natal muscle growth and protein turnover: a narrative review of current understanding

A model explaining the dietary-protein-driven post-natal skeletal muscle growth and protein turnover in the rat is updated, and the mechanisms involved are described, in this narrative review. Dietary protein controls both bone length and muscle growth, which are interrelated through mechanotransduction mechanisms with muscle growth induced both from stretching subsequent to bone length growth and from internal work against gravity. This induces satellite cell activation, myogenesis and remodelling of the extracellular matrix, establishing a growth capacity for myofibre length and cross-sectional area. Protein deposition within this capacity is enabled by adequate dietary protein and other key nutrients. After briefly reviewing the experimental animal origins of the growth model, key concepts and processes important for growth are reviewed. These include the growth in number and size of the myonuclear domain, satellite cell activity during post-natal development and the autocrine/paracrine action of IGF-1. Regulatory and signalling pathways reviewed include developmental mechanotransduction, signalling through the insulin/IGF-1-PI3K-Akt and the Ras-MAPK pathways in the myofibre and during mechanotransduction of satellite cells. Likely pathways activated by maximal-intensity muscle contractions are highlighted and the regulation of the capacity for protein synthesis in terms of ribosome assembly and the translational regulation of 5-TOPmRNA classes by mTORC1 and LARP1 are discussed. Evidence for and potential mechanisms by which volume limitation of muscle growth can occur which would limit protein deposition within the myofibre are reviewed. An understanding of how muscle growth is achieved allows better nutritional management of its growth in health and disease.

Application of elemental analysis-coupled isotope ratio mass spectrometry for protein turnover analysis using deuterium labeling: Purification and analysis of hydrogen isotope ratio of non-derivatized protein-bound alanine

RATIONALE

Heavy water can be used as a tracer for the evaluation of protein turnover. By adding heavy water (D₂ O) to the precursor pool, nonessential amino acids, including alanine, can be isotopically labeled in vivo. Protein turnover can then be quantified by measuring the hydrogen isotope ratio of protein-bound alanine.

METHODS

In this study, we constructed a novel method to apply deuterium labeling of alanine to the evaluation of protein turnover using elemental analysis-coupled isotope ratio mass spectrometry (EA-IRMS). We established a preparative high-performance liquid chromatography method to isolate alanine from protein hydrolysates. EA-IRMS was then used to determine the hydrogen isotope ratio of alanine isolated from hydrolysates of protein from mouse myoblast C2C12 cells that had been treated with D₂ O over the course of 72 h.

RESULTS

In cells treated with 4% D₂ O, the deuterium enrichment of alanine increased to approximately 0.9% over time, while that of cells treated with 0.017% D₂ O increased to approximately 0.006%. The rate of protein synthesis calculated by fitting the increase of deuterium excess to rise-to-plateau kinetics was similar regardless of the concentration of D₂ O. When C2C12 cells treated with insulin and rapamycin were analyzed 24 h after the addition of 0.017% D₂ O, protein turnover was found to be accelerated by insulin, but this effect was offset by co-treatment with rapamycin.

CONCLUSION

The derivative-free measurement of the hydrogen isotope ratio of protein-bound alanine using EA-IRMS can be applied to the evaluation of protein turnover. The proposed method is an accessible option for many laboratories to perform highly sensitive IRMS-based evaluations of protein metabolic turnover.

The effects of elective abdominal surgery on protein turnover: A meta-analysis of stable isotope techniques to investigate postoperative catabolism

BACKGROUND & AIMS

Elective surgery induces skeletal muscle wasting driven by an imbalance between muscle protein synthesis and breakdown. From examination of diverse stable isotope tracer techniques, the dynamic processes driving this imbalance are unclear. This meta-analysis aimed to elucidate the mechanistic driver(s) of postoperative protein catabolism through stable isotope assessment of protein turnover before and after abdominal surgery.

