Expression and functional behavior of troponin C in soleus muscle fibers of rat after hindlimb unloading

Mechanical interaction of myosin and native thin filament in the disused rat soleus muscle

The disuse of skeletal limb muscles occurs in a variety of conditions, yet our comprehension of the molecular mechanisms involved in adaptation to disuse remains incomplete. We studied the mechanical characteristics of actin-myosin interaction using an in vitro motility assay and isoform composition of myosin heavy and light chains by dint of SDS-PAGE in soleus muscle of both control and hindlimb-unloaded rats. 14 days of hindlimb unloading led to the increased maximum sliding velocity of actin, reconstituted, and native thin filaments over rat soleus muscle myosin by 24 %, 19 %, and 20 %, respectively. The calcium sensitivity of the "pCa-velocity" relationship decreased. There was a 26 % increase in fast myosin heavy chain IIa (MHC IIa), a 22 % increase in fast myosin light chain 2 (MLC 2f), and a 13 % increase in fast MLC 1f content. The content of MLC 1s/v, typical for slow skeletal muscles and cardiac ventricles did not change. At the same time, MLC 1s, typical only for slow skeletal muscles, disappeared. The maximum velocity of soleus muscle native thin filaments was 24 % higher compared to control ones sliding over the same rabbit myosin. Therefore, both myosin and native thin filament kinetics could influence the mechanical characteristics of the soleus muscle. Additionally, the MLC 1s and MLC 1s/v ratio may contribute to the mechanical characteristics of slow skeletal muscle, along with MHC, MLC 2, and MLC 1 slow/fast isoforms ratio.

Troponin Variants as Markers of Skeletal Muscle Health and Diseases

Ca2 +-regulated contractility is a key determinant of the quality of muscles. The sarcomeric myofilament proteins are essential players in the contraction of striated muscles. The troponin complex in the actin thin filaments plays a central role in the Ca2+-regulation of muscle contraction and relaxation. Among the three subunits of troponin, the Ca2+-binding subunit troponin C (TnC) is a member of the calmodulin super family whereas troponin I (TnI, the inhibitory subunit) and troponin T (TnT, the tropomyosin-binding and thin filament anchoring subunit) are striated muscle-specific regulatory proteins. Muscle type-specific isoforms of troponin subunits are expressed in fast and slow twitch fibers and are regulated during development and aging, and in adaptation to exercise or disuse. TnT also evolved with various alternative splice forms as an added capacity of muscle functional diversity. Mutations of troponin subunits cause myopathies. Owing to their physiological and pathological importance, troponin variants can be used as specific markers to define muscle quality. In this focused review, we will explore the use of troponin variants as markers for the fiber contents, developmental and differentiation states, contractile functions, and physiological or pathophysiological adaptations of skeletal muscle. As protein structure defines function, profile of troponin variants illustrates how changes at the myofilament level confer functional qualities at the fiber level. Moreover, understanding of the role of troponin modifications and mutants in determining muscle contractility in age-related decline of muscle function and in myopathies informs an approach to improve human health.

Preservation of metabolic flexibility in skeletal muscle by a combined use of n-3 PUFA and rosiglitazone in dietary obese mice

