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23.Lit.C: Skeletal Muscle Regulates Metabolism - Biology


Josep M. Journal of the American Medical Directors Association,
Volume 17, Issue 9, 2016, Pages 789-796,ISSN 1525-8610,https://doi.org/10.1016/j.jamda.2016.04.019.

Abstract

Skeletal muscle is recognized as vital to physical movement, posture, and breathing. In a less known but critically important role, muscle influences energy and protein metabolism throughout the body. Muscle is a primary site for glucose uptake and storage, and it is also a reservoir of amino acids stored as protein. Amino acids are released when supplies are needed elsewhere in the body. These conditions occur with acute and chronic diseases, which decrease dietary intake while increasing metabolic needs. Such metabolic shifts lead to the muscle loss associated with sarcopenia and cachexia, resulting in a variety of adverse health and economic consequences. With loss of skeletal muscle, protein and energy availability is lowered throughout the body. Muscle loss is associated with delayed recovery from illness, slowed wound healing, reduced resting metabolic rate, physical disability, poorer quality of life, and higher health care costs. These adverse effects can be combatted with exercise and nutrition. Studies suggest dietary protein and leucine or its metabolite β-hydroxy β-methylbutyrate (HMB) can improve muscle function, in turn improving functional performance. Considerable evidence shows that use of high-protein oral nutritional supplements (ONS) can help maintain and rebuild muscle mass and strength. We review muscle structure, function, and role in energy and protein balance. We discuss how disease- and age-related malnutrition hamper muscle accretion, ultimately causing whole-body deterioration. Finally, we describe how specialized nutrition and exercise can restore muscle mass, strength, and function, and ultimately reverse the negative health and economic outcomes associated with muscle loss.

Keywords

Muscle

glucose

amino acid

sarcopenia

HMB

ONS

Skeletal muscle is integral to physical movement, posture, and vital actions, such as chewing, swallowing, and breathing.1, 2 Skeletal muscle also serves as a regulator of interorgan crosstalk for energy and protein metabolism throughout the body, a less recognized but critically important role. As such, skeletal muscle is a key site for glucose uptake and storage.3 Skeletal muscle is likewise a reservoir of amino acids that can support protein synthesis or energy production elsewhere in the body when other sources are depleted.4

This review of muscle metabolism describes how amino acids stored as protein in muscle can be broken down through proteolysis for ultimate use in energy production. Such breakdown occurs when energy demands are high (as with stress-induced hypermetabolism), or when supplies are low (as in severe starvation or longer-term protein energy malnutrition). Both of these states can be hallmarks of many diseases, either directly as a result of disease-related dysregulation of metabolism (such as in the extreme case of cancer-cachexia) or, more subtly, as a result of the general illness-associated loss of appetite. Muscle is therefore crucially important during illness, both for its role in balancing the metabolic needs of other organs and for its reserves of protein for use in energy production. Yet, during illness, the maintenance of muscle mass through exercise and nutrition are often overlooked or difficult to address, and muscle atrophy develops. Even more subtle is aging-related muscle loss, which can dramatically increase morbidity and mortality of otherwise survivable illnesses in the aged. This review also illustrates the consequences of muscle atrophy in aging and illness and proposes steps to combat these challenges.

Muscle Basics

Muscle Structure and Classification

Skeletal muscle comprises the fibrillar proteins myosin (a thick filament) and actin (a thin filament) that interact to cause muscle contraction, a process requiring energy in the form of adenosine triphosphate (ATP). Different muscle types have been classified according to histochemical features, structural protein composition, and major metabolic properties.5, 6 Most commonly, skeletal muscles are referred to as either “slow” or “fast” to reflect speeds of contraction, or the shortening of myosin heavy chain (MHC) protein.6 The velocity of this shortening is dependent on the MHC isoform present; “fast” fiber isoforms MHCIIa and IIb demonstrate a higher shortening velocity than their “slow” fiber MHCI counterparts.6, 7 Classic histochemical staining methods also classify muscle as type I (slow) and type II (fast) based on the myosin ATPase enzyme forms revealed. Recently, these types have been further distinguished based on histology (types I, IC, IIC, IIAC, IIA, IIAB, and IIB).6

Muscle Metabolism and Interorgan Crosstalk

Glucose regulation is central to energy balance both within muscle fibers and throughout the body. In the cytoplasm of most cells, glucose undergoes glycolysis to produce the substrate for ATP generation. Muscle fibers are also characterized on the basis of the speed and manner in which they metabolize glucose. The terms “fast” and “slow” can indicate the type of glucose metabolism occurring within the fiber. Slow muscles, which use aerobic metabolism, contain a high density of capillaries and oxidative enzymes that allow a greater resistance to fatigue.7 Fast muscles, which depend on anaerobic metabolism, or glycolysis, can quickly generate ATP and therefore contract more readily. Fast muscles also fatigue sooner than slow fibers, as the conversion of glucose to pyruvate generates less ATP than can be generated by using the rest of central metabolism, ultimately generating CO2.

Muscle has the ability to store glucose in the form of glycogen, which facilitates the rapid initiation of energy production for contraction even when glucose is not readily available from the diet. This unique capacity, shared also by the liver and kidneys, makes skeletal muscle an important metabolic organ that helps all organs have access to essential energy substrates during fasting. Furthermore, the amino acids stored in muscle as protein can be broken down as a last resort during times of starvation or extreme energy shortfalls.4 Patterns of glucose utilization throughout the body as a whole reflect feeding status (Figure 1; Table 1). Based on a classic study of the fed state (measurement within 3 hours of eating), researchers estimated that 25% to 35% of an ingested carbohydrate load was quickly extracted from circulation and stored by the liver.3 Of the remaining glucose, approximately 40% was disposed in the muscle and 10% in the kidney.3 The brain used 15% to 20% of post-meal glucose.3

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Fig. 1. Glucose metabolism in fed, fasted and malnourished states. A, Glucose, lipids, and amino acids from the diet circulate during the fed state for use or storage in body organs. In the fasted state, glucose is released from the muscles, kidneys, and liver for whole-body metabolism, along with lipids from adipose tissue and amino acids from the muscle. B, When glucose stores have been depleted, amino acids are provided by the muscles to support crucial bodily functions.

Table 1. Glucose Metabolism in Fasted and Fed States

Fed StateFasted State
Diet-sourced glucose (exogenous glucose) is absorbed from the intestine to circulate in blood; glucose serves as an energy source in cells throughout the body. Cytoplasmic glucose undergoes glycolysis, in turn producing ATP.Little or no blood glucose from dietary sources; alternative energy sources are needed for function of tissues body-wide.
Glucose is primarily taken up by muscle and liver, where it can be used for energy or stored as glycogen (Glycogen synthesis).Glycogen stored in liver, kidney, and muscle is broken down to provide glucose as energy source (Glycogenolysis). Muscle uses glycogen-sourced glucose internally; liver and kidney can supply glucose to circulation.
Gluconeogenic substrates are stored in various organs (eg, pyruvate in liver, glycerol in fat, and amino acids in muscle).Endogenous generation of glucose from noncarbohydrate carbon substrates, such as pyruvate, lactate, glycerol, and glucogenic amino acids (Gluconeogenesis); occurs primarily in the liver and muscle, and to a lesser extent in the kidney.

In the fasted state (after 14 to 16 h without eating), the liver provides approximately 80% of glucose that is released into circulation. About half of this glucose comes from the breakdown of stored glycogen, and the rest from the metabolism of sources other than carbohydrate or glycogen, including certain amino acids, through a process known as gluconeogenesis.8 Interactions between muscle and liver are largely responsible for regulating carbohydrate metabolism and for achieving energy balance in normal fed and fasted states; the kidneys play a role similar to that of the liver, but to a lesser extent.3, 8 In addition, muscle tissue stores amino acids as protein, and adipose tissue serves as a depot of glycerol and fatty acids. As needed, amino acids and fatty acids can be metabolized to form acetyl coenzyme A for the tricarboxylic acid (TCA) cycle.

As glycogen stores become depleted, increasingly more glucose is produced by gluconeogenesis. Gluconeogenesis provides 70% of glucose released into the body 24 hours after eating, and 90% by 48 hours.8 As fasting is prolonged, the kidneys contribute increasingly higher amounts of glucose from gluconeogenesis.

Ultimately, amino acids stored in skeletal muscle are metabolized when the need for gluconeogenesis substrate is greatest. Skeletal muscle houses nearly 75% of all protein in the body and constitutes an important contributor to gluconeogenesis in states of drastic depletion. Maintenance of muscle protein content depends on the balance between protein synthesis and degradation.5 Under normal conditions, muscle protein mass gains during the fed state balance losses during the fasted state.4 However, under severe metabolic stress generated by serious illness or injury, muscle protein can become depleted by catabolism, and this can lead to harmful functional limitations.