METHODS

Meta-analysis was performed of randomized controlled trials and cohort studies in patients undergoing elective abdominal surgery that contained measurements of whole-body or skeletal muscle protein turnover using stable isotope tracer methodologies pre- and postoperatively. Postoperative changes in protein synthesis and breakdown were assessed through subgroup analysis of tracer methodology and perioperative care.

RESULTS

Surgery elicited no overall change in protein synthesis [standardized mean difference (SMD) -0.47, 95% confidence interval (CI): -1.32, 0.39, p = 0.25]. However, subgroup analysis revealed significant suppressions via direct-incorporation methodology [SMD -1.53, 95%CI: -2.89, -0.17, p = 0.03] within skeletal muscle. Changes of this nature were not present among arterio-venous [SMD 0.61, 95%CI: -1.48, 2.70, p = 0.58] or end-product [SMD -0.09, 95%CI: -0.81, 0.64, p = 0.82] whole-body measures. Surgery resulted in no overall change in protein breakdown [SMD 0.63, 95%CI: -0.06, 1.32, p = 0.07]. Yet, separation by tracer methodology illustrated significant increases in urinary end-products (urea/ammonia) [SMD 0.70, 95%CI: 0.38, 1.02, p < 0.001] that were not present among arterio-venous measures [SMD 0.67, 95%CI: -1.05, 2.38, p = 0.45].

CONCLUSIONS

Elective abdominal surgery elicits suppressions in skeletal muscle protein synthesis that are not reflected on a whole-body level. Lack of uniform changes across whole-body tracer techniques are likely due to contribution from tissues other than skeletal muscle.

Making Sense of Muscle Protein Synthesis: A Focus on Muscle Growth During Resistance Training

The acute response of muscle protein synthesis (MPS) to resistance exercise and nutrition is often used to inform recommendations for exercise programming and dietary interventions, particularly protein nutrition, to support and enhance muscle growth with training. Those recommendations are worthwhile only if there is a predictive relationship between the acute response of MPS and subsequent muscle hypertrophy during resistance exercise training. The metabolic basis for muscle hypertrophy is the dynamic balance between the synthesis and degradation of myofibrillar proteins in muscle. There is ample evidence that the process of MPS is much more responsive to exercise and nutrition interventions than muscle protein breakdown. Thus, it is intuitively satisfying to translate the acute changes in MPS to muscle hypertrophy with training over a longer time frame. Our aim is to examine and critically evaluate the strength and nature of this relationship. Moreover, we examine the methodological and physiological factors related to measurement of MPS and changes in muscle hypertrophy that contribute to uncertainty regarding this relationship. Finally, we attempt to offer recommendations for practical and contextually relevant application of the information available from studies of the acute response of MPS to optimize muscle hypertrophy with training.

Weight Loss Strategies and the Risk of Skeletal Muscle Mass Loss

With energy intake restriction and exercise remaining the key diet and lifestyle approaches to weight loss, this is not without potential negative implications for body composition, metabolic health, and quality and quantity of life. Ideally, weight loss should be derived almost exclusively from the fat mass compartment as this is the main driver of metabolic disease, however, several studies have shown that there is an accompanying loss of tissue from the fat-free compartment, especially skeletal muscle. Population groups including post-menopausal women, the elderly, those with metabolic disease and athletes may be particularly at risk of skeletal muscle loss when following a weight management programme. Research studies that have addressed this issue across a range of population groups are reviewed with a focus upon the contribution of resistance and endurance forms of exercise and a higher intake dietary protein above the current guideline of 0.8 g/kg body weight/day. While findings can be contradictory, overall, the consensus appears that fat-free and skeletal muscle masses can be preserved, albeit to varying degrees by including both forms of exercise (but especially resistance forms) in the weight management intervention. Equally, higher intakes of protein can protect loss of these body compartments, acting either separately or synergistically with exercise. Elderly individuals in particular may benefit most from this approach. Thus, the evidence supports the recommendations for intakes of protein above the current guidelines of 0.8 g/kg body weight/d for the healthy elderly population to also be incorporated into the dietary prescription for weight management in this age group.