Insulin resistance, the key defect in type 2 diabetes (T2D), is associated with a low capacity to adapt fuel oxidation to fuel availability, i.e., metabolic inflexibility. This, in turn, contributes to a further damage of insulin signaling. Effectiveness of T2D treatment depends in large part on the improvement of insulin sensitivity and metabolic adaptability of the muscle, the main site of whole-body glucose utilization. We have shown previously in mice fed an obesogenic high-fat diet that a combined use of n-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFA) and thiazolidinediones (TZDs), anti-diabetic drugs, preserved metabolic health and synergistically improved muscle insulin sensitivity. We investigated here whether n-3 LC-PUFA could elicit additive beneficial effects on metabolic flexibility when combined with a TZD drug rosiglitazone. Adult male C57BL/6N mice were fed an obesogenic corn oil-based high-fat diet (cHF) for 8 weeks, or randomly assigned to various interventions: cHF with n-3 LC-PUFA concentrate replacing 15% of dietary lipids (cHF+F), cHF with 10 mg rosiglitazone/kg diet (cHF+ROSI), cHF+F+ROSI, or chow-fed. Indirect calorimetry demonstrated superior preservation of metabolic flexibility to carbohydrates in response to the combined intervention. Metabolomic and gene expression analyses in the muscle suggested distinct and complementary effects of the interventions, with n-3 LC-PUFA supporting complete oxidation of fatty acids in mitochondria and the combination with n-3 LC-PUFA and rosiglitazone augmenting insulin sensitivity by the modulation of branched-chain amino acid metabolism. These beneficial metabolic effects were associated with the activation of the switch between glycolytic and oxidative muscle fibers, especially in the cHF+F+ROSI mice. Our results further support the idea that the combined use of n-3 LC-PUFA and TZDs could improve the efficacy of the therapy of obese and diabetic patients.

Depressed contractile performance and reduced fatigue resistance in single skinned fibers of soleus muscle after long-term disuse in rats

Long-term disuse results in atrophy in skeletal muscle, which is characterized by reduced functional capability, impaired locomotor condition, and reduced resistance to fatigue. Here we show how long-term disuse affects contractility and fatigue resistance in single fibers of soleus muscle taken from the hindlimb immobilization model of the rat. We found that long-term disuse results in depression of caffeine-induced transient contractions in saponin-treated single fibers. However, when normalized to maximal Ca2+-activated force, the magnitude of the transient contractions became similar to that in control fibers. Control experiments indicated that the active force depression in disused muscle is not coupled with isoform switching of myosin heavy chain or troponin, or with disruptions of sarcomere structure or excessive internal sarcomere shortening during contraction. In contrast, our electronmicroscopic observation supported our earlier observation that interfilament lattice spacing is expanded after disuse. Then, to investigate the molecular mechanism of the reduced fatigue resistance in disused muscle, we compared the inhibitory effects of inorganic phosphate (Pi) on maximal Ca2+-activated force in control vs. disused fibers. The effect of Pi was more pronounced in disused fibers, and it approached that observed in control fibers after osmotic compression. These results suggest that contractile depression in disuse results from the lowering of myofibrillar force-generating capacity, rather than from defective Ca2+ mobilization, and the reduced resistance to fatigue is from an enhanced inhibitory effect of Pi coupled with a decrease in the number of attached cross bridges, presumably due to lattice spacing expansion.

Mapping of O-linked beta-N-acetylglucosamine modification sites in key contractile proteins of rat skeletal muscle

O-linked beta-N-acetylglucosamine (O-GlcNAc) is a widespread modification of serine/threonine residues of nucleocytoplasmic proteins. Recently, several key contractile proteins in rat skeletal muscle (i.e., myosin heavy and light chains and actin) were identified as O-GlcNAc modified. Moreover, it was demonstrated that O-GlcNAc moieties involved in contractile protein interactions could modulate Ca(2+) activation parameters of contraction. In order to better understand how O-GlcNAc can modulate the contractile activity of muscle fibers, we decided to identify the sites of O-GlcNAc modification in purified contractile protein homogenates. Using an MS-based method that relies on mild beta-elimination followed by Michael addition of DTT (BEMAD), we determined the localization of one O-GlcNAc site in the subdomain four of actin and four O-GlcNAc sites in the light meromyosin region of myosin heavy chains (MHC). According to previous reports concerning the role of these regions, our data suggest that O-GlcNAc sites might modulate the actin-tropomyosin interaction, and be involved in MHC polymerization or interactions between MHC and other contractile proteins. Thus, the results suggest that this PTM might be involved in protein-protein interactions but could also modulate the contractile properties of skeletal muscle.