Skeletal muscle proteolysis can provide amino acid substrates for glucose and glycogen formation, notably glutamine and alanine. Alanine is released into circulation and reaches the liver, where it serves as an excellent substrate for gluconeogenesis. Glutamine also has a beneficial role in this process: the carbon skeleton of glutamine is a gluconeogenic precursor that can regulate gluconeogenesis independently of the insulin/glucagon ratio. Therefore, glutamine supplementation may also enhance glycogen synthesis and increase muscle glycogen stores even when insulin levels are low or when insulin resistance is present.9

In summary, dietary glucose is supplied by meals, and glucose is stored as glycogen in liver, kidney, and muscle for metabolic energy functions, as needed (Table 1). At times when glucose supplies are not sufficient to meet energy needs, breakdown of glycogen (glycogenolysis) occurs. When stored glucose products are no longer available, energy is released by breakdown of substrates other than glucose. In this review of muscle metabolism, we emphasize that amino acids stored as protein in muscle can be broken down by way of gluconeogenesis, ultimately entering the TCA cycle for energy production. Such breakdown occurs when energy demands are high, as with stress-induced hypermetabolism of disease, or when supplies are low, as in severe starvation or disease-associated loss of appetite. Such use can become problematic in that it reduces skeletal muscle mass and produces waste nitrogen, which requires further energy to sequester and secrete. Prolonged reliance on these processes can accelerate existing health problems and must be addressed by the health care provider.

Muscle Plasticity: Changes in Muscle Mass, Strength, and Function

Skeletal muscle is remarkably plastic. It changes continuously in response to calorie and nutrient intake, illness, and physical stress. Changes in adult skeletal muscle also may occur as fiber-type switching, which is influenced by changes in physical activity, loading, nerve stimulation, or hormone and cytokine levels.7, 10, 11, 12

Mechanisms of Muscle Loss in Aging, Inactivity, Sickness, and Frailty

This ability of skeletal muscle to change dynamically in response to body conditions is also manifest as changes resulting from injury, illness, or aging. When the metabolic demands placed on muscle outweigh the protein synthesis that occurs from dietary intake and after exercise, muscle mass is lost, metabolic storage products are depleted, and muscle fiber balance changes.

Aging may lead to a loss of muscle mass resulting from both the shrinking of muscle fibers (atrophy) and the elimination of fibers altogether (Figure 2).6 This condition is known as primary sarcopenia, the age-related loss of muscle mass and function. Although both fiber types I and II lose mass, aging causes preferential atrophy of type II fibers; the net change is thus from type II to type I fibers, or from fast to slow muscle fibers.6, 13 Because fast muscle fibers mobilize ATP and create tension more readily than slow fibers, this shift can leave older adults without the energy to perform daily tasks.14 This shift to type I slow fibers leads to a corresponding increase in their characteristic oxidative metabolism relative to the glycolytic metabolism that occurs in type II fast fibers. Exacerbating the problems caused by muscle degradation in aging, it is possible that type I oxidative fibers normally experience higher protein turnover (i. protein synthesis and degradation), are less able to grow in size, and have different responses to insufficient nutrient intake, although these fiber-type differences remain poorly understood.7, 15, 16

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Fig. 2. Effects of aging and illness on muscle mass.

Beyond aging, muscle wasting is associated with many pathological states and chronic diseases, such as malnutrition, cancer, chronic kidney disease, chronic obstructive pulmonary disease, burns, muscular dystrophies, acquired immunodeficiency syndrome, sepsis, and immune disorders, and forced immobilization and bed rest are devastating to patients who are already challenged by these factors (Figure 2).14, 17 Most of these pathological conditions are associated with variable degrees of local and/or systemic chronic inflammation, which plays a crucial role in the onset of muscle atrophy. Loss of muscle mass is frequently associated with increased production of proinflammatory cytokines. Systemic inflammation is associated with reduced rates of protein synthesis paralleled by enhanced protein breakdown, both accounting for the loss of muscle mass. The effects exerted by proinflammatory cytokines on muscle mass are partially mediated by activating the transcription factor nuclear factor κB (NF-κB).18 The transcriptional activity is regulated by the phosphorylation and consequent degradation of the inhibitor Iκ-Bα, allowing the positive regulation of muscle RING-finger protein-1 (MuRF1) and other atrophy-related genes. Proinflammatory cytokines act on muscle protein metabolism not only by activating catabolic pathways, but also by downregulating the anabolic pathways.19 Elevated tumor necrosis factor-alpha (TNF-α) and interleukin-1 (IL-1) lead to inhibition of the Akt/mTOR signal transduction pathway and a subsequent reduction in protein synthesis. The inflammatory process that takes place during trauma or fractures is controlled and finely regulated. In the short term, it can facilitate complete and efficient reconstruction of muscle fibers through the stimulation of myogenesis. However, chronic inflammation can be deleterious, driving uncontrolled muscle atrophy and affecting contraction ability. Balance between pro- and anti-inflammatory cytokines is well known to be important in regulating physiological muscle protein turnover and myogenesis, and evidence suggesting that inflammation can impair force generation in muscles is also growing.20, 21

As inflammation accelerates muscle catabolism, resting energy expenditure increases and amino acids are released from muscles to serve as substrates for gluconeogenesis in liver and elsewhere in the body (Table 2).22 The efficiency of energy production is low when amino acids are used to generate energy, so muscle is at further risk for breakdown to meet needs.23 In addition, the liver changes metabolic priorities, using amino acids to produce acute phase reactant proteins instead of normal proteins, such as serum albumin, and to support gluconeogenesis. This process continues until the cause of stress has subsided. Thus, when the dietary proteins supplied are inadequate to meet needs, muscle protein is broken down to supply amino acids throughout the body. This reaction releases waste nitrogen, which requires further energy to convert to urea, thereby exacerbating the problem of the energy shortfall.24

Table 2. Major Molecular Pathways Influencing Muscle Accretion

EffectorMediatorMajor Pathway(s)Consequence
Mammalian target of rapamycin (mTOR)+Induced by BCAAs, HMBInteracts with protein translation machinery to facilitate initiation and elongationmTOR stimulation by a number of pathways increases protein synthesis
Insulinlike growth factor (IGF1)+Stimulated by meal-induced insulinIGF1R → PI3K → AKT → mTORReduced IGF1 from decreased eating and/or exercise leads to reduced protein synthesis and to muscle wasting
+Stimulated by exercise
Myostatin/Activin+Produced by skeletal muscleActivin receptors (ACTRIIA/B) → Smad2/3 –I mTORMyostatins negatively regulate protein synthesis
–Inhibited by FollistatinACTRIIA/B → FoxO → UPS
Inflammatory cytokines (TNFα, IL-1)+Upregulated by illness, injuryCytokine receptors → NFKB, p38, JAK, Caspases, E3 ligasesInflammation leads to apoptosis or autophagy-mediated muscle cell loss
–Inhibited by exerciseFoxO transcription factors → MAFBX; MURF1 → UPS (ubiquitin-proteasome system)
Vitamin D+Levels are increased by diet and sunlightVitamin D receptor → gene expression or repression in myogenic cellsVitamin D positively influences muscle growth

AKT, protein kinase B; BCAA, branched-chain amino acid; FoxO, forkhead box protein O; HMB, β-Hydroxy β-Methylbutyrate; IGF1R, insulinlike growth factor 1 receptor; JAK, janus kinase; MURF1, muscle RING-finger protein-1 p38, mitogen-activated protein kinase; PI3K, phosphatidylinositide 3-kinase; UPS, ubiquitin-proteasome system.

Complications Associated With Loss of Muscle

As aging and illness lead to muscle breakdown and atrophy, reduced muscle mass leaves patients without a crucial reservoir of amino acids and effector molecules, such as myokines, cytokines released by muscle, to help the body combat illness, infection, and wasting (Figure 3).23, 25, 26, 27 Therefore, muscle atrophy is associated with a wide range of harmful health effects that can be life changing, especially for older people.28, 29, 30, 31, 32, 33, 34, 35, 36 The most relevant condition associated with the presence of sarcopenia in this population is a clinical syndrome called frailty. The most accepted physiological framework explaining frailty and its consequences was proposed by Walston and Fried,37 who described a relationship between sarcopenia and energy imbalance called the “frailty cycle.” This cycle affects multiple systems, especially those susceptible to changes in hormones (mainly sexual hormones, IGF-1, and insulin) and the progressive development of a proinflammatory state.38, 39, 40 Additional biomarkers have recently been identified for roles in frailty, such as those related to endothelial dysfunction or micro RNAs central to the aging process.41, 42 Frailty can be defined as an age-associated biological syndrome characterized by a decreased biological reserve resulting from a decline in multiple physiological systems that leaves the individual at risk for developing poor outcomes (disability, death, and hospitalization) in the presence of stressors.43, 44 The prevalence of frailty in people older than 65 is approximately 10%, increases with age, and is greater in women.45

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Fig. 3. Complications of lean body mass (muscle) loss.