Monitoring urinary collagen metabolite changes following collagen peptide ingestion and physical activity using ELISA with anti active collagen oligopeptide antibody

Active collagen oligopeptides (ACOP) are bioactive collagen-derived peptides detected by a recently-established ELISA. To facilitate studies of the function and metabolism of these products, this study aims to determine which of these peptides is recognized by a novel anti-ACOP antibody used in this ELISA. We then investigate the effect of collagen peptide (CP) ingestion and exercise on urinary ACOP concentrations in a cohort of university student athletes using colorimetric, LC–MS/MS, and ELISA. We observed that the antibody showed strong cross-reactivity to Pro-Hyp and Gly-Pro-Hyp and weak cross-reactivity to commercial CP. CP ingestion increased the urinary level of ACOP over time, which correlated highly with urinary levels of peptide forms of Hyp and Pro-Hyp. Physical activity significantly decreased the urinary ACOP level. This study demonstrates changes in urinary ACOP following oral CP intake and physical activity using ELISA with the novel anti-ACOP antibody. Thus, ACOP may be useful as a new biomarker for collagen metabolism.

Interactions between Growth of Muscle and Stature: Mechanisms Involved and Their Nutritional Sensitivity to Dietary Protein: The Protein-Stat Revisited

Childhood growth and its sensitivity to dietary protein is reviewed within a Protein-Stat model of growth regulation. The coordination of growth of muscle and stature is a combination of genetic programming, and of two-way mechanical interactions involving the mechanotransduction of muscle growth through stretching by bone length growth, the core Protein-Stat feature, and the strengthening of bone through muscle contraction via the mechanostat. Thus, growth in bone length is the initiating event and this is always observed. Endocrine and cellular mechanisms of growth in stature are reviewed in terms of the growth hormone-insulin like growth factor-1 (GH-IGF-1) and thyroid axes and the sex hormones, which together mediate endochondral ossification in the growth plate and bone lengthening. Cellular mechanisms of muscle growth during development are then reviewed identifying (a) the difficulties posed by the need to maintain its ultrastructure during myofibre hypertrophy within the extracellular matrix and the concept of muscle as concentric "bags" allowing growth to be conceived as bag enlargement and filling, (b) the cellular and molecular mechanisms involved in the mechanotransduction of satellite and mesenchymal stromal cells, to enable both connective tissue remodelling and provision of new myonuclei to aid myofibre hypertrophy and (c) the implications of myofibre hypertrophy for protein turnover within the myonuclear domain. Experimental data from rodent and avian animal models illustrate likely changes in DNA domain size and protein turnover during developmental and stretch-induced muscle growth and between different muscle fibre types. Growth of muscle in male rats during adulthood suggests that "bag enlargement" is achieved mainly through the action of mesenchymal stromal cells. Current understanding of the nutritional regulation of protein deposition in muscle, deriving from experimental studies in animals and human adults, is reviewed, identifying regulation by amino acids, insulin and myofibre volume changes acting to increase both ribosomal capacity and efficiency of muscle protein synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) and the phenomenon of a "bag-full" inhibitory signal has been identified in human skeletal muscle. The final section deals with the nutritional sensitivity of growth of muscle and stature to dietary protein in children. Growth in length/height as a function of dietary protein intake is described in the context of the breastfed child as the normative growth model, and the "Early Protein Hypothesis" linking high protein intakes in infancy to later adiposity. The extensive paediatric studies on serum IGF-1 and child growth are reviewed but their clinical relevance is of limited value for understanding growth regulation; a role in energy metabolism and homeostasis, acting with insulin to mediate adiposity, is probably more important. Information on the influence of dietary protein on muscle mass per se as opposed to lean body mass is limited but suggests that increased protein intake in children is unable to promote muscle growth in excess of that linked to genotypic growth in length/height. One possible exception is milk protein intake, which cohort and cross-cultural studies suggest can increase height and associated muscle growth, although such effects have yet to be demonstrated by randomised controlled trials.