Phenotypical transitions and Ca2+activation properties in human muscle fibers: effects of a 60-day bed rest and countermeasures

Muscle biopsies were taken from soleus and vastus lateralis before and after a 60-day bed rest (BR) to examine expression changes in the regulatory proteins of the thin filament and in contractile function. Twenty-four women separated in three groups were submitted to BR or a combined protocol of resistance and aerobic exercises during BR or received a supplementation of amino acids during BR. Ca2+-tension relationships were established in single skinned fibers identified by their myosin heavy chain and troponin C isoform expressions. Expression patterns of regulatory proteins were analyzed on muscle pieces. For both muscles, BR produced similar decreases in slow and fast fiber diameters but larger decreases in P0maximal forces in slow than in fast fibers. Specific forces were decreased in slow soleus and vastus fibers, which displayed a reduction in Ca2+affinity. These changes were accompanied by slow-to-fast transitions in regulatory proteins, with troponins C and T appearing as sensitive markers of unloading. Exercises prevented the changes in fiber diameters and forces and counteracted most of the slow-to-fast transitions. The nutrition program had a morphological beneficial effect on slow fibers. However, these fibers still presented decreases in specific P0after BR. Phenotypical transitions due to BR were not prevented by amino acids. Finally, in vastus lateralis muscle, BR induced a decrease in O-glycosylation level that was prevented by exercise and attenuated by nutrition. In conclusion, this study has addressed for the first time in women the respective efficiencies of two countermeasures associated with BR on muscle properties and regulatory protein expression.

O-linked N-acetylglucosaminylation is involved in the Ca2+ activation properties of rat skeletal muscle

O-Linked N-acetylglucosaminylation termed O-GlcNAc is a dynamic cytosolic and nuclear glycosylation that is dependent both on glucose flow through the hexosamine biosynthesis pathway and on phosphorylation because of the existence of a balance between phosphorylation and O-GlcNAc. This glycosylation is a ubiquitous post-translational modification, which probably plays an important role in many aspects of protein functions. We have previously reported that, in skeletal muscle, proteins of the glycolytic pathway, energetic metabolism, and contractile proteins were O-GlcNAc-modified and that O-Glc-NAc variations could control the muscle protein homeostasis and be implicated in the regulation of muscular atrophy. In this paper, we report O-N-acetylglucosaminylation of a number of key contractile proteins (i.e. myosin heavy and light chains and actin), which suggests that this glycosylation could be involved in skeletal muscle contraction. Moreover, our results showed that incubation of skeletal muscle skinned fibers in N-acetyl-d-glucosamine, in a concentration solution known to inhibit O-GlcNAc-dependent interactions, induced a decrease in calcium sensitivity and affinity of muscular fibers, whereas the cooperativity of the thin filament proteins was not modified. Thus, our results suggest that O-GlcNAc is involved in contractile protein interactions and could thereby modulate muscle contraction.

Differential regulation of myofilament protein isoforms underlying the contractility changes in skeletal muscle unloading

Weight-bearing skeletal muscles change phenotype in response to unloading. Using the hindlimb suspension rat model, we investigated the regulation of myofilament protein isoforms in correlation to contractility. Four weeks of continuous hindlimb unloading produced progressive atrophy and contractility changes in soleus but not extensor digitorum longus muscle. The unloaded soleus muscle also had decreased fatigue resistance. Along with the decrease of myosin heavy chain isoform I and IIa and increase of IIb and IIx, coordinated regulation of thin filament regulatory protein isoforms were observed: gamma- and beta-tropomyosin decreased and alpha-tropomyosin increased, resulting in an alpha/beta ratio similar to that in normal fast twitch skeletal muscle; troponin I and troponin T (TnT) both showed decrease in the slow isoform and increases in the fast isoform. The TnT isoform switching began after 7 days of unloading and TnI isoform showed detectable changes at 14 days while other protein isoform changes were not significant until 28 days of treatment. Correlating to the early changes in contractility, especially the resistance to fatigue, the early response of TnT isoform regulation may play a unique role in the adaptation of skeletal muscle to unloading. When the fast TnT gene expression was upregulated in the unloaded soleus muscle, alternative RNA splicing switched to produce more high molecular weight acidic isoforms, reflecting a potential compensation for the decrease of slow TnT that is critical to skeletal muscle function. The results demonstrate that differential regulation of TnT isoforms is a sensitive mechanism in muscle adaptation to functional demands.