Frailty is now a recognized clinical medical syndrome that provides a biological framework for understanding vulnerabilities resulting from aging or chronic conditions.44, 46 It is clinically important to detect frailty in those at risk of developing disability. As aging progresses, frailty increases as the prognostic factor for death and incident disability.47, 48 Frailty and its underlying sarcopenia have been shown to predict risk of death, disability, and other adverse outcomes, including muscle mass atrophy, metabolic deterioration, slowed wound or postsurgical healing, and delayed recovery from illness.32, 34, 35 Frailty and the weakness that follows muscle loss lead to higher risk of falls, fractures,30 physical disability,29 need for institutional care,29 reduced quality of life,36 and heightened mortality.29, 33 Early identification of frailty risk provides the opportunity to provide interventions and avoid or delay disability as well as enhance recovery.

Loss of muscle associated with disease, injury, disuse, or aging significantly increases the cost of health care.34, 49, 50 Results of a recent study showed that older adults (mean age=70 years) who were very frail spent (Euro) 1917 more on total health costs in an interval of 3 months than did those who were not frail.51 In the United States, the direct cost of cachexia/sarcopenia to health care was reported to be 1.5% of annual total health care expenses.50 Such costs arise from the increased rate of hospitalization, incidence of complications, lengths of stay, and likelihood of readmission.52, 53 In the face of an aging population, the importance of identifying, preventing, and treating muscle loss cannot be overstated.

Detection and Treatment of Muscle Loss

Who Is at Risk of Muscle Atrophy, and How Do We Identify It?

Screening is crucial for predicting risk, and proper, timely intervention can reduce or eliminate the ensuing muscle mass and metabolic atrophy, substantially affecting morbidity, mortality, and cost. Special attention should be paid to the main risk categories: people who are malnourished or at risk of malnutrition for any reason33, 54; frail adults, especially the very old; people who become deconditioned and lose muscle due to age- and disability-related physical inactivity35; those with diseases or conditions with inflammatory components, such as chronic heart failure,55 chronic or acute kidney disease,56 cancer,57, 58, 59 severe infection and sepsis,60 insulin resistance/diabetes,61 intensive care unit–acquired weakness,25 and wound/surgical recovery.34

Reaching an accurate diagnosis of age- or disease-related muscle atrophy is difficult, and a number of criteria have been proposed but have not yet assessed in the clinical setting.14 Nonetheless, specific criteria and measures can be used to diagnose sarcopenia or cachexia.13, 27, 62, 63 Sarcopenia can be diagnosed when a patient has muscle mass that is at least 2 SDs below the relevant population mean and also presents with a low gait (walking) speed. In addition, low muscle strength and general physical performance may be taken into consideration.14 Cachexia can be diagnosed when at least 5% of body weight is lost within 12 months in the presence of underlying illness, and 3 of the following criteria are also met: decreased muscle strength, increased fatigue, anorexia, low fat-free mass index, abnormal biochemistry, increased inflammatory markers C-reactive protein (>5.0 mg/L) or IL-6 (>4.0 pg/mL), anemia (<12 g/dL), or low serum albumin (<3.2 g/dL).

Recent research into the molecular adaptations associated with the development of or that result from muscle atrophy and metabolic depletion may lead to the identification of biomarkers and, therefore, improvements in early detection (Table 2). A variety of signaling pathways known to positively influence muscle growth (bone morphogenetic proteins, brain-derived neurotrophic factors, follistatin, and irisin), as well as those known to negatively regulate muscle growth (transforming growth factor β, myostatin, activins, and growth and differentiation factor-15) and factors associated with muscle function and dysfunction (C-terminal agrin fragment and skeletal muscle specific troponin T) may emerge as biomarkers for muscle atrophy in aging and disease.64 To date, there is no universally recognized biomarker for muscle atrophy, but recent research in the field suggests that the combination of several biomarkers may facilitate the adequate diagnosis of muscle atrophy. Identification of such biomarkers and their incorporation into validated testing instruments should allow early identification of muscle atrophy (improving prognosis, and likely reducing cost to health care systems), but may also provide exciting targets for the development of new medications.

Nutritional Strategies for Maintaining and Rebuilding Muscle

Treatment of patients at risk can prevent or delay onset of muscle atrophy, or even target rebuilding of muscle when muscle atrophy is already evident (Figure 4).65 As a first step, treatment must provide adequate energy so that muscle proteins and their constituent amino acids are spared as an energy source. In addition, high protein intake is vital to treatment of muscle atrophy or for delaying its onset.7, 66, 67, 68, 69 It should be noted that the range of protein needs can vary widely from patient to patient. Because muscle mass may decrease or remain the same (based largely on how much protein synthesis outpaces protein degradation), the most direct way to prevent muscle loss is to ensure that sufficient protein is ingested. Use of high-protein oral nutritional supplements (ONS; ≥20% of total calories as protein) may be beneficial to such patients.70

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Fig. 4. Treatments for sarcopenia. It is currently recommended that patients at risk of or suffering from sarcopenia consume a diet high in protein, engage in resistance exercise, and take supplements of the leucine metabolite HMB.

By definition, the essential amino acids (EAAs) play a central role in protein nutritional status. Some amino acids play roles that are distinct from the traditional one of protein building blocks; many of these have little or nothing to do with protein synthesis, and are thus not included here. However, of central importance to the current discussion are the branched-chain amino acids (BCAAs), especially leucine.65, 71, 72 BCAAs promote protein synthesis in the muscles through a number of pathways.66 In particular, they are now known to have a key role in altering tissue response to a meal, the post-prandial response, especially in muscle, where they signal a reduction in protein breakdown and an increase in protein synthesis, resulting in net accretion of protein in muscle and helping to regulate blood amino acid levels. However, aspects of this postprandial regulation are not as robust in aged muscle, and muscle in hypercatabolic conditions, such as cancer, is challenged and its normal system is overwhelmed. In these cases, a substantial body of research suggests that significantly more of these amino acids are required in the diet to overcome resistance to protein anabolism; very high doses, such as 10 to 15 g of BCAAs, or 3 g or more of leucine per meal, have been studied to combat muscle loss in the elderly,44 although this may be a result of improved protein synthesis that does not lead to muscle mass accretion.73, 74, 75

This resistance to the normal of BCAAs in muscle protein homeostasis has prompted studies into leucine's mechanism of action. These have identified the leucine metabolite β-hydroxy β-methylbutyrate (HMB) as a potent stimulator of protein synthesis as well as an inhibitor of protein breakdown in the extreme case of cachexia.65, 72, 76, 77, 78, 79, 80, 81, 82, 83, 84 A growing body of evidence suggests HMB may help slow, or even reverse, the muscle loss experienced in sarcopenia and improve measures of muscle strength.44, 65, 72, 76, 77, 78, 79, 80, 81, 82, 83, 84 However, dietary leucine does not provide a large amount of HMB: only a small portion, as little as 5%, of catabolized leucine is metabolized into HMB.85 Thus, although dietary leucine itself can lead to a modest stimulation of protein synthesis by producing a small amount of HMB, direct ingestion of HMB more potently affects such signaling, resulting in demonstrable muscle mass accretion.71, 80 Indeed, a vast number of studies have found that supplementation of HMB to the diet may reverse some of the muscle loss seen in sarcopenia and in hypercatabolic disease.65, 72, 83, 86, 87 The overall treatment of muscle atrophy should include dietary supplementation with HMB, although the optimal dosage for each condition is still under investigation.68

In addition to dietary protein, EAAs/BCAAs including leucine, the leucine metabolite HMB, a number of other dietary or supplemental components have been explored for their ability to positively influence muscle mass during sarcopenia. These include creatine monohydrate, a variety of antioxidants, ornithine α-ketoglutarate, omega-3 fatty acids, ursolic acid, and nitrates.68, 88, 89, 90 Given the length of this list, its growing nature, and the difficulty many aged individuals experience ingesting proper calories and nutrients, additional studies will be needed to determine which components are most beneficial to maintaining muscle mass, as well as their optimal doses and administration routes both in isolation and in combination.

Physical Activity Is Also Key

Nutrition is important and can counteract metabolic alterations induced during periods of significant stress and inflammation; however, sufficient exogenous provisions of protein and energy substrates alone cannot completely eliminate or reverse the deteriorations associated with aging or the deleterious impact inadequate control and regulation of inflammation have on muscle.23, 66, 69 Protein synthesis occurs in muscle fibers following their contraction,91 and physical activity has been shown to induce a number of anabolic signaling pathways.92 Physical activity can likewise reduce degradation of muscle protein.93, 94 Even more, a lack of physical activity increases the resistance of muscle to anabolism, particularly the synthesis of proteins from amino acids.95 An exercise component to muscle atrophy treatment is therefore highly recommended, and exercise also may prevent the onset of sarcopenia later in life, possibly by increasing the presence of type I fibers that are less susceptible to degradation during sarcopenia.7, 25, 66, 89 Although aerobic and other types of exercise are all preferable to a lack of physical activity, resistance exercise in particular has been shown repeatedly to improve rates of protein synthesis and reverse muscle loss.88, 89, 94 This may be attributable to differential effects on muscle fiber types.7 It is therefore recommended that patients with muscle atrophy or at risk of developing muscle atrophy engage a regular exercise program containing both aerobic and anaerobic components, and the importance of appropriate resistance training cannot be overstated. Although this must be tailored to the individual's current physical status, it should also periodically be reviewed and increased to maximize its impact.