More than just a garbage can: emerging roles of the lysosome as an anabolic organelle in skeletal muscle

Skeletal muscle is a highly plastic tissue capable of remodeling in response to a range of physiological stimuli, including nutrients and exercise. Historically, the lysosome has been considered an essentially catabolic organelle contributing to autophagy, phagocytosis, and exo-/endocytosis in skeletal muscle. However, recent evidence has emerged of several anabolic roles for the lysosome, including the requirement for autophagy in skeletal muscle mass maintenance, the discovery of the lysosome as an intracellular signaling hub for mechanistic target of rapamycin complex 1 (mTORC1) activation, and the importance of transcription factor EB/lysosomal biogenesis-related signaling in the regulation of mTORC1-mediated protein synthesis. We, therefore, propose that the lysosome is an understudied organelle with the potential to underpin the skeletal muscle adaptive response to anabolic stimuli. Within this review, we describe the molecular regulation of lysosome biogenesis and detail the emerging anabolic roles of the lysosome in skeletal muscle with particular emphasis on how these roles may mediate adaptations to chronic resistance exercise. Furthermore, given the well-established role of amino acids to support muscle protein remodeling, we describe how dietary proteins "labeled" with stable isotopes could provide a complementary research tool to better understand how lysosomal biogenesis, autophagy regulation, and/or mTORC1-lysosomal repositioning can mediate the intracellular usage of dietary amino acids in response to anabolic stimuli. Finally, we provide avenues for future research with the aim of elucidating how the regulation of this important organelle could mediate skeletal muscle anabolism.

A novel stable isotope tracer method to simultaneously quantify skeletal muscle protein synthesis and breakdown

Background/aims

Methodological challenges have been associated with the dynamic measurement of muscle protein breakdown (MPB), as have the measurement of both muscle protein synthesis (MPS) and MPB within the same experiment. Our aim was to use the transmethylation properties of methionine as proof-of-concept to measure rates of MPB via its methylation of histidine within skeletal muscle myofibrillar proteins, whilst simultaneously utilising methionine incorporation into bound protein to measure MPS.

Results

During the synthesis measurement period, incorporation of methyl[D₃]-¹³C-methionine into cellular protein in C2C12 myotubes was observed (representative of MPS), alongside an increase in the appearance of methyl[D₃]-methylhistidine into the media following methylation of histidine (representative of MPB). For further validation of this approach, fractional synthetic rates (FSR) of muscle protein were increased following treatment of the cells with the anabolic factors insulin-like growth factor-1 (IGF-1) and insulin, while dexamethasone expectedly reduced MPS. Conversely, rates of MPB were reduced with IGF-1 and insulin treatments, whereas dexamethasone accelerated MPB.

Conclusions

This is a novel stable isotope tracer approach that permits the dual assessment of muscle cellular protein synthesis and breakdown rates, through the provision of a single methionine amino acid tracer that could be utilised in a wide range of biological settings.

The Importance of Isotopic Turnover for Understanding Key Aspects of Animal Ecology and Nutrition

Stable isotope-based methods have proved to be immensely valuable for ecological studies ranging in focus from animal movements to species interactions and community structure. Nevertheless, the use of these methods is dependent on assumptions about the incorporation and turnover of isotopes within animal tissues, which are oftentimes not explicitly acknowledged and vetted. Thus, the purpose of this review is to provide an overview of the estimation of stable isotope turnover rates in animals, and to highlight the importance of these estimates for ecological studies in terrestrial, freshwater, and marine systems that may use a wide range of stable isotopes. Specifically, we discuss 1) the factors that contribute to variation in turnover among individuals and across species, which influences the use of stable isotopes for diet reconstructions, 2) the differences in turnover among tissues that underlie so-called ‘isotopic clocks’, which are used to estimate the timing of dietary shifts, and 3) the use of turnover rates to estimate nutritional requirements and reconstruct histories of nutritional stress from tissue isotope signatures. As we discuss these topics, we highlight recent works that have effectively used estimates of turnover to design and execute informative ecological studies. Our concluding remarks suggest several steps that will improve our understanding of isotopic turnover and support its integration into a wider range of ecological studies.