Hypergravity from conception to adult stage: effects on contractile properties and skeletal muscle phenotype

This study examined the effects of an elevation of the gravity factor(hypergravity – 2 g) on the molecular and functional characteristics of rat soleus and plantaris muscles. Long Evans rats were conceived, born and reared (CBR) continuously in hypergravity conditions until the age of 100 days. Whole muscle morphological parameters, Ca2+activation characteristics from single skinned fibers, troponin (Tn) subunit and myosin heavy (MHC) and light (MLC) chains isoform compositions were examined in CBR and control muscles from age-paired terrestrial rats. Decreases in body and muscle mass in soleus and plantaris muscles were observed and associated, in the soleus, with a decrease in fiber diameter. The specific force of CBR soleus fibers was increased, and correlated with the elevation of Ca2+ affinity. This was accompanied by slow-to-slower TnC and TnI isoform transitions and a rearrangement in TnT fast isoform content. The MHC transformations of the soleus after hypergravity were associated with the up (down)-regulation of the MHCI (MHCIIa) mRNA isoforms. The MLC2 phosphorylation state remained unchanged in the soleus muscle. The results suggested that the gravity factor could interact with rat muscle development and that hypergravity experiments could provide good tools for the study of myofibrillar protein plasticity and their associated pathways of regulation.

Contractile properties of rat single muscle fibers and myosin and troponin isoform expression after hypergravity

The effects of 19 days of hypergravity (HG) were investigated on the biochemical and physiological properties of the slow soleus muscle and its fast agonist, the plantaris. HG was induced by rotational centrifugation that led to a 2-G gravity level. The HG rats were characterized by a slower body growth than control, whereas the soleus muscle mass was increased by 15%. Using electrophoretic techniques, we showed that the distribution of myosin heavy chain and troponin T isoforms was not modified after HG in both soleus and plantaris. In contrast, the isoform expression pattern of two troponin subunits, troponin I and troponin C, was changed in a slow-to-fast manner only in the soleus. From tension-pCa relationships, changes in Ca(2+) activation threshold by 0.18 pCa unit indicated a decrease in Ca(2+) sensitivity and an increase in the slope of the curve, attesting to a higher cooperativity along the thin filament after HG. Comparison of our HG data with previous results in microgravity conditions indicated that muscle characteristics, except muscle mass, did not evolve linearly from 0 to 2 G.

Protein turnover in atrophying muscle: from nutritional intervention to microarray expression analysis

PURPOSE OF REVIEW

In response to decreased usage, skeletal muscle undergoes adaptive reductive remodeling due to the decrease in tension on the weight bearing components of the musculo-skeletal system. This response occurs with uncomplicated disuse (e.g. bed rest, space flight), as a secondary consequence of several widely prevalent chronic diseases for which activity is reduced (e.g. chronic obstructive pulmonary disease and chronic heart failure) and is part of the aging process. The problem is therefore one of considerable clinical importance.

RECENT FINDINGS

The impaired function and exercise intolerance is related more to the associated muscle wasting rather than to the specific organ system primarily impacted by the disease. Progress has continued in describing the use of anabolic drugs and dietary manipulation. The major advance in the field has been: (i) the discovery of the atrogin-1 gene and (ii) the application of microarray expression analysis and proteomics with the objectives of obtaining comprehensive understanding of the pathways changed with disuse atrophy.

SUMMARY

Disuse atrophy is a common clinical problem. There is a need for therapeutic interventions that do not involve exercise. A better understanding of the changes, particularly at the molecular level, could indicate hitherto unsuspected sites for nutritional and pharmacological intervention.