Although resistance exercise and general physical activity are important to the stimulation of protein synthesis from amino acids in the diet, some aged and ill individuals experiencing extensive muscle atrophy are likely to have difficulty engaging in physical activities because of low energy and other medical complications. Nutrition and some supplements can be used to bolster results of exercise, both preventively and during sarcopenia. For example, bioactive substances known as nutraceuticals that mimic the molecular effects of exercise can induce signaling pathways that are thought to support or even underlie exercise's effects on health and muscle mass accretion. Found in a variety of foods, including some common fruits, green tea, and even red wine, these compounds can be isolated and added to nutritional supplements used in the treatment of muscle atrophy.44

Summary and Conclusions

The classic physical functions of skeletal muscle are well known, but skeletal muscle is increasingly recognized as a one of the key regulators of energy and protein metabolism by way of metabolic crosstalk between body organs. Skeletal muscle is the primary site for glucose uptake and storage, and it is likewise a reservoir of amino acids that sustain protein synthesis in all other body sites. When dietary glucose intake decreases or metabolic needs increase, stored glucose is mobilized from liver, while energy is released from fat depots. When these energy supplies are depleted, the muscle reservoir of amino acids stores is tapped, and muscle proteins are broken down to provide amino acids for gluconeogenesis, thereby supplying energy to other parts of the body.

Undernutrition and resultant muscle loss (muscle atrophy), as associated with aging and disease, can lead to adverse health and economic consequences. Conditions and diseases that lower dietary intake and increase nutrient needs are associated with catabolism of skeletal muscle, which in turn limits availability of protein and energy throughout the body. Loss of muscle mass, strength, and function has adverse consequences: slowed wound healing and recovery from illness, physical disability (due both to overall reduction of muscle status), as well as selective losses in type I fibers, which are essential for balance recovery (and thus fall prevention), poorer quality of life, and higher health care costs.

Nutrition and exercise are key to growth and maintenance of muscle promoting overall health, well-being, and recovery from disease. A wealth of research underscores the importance of a few key dietary components: protein (EAAs/BCAAs in particular), and the leucine metabolite HMB. Others will very likely be added to this list as our knowledge base grows. In addition, physical activity, especially resistance strength training, is essential to the treatment of muscle atrophy. Considerable evidence shows that ONS and enteral feeding formulations can help maintain and rebuild muscle mass and strength. Further studies are needed to show support for functional outcomes, such as ability to perform activities of daily living and maintain or restore independence.

Acknowledgments

The authors thank Jeffrey H. Baxter and Abby Sauer from ANR&D for their critical review of this article, as well as Cecilia Hofmann, PhD, and Hilary North Scheler, PhD (C Hofmann & Associates, Western Springs, IL), for valuable assistance with efficient compilation of the medical literature and with editing this English-language review article.

References


Skeletal muscle-specific eukaryotic translation initiation factor 2α phosphorylation controls amino acid metabolism and fibroblast growth factor 21-mediated non-cell-autonomous energy metabolism

The eukaryotic translation initiation factor 2a (eIF2α) phosphorylation-dependent integrated stress response (ISR), a component of the unfolded protein response, has long been known to regulate intermediary metabolism, but the details are poorly worked out. We report that profiling of mRNAs of transgenic mice harboring a ligand-activated skeletal muscle–specific derivative of the eIF2α protein kinase R-like ER kinase revealed the expected up-regulation of genes involved in amino acid biosynthesis and transport but also uncovered the induced expression and secretion of a myokine, fibroblast growth factor 21 (FGF21), that stimulates energy consumption and prevents obesity. The link between the ISR and FGF21 expression was further reinforced by the identification of a small-molecule ISR activator that promoted Fgf21 expression in cell-based screens and by implication of the ISR-inducible activating transcription factor 4 in the process. Our findings establish that eIF2α phosphorylation regulates not only cell-autonomous proteostasis and amino acid metabolism, but also affects non-cell-autonomous metabolic regulation by induced expression of a potent myokine.—Miyake, M., Nomura, A., Ogura, A., Takehana, K., Kitahara, Y., Takahara, K., Tsugawa, K., Miyamoto, C., Miura, N., Sato, R., Kurahashi, K., Harding, H. P., Oyadomari, M., Ron, D., Oyadomari, S. Skeletal muscle-specific eukaryotic translation initiation factor 2a phosphorylation controls amino acid metabolism and fibroblast growth factor 21-mediated non-cell-autonomous energy metabolism. FASEB J. 30, 798–812 (2016). www.fasebj.org

Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.


A Calcineurin-NFATc3-Dependent Pathway Regulates Skeletal Muscle Differentiation and Slow Myosin Heavy-Chain Expression

Fig. 1 . Calcineurin phosphatase activity peaks at an early stage of myocyte differentiation. (A) Calcineurin phosphatase activity assays were performed in C2C12 extracts prepared at the indicated times after transfer into DM as described in Materials and Methods. After 14 h in DM, calcineurin activity was increased nearly threefold. The data represent the means of three independent experiments, each done in duplicate, and SEM. ∗, P < 0.05. (B) Representative Western blot of total calcineurin A protein from myoblasts in GM (0 h) or at the indicated times in DM. Identical results were obtained in three independent experiments. (C) C2C12 cells were harvested at 0, 6, 10, 12, and 24 h after switching to DM, and whole-cell protein extracts were generated for NFATc3 Western blotting. The data demonstrate the appearance of a faster-migrating band (lower arrow), suggestive of enhanced calcineurin activity in vivo as differentiation progresses. (D) Infection of C2C12 cells with a calcineurin-inhibitory adenovirus, Adcain, blocked the increase in calcineurin activity at 14 h, while Adβgal infection had no effect. ∗, P < 0.05 versus the zero-time point †, P < 0.05 versus Adβgal infection.

Calcineurin promotes differentiation of C2C12 and Sol8 myoblasts.

Fig. 2 . Calcineurin enhances myogenic differentiation in myoblast cell lines. (A, E, and I) C2C12 myocytes were placed in DM for 24, 48, or 72 h without adenovirus infection. (B, F, and J) Adβgal infection did not influence the degree of myotube formation, the size of myotubes, or their degree of multinucleation. (C, G, and K) AdCnA-infected C2C12 myocytes displayed enhanced differentiation characterized by increased numbers of myosin-expressing cells and increased multinucleated cells. (D, H, and L) Inhibition of calcineurin activity by Adcain infection attenuated myocyte differentiation. Total myosin (MF-20 antibody) is shown in green, and nuclei are shown in blue. (M) Western blotting for total MyHC protein was assessed 72 h after adenovirus infection of either C2C12 or Sol8 myocytes. (N) Western blot quantitation of MyHC protein expression in C2C12 cells was averaged from five independent experiments, and the mean values and SEM are shown.

Calcineurin induces NFATc3 nuclear translocation in myoblasts.

Fig. 3 . Calcineurin induces nuclear translocation of NFATc3 in C2C12 myoblasts. C2C12 myoblasts were immunostained 13 h after adenovirus infection with an antibody against NFATc3, and nuclei were counterstained with bisbenzimide (blue). (A and B) In Adβgal-infected cells, NFATc3 was localized mostly to the cytoplasm. (C to F) Infection with AdCnA induced nuclear translocation of NFATc3 (C and D), similar to calcium mobilization by thapsigargin treatment (E and F). (G) Protein fractionation followed by Western blotting revealed a loss of NFATc3 (arrow) from the cytoplasm and a redistribution to the nucleus in AdCnA-infected C2C12 cells (asterisks). In contrast, Adcain infection was associated with a mild increase in cytoplasmic NFATc3 and less in the nucleus (arrowheads). NFATc3-transfected 10T1/2 cell extract is shown as a mobility control.

Calcineurin is sufficient to induce myogenic differentiation in the absence of IGFs.

Fig. 4 . Calcineurin is sufficient to induce differentiation in an IGF-inhibited cell line. (A to D) C2BP-5 cells failed to differentiate in DM after 48 or 96 h in the absence of exogenous IGF-1 (A and B) however, supplementation with IGF-1 rescued differentiation by 96 h but not by 48 h (C and D). (E and F) AdCnA infection of C2BP-5 myoblasts maintained in DM demonstrated noticeable differentiation by 48 and 96 h. (G and H) AdCnA infection in the presence of IGF-1 promoted even greater differentiation and increased the myotube size by 96 h. (I) Western blotting for total MyHC protein demonstrated that Adβgal-infected C2BP-5 cells completely lacked MyHC protein at 48 or 96 h in the absence of IGF-1 (lanes 3 and 9). However, AdCnA-infected cells displayed abundant MyHC protein expression at 48 and 96 h in the absence of IGF-1 (lanes 5 and 11).

Calcineurin cooperates with MyoD to induce myogenic conversion of 10T1/2 fibroblasts.

Fig. 5 . Calcineurin enhances the differentiation of 10T1/2 cells transfected with MyoD. (A) 10T1/2 fibroblasts were transiently transfected with an expression vector encoding a constitutively active calcineurin Aα protein and subsequently immunostained for total MyHC protein (MF20 antibody). No MyHC was detected after 6 days in DM. (B) In contrast, transient transfection of an MyoD-encoding expression vector induced the conversion of fibroblasts into MyHC-expressing myotubes (green stain). (C) Cotransfection of MyoD and calcineurin resulted in a dramatic enhancement in the MyHC immunoreactivity and size of each converted cell. (D) However, inhibition of calcineurin activity with a cain (194-amino-acid fragment) expression vector blocked MyoD-directed differentiation. (E) Western blot quantitation revealed a dramatic increase in the amount of total MyHC protein between MyoD and calcineurin, while cain blocked MyHC expression. Identical results were obtained in four independent experiments. Fig. 6 . Myogenin immunocytochemistry reveals that calcineurin does not alter myocyte commitment. (A) 10T1/2 cells transiently transfected with MyoD become immunoreactive for the muscle-specific marker myogenin (arrowhead, green staining). (B) Even though cain cotransfection blocks MyoD-directed MyHC expression, these cells still express myogenin, suggesting no loss of transfected cells or in their respecification to the myogenic lineage. (C) Cotransfection of MyoD and calcineurin (CnA) also does not alter the number of myogenin-positive cells. Transfected cells were shifted to DM for 6 days before being immunostained for myogenin.

NFATc3 enhances the myogenic activity of MyoD.

Fig. 7 . NFATc3 collaborates with MyoD to induce myogenic conversion in transiently transfected 10T1/2 cells. (A) 10T1/2 cells were grown in DM for 6 days after transfection with MyoD and stained for total MyHC protein (MF20 antibody). (B and C) Cotransfection of MyoD with either a constitutively nuclear or wild-type (not shown) NFATc1- or NFATc4-encoding vector did not significantly enhance 10T1/2 cell differentiation or MyHC immunostaining. (D) In contrast, cotransfection of a expression vector encoding full-length NFATc3 produced a noticeable enhancement of myogenesis (extent of differentiation). (E) Western blot analysis for total MyHC protein levels demonstrated that only NFATc3 enhanced the effects of MyoD. These results were similar in three independent experiments. CnA, calcineurin.

Calcineurin promotes slow-fiber-specific MyHC expression in vitro and in vivo.

Fig. 8 . Calcineurin promotes the expression of slow fiber MyHC isoform in C2C12 and 10T1/2 cells. (A) Extracts from transiently transfected 10T1/2 cells were probed with antibodies against total, slow, or fast MyHC antibody. Calcineurin (CnA) cotransfection induced slow myosin but not fast myosin. In contrast, mitogen-activated protein kinase kinase 6 (MKK6) cotransfection induced fast myosin but not slow myosin. These data indicate that increased slow MyHC protein expression is specific to calcineurin and is not the result of a general enhancement of differentiation. (B and C) Quantitation of these effects from multiple independent experiments demonstrates augmented slow MyHC protein levels in both 10T1/2 cells and C2C12 cells infected with AdCnA. Fig. 9 . Adenovirus-mediated gene transfer of activated calcineurin in the rat gastrocnemius induces slow MyHC expression in vivo. (A) Immunostaining with calcineurin (CnA)-specific antibody (which readily detects the activated form of calcineurin) on histological sections from an injected rat gastrocnemius demonstrates a large region of expression (red). (B) Coimmunostaining with slow MyHC antibody (green) demonstrates largely coincident staining (see arrowheads). (C) As a control, Adβgal infection was performed followed by immunostaining with a β-galactosidase (β-gal) antibody (nuclear staining in red). (D) Slow MyHC protein (green) was not coincident with Adβgal infection.

The Molecular Biology of Arrestins

1 A Short Introduction to Metabolic Regulation

Metabolic regulation is the physiological mechanism by which the body takes in nutrients and delivers energy as required. Metabolic regulation works ultimately at a molecular level, mainly by modulation of enzyme activities that function together as a whole system to sense the balance of energy coming in and energy required. The different organs in the body have their own characteristic patterns of metabolism according to their functions in the body. Thus, it is critical that metabolic pathways interact in a dynamic sense, in the entire organism. Furthermore, the endocrine and nervous systems need to precisely coordinate to control the flow of energy within the body.

Much of the metabolic regulation is governed by hormones that are delivered through the bloodstream and act through specific cellular receptors. Both the cell-surface receptors (that usually bind peptide hormones) and the nuclear receptors (that bind thyroid hormones, steroid hormones, and other membrane-permeant ligands) play critical roles in metabolic regulation. Hormones acting through cell-surface receptors are involved in rapid metabolic adjustments. These receptors signal via the small molecule cyclic adenosine 3,5-monophosphate (cyclic AMP or cAMP) and the membrane lipid phosphatidylinositol (3,4,5) trisphosphate. Following this, the activities of downstream metabolic enzymes are regulated by covalent modification, especially phosphorylation and dephosphorylation, and/or translocation of enzymes within the cell. Activation of the nuclear receptors by their ligands, on the other hand, directly controls the transcription of metabolic genes and leads to long-term metabolic regulation.

Studies carried out with several different families of receptors have pointed out that β-arrestins determine the specificity, spatiality, and temporality of cellular signals as well as the intracellular movement of receptors and other signal complexes. 1–4 Binding of β-arrestins to ligand-bound G protein-coupled receptors (GPCRs) physically uncouples the G protein from the receptor and effectively terminates G protein-mediated signaling. By coupling to a ligand-activated receptor, β-arrestins also initiate GPCR signaling in a G protein-independent manner. Furthermore, β-arrestins scaffold diverse signal complexes, thereby linking activated receptors with distinct sets of accessory and effector proteins. Considering the intricate metabolic regulatory network composed of a variety of hormones and their specific receptors, it should not be a surprise that proper functioning of β-arrestins is indispensable for the body’s metabolic function. This chapter summarizes the function of β-arrestins in metabolic regulation and also discusses their association with metabolic syndromes including insulin resistance, type 2 diabetes, and obesity.


Genes That Define Skeletal Muscle Phenotype

Skeletal muscle fiber-type phenotype is regulated by several independent signaling pathways (Figure 3). These include pathways involved with the Ras/mitogen-activated protein kinase (MAPK) (Murgia et al. 2000), calcineurin (Chin et al. 1998 Naya et al. 2000), calcium/calmodulin-dependent protein kinase IV (Wu et al. 2002), and the peroxisome proliferator γ coactivator 1 (PGC-1) (Lin et al. 2002). The Ras/MAPK signaling pathway links the motor neurons and signaling systems, coupling excitation and transcription regulation to promote the nerve-dependent induction of the slow program in regenerating muscle (Murgia et al. 2000). Calcineurin, a Ca 2+ /calmodulin-activated phosphatase implicated in nerve activity-dependent fiber-type specification in skeletal muscle, directly controls the phosphorylation state of the transcription factor NFAT, allowing for its translocation to the nucleus and leading to the activation of slow-type muscle proteins in cooperation with myocyte enhancer factor 2 (MEF2) proteins and other regulatory proteins (Chin et al. 1998 Serrano et al. 2001). Calcium-dependent Ca 2+ /calmodulin kinase activity is also upregulated by slow motor neuron activity, possibly because it amplifies the slow-type calcineurin-generated responses by promoting MEF2 transactivator functions and enhancing oxidative capacity through stimulation of mitochondrial biogenesis (Wu et al. 2002).

Contraction-induced changes in intracellular calcium or reactive oxygen species provide signals to diverse pathways that include the MAPKs, calcineurin and calcium/calmodulin-dependent protein kinase IV to activate transcription factors that regulate gene expression and enzyme activity in skeletal muscle.

PGC1-α, a transcriptional coactivator of nuclear receptors important to the regulation of a number of mitochondrial genes involved in oxidative metabolism, directly interacts with MEF2 to synergistically activate selective ST muscle genes and also serves as a target for calcineurin signaling (Lin et al. 2002 Wu et al. 2001). New data presented in this issue of PLoS Biology (Wang et al. 2004) reveals that a peroxisome proliferator-activated receptor δ (PPARδ)-mediated transcriptional pathway is involved in the regulation of the skeletal musclefiber phenotype. Mice that harbor an activated form of PPARd display an “endurance” phenotype, with a coordinated increase in oxidative enzymes and mitochondrial biogenesis and an increased proportion of ST fibers. Thus—through functional genomics—calcineurin, calmodulin-dependent kinase, PGC-1α, and activated PPARδ form the basis of a signaling network that controls skeletal muscle fiber-type transformation and metabolic profiles that protect against insulin resistance and obesity.