Energy metabolism pathways in rat muscle under conditions of simulated microgravity

Evidence from rats flown in space suggests that there is a decrease in the ability of the soleus muscle to oxidize long chain fatty acids during space flight. The observation suggests that a shift in the pathways involved in muscle fuel utilization in the absence of load on the muscle has occurred. It is also possible that the reduction is part of a general down-sizing of metabolic capacity since energy needs of inactive muscle are necessarily less. The rodent hind limb suspension model has proved to be a useful ground based model for studying the musculo-skeletal systems changes that occur with space flight. Microarray technology permits the screening of a large number of the enzymes of the relevant pathways thereby permitting a distinction to be made between a shift fuel utilization pattern or a general decrease in metabolic activity. The soleus muscle was isolated from 5 control and 5 hindlimb suspended rats (21 days) and the Affymetrix system for assessing gene expression used to determine the impact of hindlimb unloading on fuel pathways within the muscle of each animal. RESULTS: Suspended rats failed to gain weight at the same rate as the controls (337 +/- 5 g vs 318 +/- 6 g, p < 0.05) and muscle mass from the soleus was reduced (135 +/- 3 mg vs 48 +/- 4 mg, p < 0.05). There was a consistent decrease (p < 0.05) in gene expression of proteins involved in fatty acid oxidation in the suspended group whereas glycolytic activity was increased (p < 0.05). Gene expressions of individual key regulatory enzymes reflected these changes. Carnitine palmitoyltransferase I and II were decreased (p < 0.05) whereas expression of hexokinase, phosphofructokinase and pyruvate kinase were increased (p < 0.05). CONCLUSION: Disuse atrophy is associated with a change in mRNA levels of enzymes involved in fuel metabolism indicative of a shift in substrate utilization away from fat towards glucose.

Time-dependent changes in expression of troponin subunit isoforms in unloaded rat soleus muscle

This study focuses on the effects of mechanical unloading of rat soleus muscle on the isoform patterns of the three troponin (Tn) subunits: troponin T (TnT), troponin I (TnI), and troponin C (TnC). Mechanical unloading was achieved by hindlimb unloading (HU) for time periods of 7, 15, and 28 days. Relative concentrations of slow and fast TnT, TnI, and TnC isoforms were assessed by electrophoretic and immunoblot analyses. HU induced profound slow-to-fast isoform transitions of all Tn subunits, although to different extents and with different time courses. The effectiveness of the isoform transitions was higher for TnT than for TnI and TnC. Indeed, TnI and TnC encompassed minor partial exchanges of slow isoforms with their fast counterparts, whereas the expression pattern of fast TnT isoforms (TnTf) was largely increased after HU. Moreover, slow and fast isoforms of the different Tn were not affected in the same manner by HU. This suggests that the slow and fast counterparts of the Tn subunit isoforms are regulated independently in response to HU. The changes in TnTf composition occurred in parallel with previously demonstrated transitions within the pattern of the fast myosin heavy chains in the same muscles.

Hybrid skeletal muscle fibres: a rare or common phenomenon?

1. The main aim of the present review is to raise awareness of the molecular complexity of single skeletal muscle fibres from "normal" and "transforming" muscles, in recognition of the many types of hybrids that have been observed in vertebrate skeletal muscle. The data used to illustrate various points made in the review were taken from studies on mammalian (mostly rat) and amphibian muscles. 2. The review provides a brief overview of the pattern and extent of molecular heterogeneity in hybrid muscle fibres and of the methodological problems encountered when attempting to identify and characterize such fibres. Particular attention is given to four types of skeletal muscle hybrids: (i) myosin heavy chain (MHC) hybrids; (ii) mismatched MHC-myosin light chains (MLC) hybrids; (iii) mismatched MHC-regulatory protein hybrids; and (iv) hybrids containing mismatched MHC-sarcoplasmic reticulum protein isoforms. 3. Some of the current ideas regarding the functional significance, origin and cognitive value of hybrid fibres are examined critically.