The transition from aerobic to anaerobic metabolism during intense work requires that several systems are rapidly activated to ensure a constant supply of ATP for the working muscles. These include a switch from fat-based to carbohydrate-based fuels, a redistribution of blood flow from nonworking to exercising muscles, and the removal of several of the byproducts of anaerobic metabolism, such as carbon dioxide and lactic acid. Some of these responses are governed by transcriptional control of the FT glycolytic phenotype. For example, skeletal muscle reprogramming from a ST glycolytic phenotype to a FT glycolytic phenotype involves the Six1/Eya1 complex, composed of members of the Six protein family (Grifone et al. 2004). Moreover, the Hypoxia Inducible Factor-1α (HIF-1α) has been identified as a master regulator for the expression of genes involved in essential hypoxic responses that maintain ATP levels in cells. In this issue of PLoS Biology (Mason et al. 2004), a key role for HIF-1α in mediating exercise-induced gene regulatory responses of glycolytic enzymes is revealed. Ablation of HIF-1α in skeletal muscle was associated with an increase in the activity of rate-limiting enzymes of the mitochondria, indicating that the citric acid cycle and increased fatty acid oxidation may be compensating for decreased flow through the glycolytic pathway in these animals. However, hypoxia-mediated HIF-1α responses are also linked to the regulation of mitochondrial dysfunction through the formation of excessive reactive oxygen species in mitochondria.


Peggy Biga

Research and Teaching Interests: Comparative Growth Biology, Developmental Physiology, Diet-Epigenetic Interactions, Skeletal Muscle Growth Regulation, Science Community Outreach, k-12 Science Engagement

Office Hours: By appointment

  • B.A., Angelo State University, Animal Science
  • M.A., Angelo State University, Nutrition
  • Ph.D., University of Idaho, Nutritional Physiology
  • Post-doctoral Training: Marine Biological Laboratory, Woods Hole, MA, Comparative Physiology

Dr. Peggy Biga is a broadly trained comparative endocrine physiologist, with primary research interests focusing on the mechanisms regulating growth patterns in animals. Her research questions revolve around what molecular and epigenetic mechanisms regulate skeletal muscle proliferation, differentiation, and atrophy. She uses comparative biology to understand the plasticity of regulatory mechanisms and how they translate to variability in overall organismal growth.

For example, most, and arguably all, terrestrial mammals reach a growth plateau around the time they reach sexual maturity which is characterized by a lack of nascent (or new) muscle fiber development post-embryonic growth. Alternatively, many aquatic vertebrates exhibit an opposing growth paradigm where no true growth plateau is reached, and skeletal muscle continues to growth through the addition of nascent muscle fibers throughout their life. The main focus of Dr. Biga’s lab for many years has been to identify molecular pathways and mechanisms that regulate the ability of some animals to continually grow (by adding NEW muscle fibers) throughout their lives. As a post-doctoral scientist, Dr. Biga identified and verified a comparative model system that can be used to ask these questions. She demonstrated that two closely related fish species, the zebrafish and giant danio, exhibit differential growth paradigms. The zebrafish, a commonly used model organism, exhibits a growth pattern that closely mirrors what is seen in human muscle growth, where muscle growth is accomplished post-birth/hatch with little to no addition of new muscle fibers, but instead through the enlargement of pre-existing fibers. Alternatively, a close relative to the zebrafish, the giant danio, exhibit continual addition of nascent muscle fibers throughout their lives. By juxtaposing the growth of these two fish species, Dr. Biga has identified transcription factors and myogenic regulatory factors that are differentially regulated between the growth types.

Dr. Biga’s research interests also focus on the endocrine regulation of growth biology, with particular focus on the GH-IGF system in relation to myostatin control of cell proliferation, cell differentiation, and energy metabolism. Myostatin is a negative regulator of muscle growth, and is known to be sensitive to GH and IGF signaling in muscle tissue. In addition, Dr. Biga has shown that myostatin is also responsive to stress hormones, like cortisol, which is likely to be involved in stress-induced muscle atrophy. Also, Dr. Biga is interested in the direct action GH might have on muscle cells in relation to cell proliferation and differentiation, and cellular respiration. This work is primarily conducted using the rainbow trout as a model, as this fish species is an important species for the US aquaculture industry.

In addition, Dr. Biga’s research also focuses on how diet influences the mechanisms that regulation growth and metabolism. Her lab researches questions related to how individual nutrients influence growth and metabolic physiology, and how these nutrients alter the epigenome to regulate changes in physiology. Within this area of research, Dr. Biga has demonstrated that amino acid (ex., methionine) restriction alters muscle cell proliferation and induces autophagy in vitro. In addition, Dr. Biga’s lab has also demonstrated that methionine restriction affects glucose metabolism that is likely regulated through changes in miRNA expression in a tissue-specific manner. On the other side of the diet-epigenetic interaction research focus, Dr. Biga is interested in evaluating how methyl-donor amino acid supplementation can affect growth physiology through maternal imprinting.

Dr. Biga participates in several collaborations with scientists in France, Canada, and the US on research projects that focus on how endocrinology, molecular biology, epigenetics, and physiology interact to regulate growth physiology.

The overall goal of my research program is to identify mechanisms regulating organismal growth potential, with specific interest on mechanisms allowing for continual growth throughout an organism’s life (indeterminate growth). My lab addresses this goal using many approaches that range from cellular to organismal: molecular biology, cell biology, endocrinology, physiology, and morphology. Generally, my lab utilizes piscine species as model organisms because they offer diverse growth potentials and serve as excellent comparative platforms. The following projects are currently active and are being primarily driven by graduate and undergraduate students in my lab:

Epigenetic regulation of myogenesis is regulated by specific nutrients, namely amino acids.

Working closely with Dr. Jean-Charles Gabillard (INRA, Rennes, France) and Dr. Iban Seiliez (INRA, St. Pee, France) we have characterized the histone methylation profile related to pax7 and myogenin expression during in vitro myogenesis in Rainbow trout, an indeterminate growing fish. We recently also demonstrated that methionine depletion specifically alters this epigenetic profile, as well as reverts myoblasts to the quiescent state, suggesting a role of histone methylation in myogenic progression regulation. This quiescence appears to be reversible with addition of methionine. We are currently investigating the role of microRNAs as part of this mechanism as well.

Nutritional state regulates atrophy/hypertrophy balance in myogenic cells in vitro.

Working closely with Dr. Jean-Charles Gabillard (INRA, Rennes, France) and Dr. Iban Seiliez (INRA, St. Pee, France) we have characterized a novel in vitro model of amino acid depletion induced autophagy using zebrafish as a model organism. Using an amino acid depleted media, we can induce autophagy without apoptosis during myogenesis in vitro. We have characterized the histone methylation profiles affected by this cell phenotype switch and identified Atg4b, p62/sqstrm1, and lc3b as tightly regulated by starvation during the onset of autophagy.

Maternal nutritional transfer regulates growth via epigenetic mechanisms.

Working closely with Dr. Beth Cleveland (USDA, ARS, Leetown, WV USA) we have demonstrated that supplementing maternal broodstock diets with choline results in enhanced offspring growth performance. Also, we recently demonstrated that choline supplemented diet intake results in increased levels of choline into the pre-fertilized eggs. We hypothesized that maternal dietary intake regulates growth performance through changes in epigenetic mechanisms regulating growth. Choline serves as a methyl donor, and we are currently evaluating the role of choline supplementation on methylome changes.

The role of paired box transcription factors (Pax) in regulating myogenic stem cell populations.

My lab has recently demonstrated that indeterminate growing fish species exhibit a unique a pax3 expression profile in adult myogenic progenitor cells (MPCs muscle stem cells) compared to determinate growing organisms. MPCs from adult indeterminate growing danios are pax3+/+, while determinate growing danios’ MPCs are pax3-/- (similar to adult mammalian MPCs) suggesting a potential role of pax3 in regulate MPC function. We are currently working to empirically test a role of pax3 in MPC function by knocking it down (morpholino and siRNA) in isolated MPCs.

Myogenic precursor cell contribution to muscle repair across the life-course in indeterminately growing species. Question: Does repair capacity decrease with age in indeterminate growing species?

We are currently characterizing the muscle repair program in indeterminately growing fish species (trout and danios) to establish a baseline understanding of the cells, genes, and pathways that play key roles in muscle repair in juvenile, sexually mature, and aged organisms. We hypothesize that species with high pax3 expression in MPCs as adults will have an enhanced repair capacity compared to species lacking pax3 expression as adults. Additionally, we will examine the role of growth hormone, IGF-I, IGF-II, and myostatin in muscle repair related to aging decline (or lack thereof).

The role of Teneurin C-terminal Associated Peptide (TCAP) in muscle function and metabolism during aging.

In collaboration with Dr. David Lovejoy (University of Toronto, Canada) and his PhD student Andrea D’Aquila we are investigating the conserved function of TCAP in muscle hypertrophy and metabolic control in teleosts. In addition, we have pilot funds from the Nathan Shock Center to examine the role TCAP plays in regulating muscle function decline during aging in the short-lived killifish model. We are also examining the effects of chronic TCAP treatment on zebrafish muscle hypertrophy and metabolic regulation. We will also begin evaluating the role TCAP plays in starvation-induced autophagy in primary myotubes in vitro. This work is specifically and uniquely informative for human muscular repair/regeneration and wasting disorders.

These research projects cover the general topic of mechanisms regulating muscle growth and repair, the overarching theme of my research program. This work is translatable to human health as mammals lose their ability to adequately repair their muscle tissue with age. In some teleost species, this functional decline in muscle structure and function is not observed and we hypothesize that the mechanisms that allow for continued growth throughout the lives of these organisms plays an important role in delaying muscle senescence (or wasting). In addition, improving adult muscle repair capabilities is extremely important in wound healing. In addition, this work is translatable to production agriculture, as further understanding of mechanisms regulating fish growth, from epigenetics to endocrinology, has direct applicability to the production efficacy to several finfish industries (including rainbow trout).

  • BY 429, BY 491, BY 629: Evolution
  • BY 245: Fundamentals of Scientific Investigation
  • Comparative Developmental Biology
  • Introductory Biology I
  • Non-thesis research credit (Science Mentorship in Inner City Schools)

Post-Doctoral Scientist, Current

Graduate Students, Current

  • Lauren Amber Requena, M.S. 2019
  • Mary N. Latimer, Ph.D. 2018
  • Nicholas J. Galt, Ph.D. 2014
  • Jacob M. Froehlich, Ph.D. 2014
  • Ben Meyer, M.S. 2012
  • Cleveland, B.M., T.D. Leeds, M.J. Picklo, C. Brentesen, J. Frost, and P.R. Biga. Supplementing rainbow trout (Oncorhynchus mykiss) broodstock diets with choline and methionine improves growth in offspring. 2019. Journal of the World Aquaculture Society. 1-16. Doi: 10.1111/jwas.12634
  • Latimer, M.N., R.M. Reid, P.R. Biga, and B.M. Cleveland. Glucose regulates protein turnover and growth-related mechanisms in rainbow trout myogenic precursor cells. 2019. Comp. Biochem. Physiol. A. 232:91-97. Doi:10.1016/j.cbpa.2019.03.010. PMID30904682
  • Reid, R.M., K.W. Freij, J.C. Maples , and P.R. Biga. 2019. Teneurins and teneurins C-terminal associated peptide (TCAP) metabolism: What’s known in fish? Front. Neurosci. 13:177. Doi:10.3389/fnins.2019.00177. PMID:30890915
  • Latimer, M.N., K.W. Freij, B. Cleveland, and P.R. Biga. 2018. Physiological and molecular mechanisms of methionine restriction. Frontiers in Endocrinology Experimental Endocrinology doi: 10.3389/fendo.2018.00217. PMID:29780356
  • Reid, R., A. D’Aquila, and P.R. Biga. 2018. The validation of a sensitive, non-toxic in vivo metabolic assay applicable across zebrafish life stages. Comp. Biochem. Physiol. C. (E-pub ahead of print, Nov. 2017) doi: 10.1016/j.cbpc.2017.11.004 PMID: 29162498
  • Latimer, M., B.M. Cleveland, and P.R. Biga. 2018. Dietary Methionine Restriction: Effects on Glucose Tolerance, Lipid Content and micro-RNA composition in the muscle of Rainbow Trout. Comp. Biochem. Physiol. C. (E-pub ahead of print, Oct. 2017) doi: 10.1016/j.cbpc.2017.10.012 PMID: 29100953
  • Galt, N.J., J.M. Froehlich, S.D. McCormick, and P.R. Biga. 2018. A comparative evaluation of crowding stress on muscle HSP90 and myostatin expression in salmonids. Aquaculture. 483:141-148. doi: 10.1016/j.aquaculture.2017.10.019
  • Biga, P.R., M.N. Latimer, J.M. Froehlich, J.C. Gabillard, and I. Seiliez. 2017. Distribution of H3K27me3, H3K9me3, and H3K4me3 along authophagy-related genes highly expressed in starved zebrafish myotubes. Biol. Open 6(11):1720-1725. doi: 10.1242/bio.029090 PMID: 29025701
  • Latimer, M.N., N. Sabin, A. Le Cam, I. Seiliez, P. Biga, and J.C. Gabillard. 2017. miR-210 expression is associated with methionine-induced differentiation of trout satellite cells. J Exp. Biol. 220(Pt 16):2932-2938. doi:10.1242/jeb.154484 PMID: 28576820.
  • Galt, N.J., S.D. McCormick, J.M. Froehlich, and P.R. Biga. 2016. A comparative examination of cortisol effects on muscle myostatin and HSP90 gene expression in salmonids. General and Comparative Endocrinology. 237:19-26. doi:10.1016/j.gcen.2016.07.019 PMID: 27444129.
  • Seiliez, I., J.M. Froehlich, L. Marandel, J.C. Gabillard, and P.R. Biga. 2015. Evolutionary history and epigenetic regulation of the three paralogous pax7 genes in rainbow trout. Cell Tissue Research. 359(3):715-27. PMID: 25487404.
  • Allison DB, Antoine LH, Ballinger SW, Bamman MM, Biga P, Darley-Usmar VM, Fisher G, Gohlke JM, Halade GV, Hartman JL, Hunter GR, Messina JL, Nagy TR, Plaisance RP, Roth KA, Sandel MW, Schwartz TS, Smith DL, Sweatt JD, Tollefsbol TO, Watts SA, Yang Y, Zhang J, Austad, S, and Powell ML. 2014. Aging and energetics’ ‘Top 40’ future research opportunities 2010-2013 [v1 ref status: indexed http://f1000r.es/4ae] F1000Research, 3:219 doi: 10.12688/f1000research.5212.1
  • Galt, N.J., J.M. Froehlich, E.A. Remily , S.R. Romero , and P.R. Biga. 2014. The effects of exogenous cortisol on myostatin transcription in rainbow trout, Oncorhynchus mykiss. Comp. Biochem. Phsyiol. A. Mol Intergr. Physiol. 175:57-63. PMID: 24875565.
  • Picha, M.E., P.R. Biga, N. Galt, A.S. McGinty, K. Gross, V.S. Hedgepeth, T.D. Siopes, and R.J. Borski. 2014. Overcompensation of circulating and local insulin-like growth factor-I during catch-up growth in hybrid striped bass (Morone chrysops X Morone saxatilis) following temperature and feeding manipulation. Aquaculture. 428-429:174-183.
  • Froehlich, J.M., I. Seiliez, J.C. Gabillard, and P.R. Biga. 2014. Preparation of Primary Myogenic Precursor Cell/Myoblast Cultures from Basal Vertebrate Lineages. Journal of Visualized Experiments. Apr 30(86). doi:10.3791/51354. PMID: 24835774.
  • Goetz, F.W., A. Jasonowicz, R. Johnson, P. Biga, G. Fischer, and S. Sitar. 2014. Physiological differences between siscowet and lean trout morphotypes: Are these metabolotypes? Canadian Journal of Fisheries and Aquatic Sciences. 71(3):427-435.
  • Galt, N.J., J.M. Froehlich, B.M. Meyer, F.T. Barrows, and P.R. Biga. 2014. High-fat diet reduces local myostatin-1 paralog expression and alters skeletal muscle lipid content in rainbow trout, Oncorhynchus mykiss. Fish Physiology and Biochemistry. 40(3):875-86. PMID: 24264425.
  • Gabillard, J.C., P.R. Biga, P.Y. Rescan, and I. Seiliez. 2013. Revisiting the paradigm of myostatin in vertebrates: insights from fishes. Gen. Comp. Endocrinol. 194C:45-54. PMID: 24018114.
  • Froehlich, J.M., Z.G. Fowler , N.J. Galt, D.L. Smith Jr., and P.R. Biga. 2013. Sarcopenia and piscines: the case for indeterminate-growing fish as unique genetic model organisms in aging and longevity research. Frontiers of Genetics in Aging. 4:159. PMID: 23967015.
  • Froehlich, J.M., N.J. Galt, M.J. Charging , B.M. Meyer, and P.R. Biga. 2013. In vitro indeterminate teleost myogenesis appears to be dependent on Pax3. In vitro Cellular and Developmental Biology- Animal. 49(5):371-385. PMID: 23613306.
  • Biga, P.R., J.M. Froehlich, K.J. Greenlee, N.J. Galt, B.M. Meyer, and D.J. Christensen . 2013. Gelatinases impart susceptibility to high-fat diet induced obesity in mice. Journal of Nutritional Biochemistry. 24(8):1462-8. PMID: 23465590.
  • Meyer, B.M., J.M. Froehlich, N.J. Galt and P.R. Biga. 2013. Inbred strains of zebrafish exhibit variation in growth performance and myostatin expression following fasting. Comparative Biochem. Physiol. A. 164(1):1-9. PMID: 23047051.

Underlined names represent undergraduate student co-authors.

  • Science & Technology Policy Fellow, American Association for the Advancement of Science, AAAS, 2019-2020
  • Altruism Award, UAB Department of Biology, 2017
  • Humble Hero Award, City of Birmingham, Division of Youth Services, Youth First Program, Mayor William A. Bell, Sr and Cedric Sparks
  • Creativity is a Decision Prize, Nutrition Obesity Research Center, 2016
  • Stop Obesity Challenge Winner, Mid-South Transdisciplinary Collaborative Center for Health Disparities Research, UAB Minority Health and Health Disparities Research Center, 2015
  • Named New Investigator, Nutrition Obesity Research Center, UAB, 2013
  • North American Society for Comparative Endocrinology (NASCE)
  • Society for Integrative and Comparative Biology (SICB)
  • American Institute of Biological Sciences (AIBS)
  • American Association for the Advancement of Science (AAAS)
  • American Fisheries Society, Physiology Section (AFS, PS)
  • World Aquaculture Society (WAS)
  • American Physiological Society (APS)
  • Association for Women in Science (AWIS)
  • Scholars Strategy Network (SSN) – Co-Leader Alabama SSN

I co-host the department’s annual Darwin Day. If you are interested in this celebration of science, please contact me!

I organize and manage an outreach program that focuses on science engagement and achievement in the Birmingham Public Schools. This program matches UAB (graduate) students with a local k-5 or k-8 public school, where the UAB student works with science teachers to enhance science learning and exposure. Contact me if you are interested!

I serve as the Faculty Advisor for STEMO, a student group focused on STEM education outreach.


  • Biochemical and molecular biological research on skeletal muscle-specific proteins.
  • Skeletal muscle cells in normal and altered states: excitation-contraction coupling, and calcium regulation muscle biomechanics cell-cell/cell-matrix interactions including pathways of signal transduction physiological evaluation of skeletal muscle gene function stem and satellite cell biology oxidative stress mitochondrial dysfunction autophagy proteosomal degradation regulation of skeletal muscle energy and substrate metabolism including mitochondrial function.
  • Skeletal muscle as a tissue: molecular and cellular mechanisms of skeletal muscle adaptation, growth, injury, repair, degeneration, and regeneration effects of exercise and inactivity, maturation, nutrition, and the aging process on skeletal muscle function, protein turnover, and metabolism normal and abnormal neural control of muscle fiber type and molecular phenotype non-invasive imaging of skeletal muscle properties, metabolism, and mechanical dynamics skeletal muscle biology of sarcopenia and cachexia.
  • Integrative functions: effects of exercise on maintenance of functional capacity of muscle and on pathology due to inherited disease, aging, and inactivity physiologic interactions between skeletal muscle and other organ systems in normal and disease states when skeletal muscle function is the primary focus.
  • Skeletal muscle diseases: evaluation of genetics and epigenetics, gene function, and development of vertebrate and invertebrate genetic models pathophysiology of skeletal muscle disorders and diseases, including the muscular dystrophies, atrophy, myotonia, periodic paralysis, malignant hyperthermia, and inflammatory myopathies pharmacological interventions and pre-clinical approaches cell and gene therapies for skeletal muscle diseases.

There are shared interests with Musculoskeletal Rehabilitation Sciences (MRS) in the investigation of muscle function and exercise. Grant applications that focus on rehabilitation interventions to improve muscle function, increase muscle mass, or identify muscles responsible to functional decline may be assigned to MRS. Applications that focus on molecular and cellular mechanisms of muscle function and related animal models may be assigned to SMEP.

There are shared interests with Musculoskeletal Tissue Engineering (MTE) in the investigation of skeletal muscle regeneration. Grant applications that focus on the use of scaffolds and muscle stem cells for skeletal muscle regeneration may be assigned to MTE. Applications that focus on in situ repair and regeneration of diseased skeletal muscle using skeletal muscle satellite or stem cells may be assigned to SMEP.

There are shared interests with Aging Systems and Geriatrics (ASG) in the investigation of sarcopenia and skeletal muscle function. Grant applications that focus on mechanisms of sarcopenia, or on skeletal muscle endpoints as consequences of aging sydromes such as multi-morbidity and polypharmacy may be reviewed in ASG. Applications to evaluate pleiotropic interventions that include skeletal muscle endpoints (along with multiple others) as outcomes in older adults may also be assigned to ASG (exercise studies, for example). Grant applications with primary focus on skeletal muscle biology or function in response to sarcopenia and aging, including exercise interventions that focus on muscle, may be reviewed in SMEP.

There are shared interests with Pathophysiology of Obesity and Metabolic Disease (POMD) in the investigation of metabolic pathways and mitochondrial function in skeletal muscle. Grant applications that focus on insulin action, cytokines, adipokines and inflammatory regulation of metabolic and energy control of skeletal muscle related to obesity and diabetes may be assigned to POMD. Applications that focus on oxidative stress, mitochondrial dysfunction, energy and substrate metabolism in normal and disease states when skeletal muscle function is the primary focus, may be assigned to SMEP.

There are shared interests with Nutrition and Metabolism in Health and Disease (NMHD) in the investigation of nutrients effects on muscle function. Grant applications that focus on signaling pathways that modulate nutrients effects on muscle physiology may be assigned to NMHD. Applications focused on effects of vitamins and nutrients on skeletal muscle diseases may be reviewed by SMEP.

There are shared interested with Cellular Mechanisms in Aging and Development (CMAD) in the investigation of metabolic and physiologic regulation of aging muscle. Grant applications that focus on nutrient sensing or signaling, mammalian target of rapamycin (mTOR), sirtuins, insulin/IGF/GH pathways and mitochondria function with specific focus to study metabolic and physiologic mechanisms that regulate aging may be assigned to CMAD. Applications on sarcopenia as well as aging of skeletal muscle stem cells and their niche may also be assigned to CMAD. Applications with primary focus on skeletal muscle biology and function in response to sarcopenia, aging and inactivity may be assigned to SMEP.

There are shared interests with Integrative Myocardial Physiology/Pathophysiology A (MPPA) in the investigation related to muscle contractility. Grant applications that focus on contractile proteins and contractile systems in the context of cardiac contractile function, hypertrophy and heart failure may be assigned to MPPA. Applications that focus on contractile function within the context of skeletal muscle and muscular dystrophy may be assigned to SMEP.


AMP-activated protein kinase control of fat metabolism in skeletal muscle

AMP-activated protein kinase (AMPK) has emerged as a key regulator of skeletal muscle fat metabolism. Because abnormalities in skeletal muscle metabolism contribute to a variety of clinical diseases and disorders, understanding AMPK’s role in the muscle is important. It was originally shown to stimulate fatty acid (FA) oxidation decades ago, and since then much research has been accomplished describing this role. In this brief review, we summarize much of these data, particularly in relation to changes in FA oxidation that occur during skeletal muscle exercise. Potential roles for AMPK exist in regulating FA transport into the mitochondria via interactions with acetyl-CoA carboxylase, malonyl-CoA decarboxylase, and perhaps FA transporter/CD36 (FAT/CD36). Likewise, AMPK may regulate transport of FAs into the cell through FAT/CD36. AMPK may also regulate capacity for FA oxidation by phosphorylation of transcription factors such as CREB or coactivators such as PGC-1α.


Biology Chapter 15: Endocrine System

Even after we stop growing, adults still need growth hormone. Growth hormone is a protein made by the pituitary gland and released into the blood.

Growth hormone plays a role in healthy muscle, how our bodies collect fat (especially around the stomach area), the ratio of high density to low density lipoproteins in our cholesterol levels and bone density. In addition, growth hormone is needed for normal brain function.

A person who has too little adult growth hormone will have symptoms that include:

A higher level of body fat, especially around the waist
Anxiety and depression
Decreased sexual function and interest
Fatigue
Feelings of being isolated from other people
Greater sensitivity to heat and cold
Less muscle (lean body mass)
Less strength, stamina and ability to exercise without taking a rest
Reduced bone density and a tendency to have more bone fractures as they get older
Changes in the make up of the blood cholesterol.


Watch the video: ANATOMY MUSCULAR SYSTEM (December 2021).