Ionic interventions that alter the association of troponin C C-domain with the thin filaments of vertebrate striated muscle

Effect of thermal manipulation during embryogenesis on the promoter methylation and expression of myogenesis-related genes in duck skeletal muscle

Avian embryos are an ideal system to investigate the effect of incubation temperature on embryonic development, but the characteristics and mechanisms of temperature effects on poultry embryonic myogenesis are unclear. In this study, we investigated the effect of increasing the incubation temperature by 1 °C on the expression of nine myogenesis-related genes in ducks and then explored the correlation between the alteration of promoter methylation and the expression of two of the nine genes under thermal manipulation (TM). The qRT-PCR results showed that TM during embryonic days (ED) 1-10 promoted (P < 0.05) the expression of genes in breast muscle (PAX3, PAX7, MYOG, MCK, SIX1, TNNC1) and leg muscle (MYOD, MYOG, MYF5, MCK, AKIRIN2, TNNC1). TM during ED10-20 promoted the expression of PAX3, MYF5 and MCK and inhibited AKIRIN2 expression in breast muscle (P < 0.05); however, it inhibited the expression of PAX3, PAX7, MYOD, MYOG, MYF5, SIX1, AKIRIN2 and TNNC1 and promoted MCK expression in leg muscle (P < 0.05). TM during ED20-27 inhibited the expression of genes in breast muscle (PAX7) and leg muscle (MYOD, MYOG, MYF5, TNNC1) and promoted MCK expression in breast and leg muscle (P < 0.05). Furthermore, with the Sequenom MassARRAY platform, it was observed that the average methylation level of AKIRIN2 (ED10) and TNNC1 (ED20) in leg muscle decreased (P < 0.05) after TM. Notably, we found significant (P < 0.05) inverse correlations between the methylation and mRNA levels of AKIRIN2 under TM during ED1-10 (r = - 0.969) and ED10-20 (r = - 0.805). Taken together, TM during ED1-10 was more favorable for improving duck myogenesis-related gene expression than TM during ED10-20 and ED20-27. TM during duck embryogenesis seemed to have a greater effect on the development of leg muscle than breast muscle and might alter AKIRIN2 expression by changing its promoter methylation status. These findings may be helpful to understand temperature effects on the muscle development of avian embryos and to explore the role of epigenetic regulation during this process.

Development and validation of a discriminative dissolution test for betamethasone sodium phosphate and betamethasone dipropionate intramuscular injectable suspension

The intramuscular administration of the injectable suspension betamethasone sodium phosphate (BSP) and betamethasone dipropionate (BD) has immediate therapeutic activity due to solubilized BSP and prolonged activity resulting from the slow release of BD micro-crystals. The purpose of this study was to develop and validate a dissolution method for BD in intramuscular injectable suspensions with detection by high-performance liquid chromatography (HPLC) method. Five commercial products presented a distribution of particle sizes, ranging between 7.43 and 40.25 μm as measured by laser diffraction. It was also found that particle sizes differed between batches of the same product. The different products were tested using the paddle apparatus, with stirring speeds of 25 and 50 rpm in 300 mL of phosphate buffer; simulated body fluid, muscle fluid, and synovial fluid were used as biorelevant dissolution media at 37±0.5°C. It was verified that not only does average particle size affect the dissolution rate, but also the mode and the polydispersity index of the particles. Discriminatory power was obtained using the in vitro dissolution method with 0.1 M sodium phosphate buffer pH 7.4 containing 0.1% sodium lauryl sulfate and a stirring speed of 50 rpm. The HPLC-method is linear, precise, selective, and accurate for the quantification of BSP and BD in dissolution profile testing. This dissolution method can be utilized as a method to control the quality of these injectable suspensions.

Redox state of troponin C cysteine in the D/E helix alters the C-domain affinity for the thin filament of vertebrate striated muscle

BACKGROUND

Despite a broad spectrum of structural studies, it is not yet clear whether the D/E helix of troponin C (TnC) contributes to the interaction of TnC with troponin I (TnI). Redox modifications at Cys 98 in the D/E helix were explored for clues to TnC binding to the thin filament off-state, using recombinant wild-type TnC and an engineered mutant without Cys (Cys98Leu).

METHODS

Recombinant proteins and rabbit psoas skinned fibres were reduced with dithiothreitol (DTT) and variously recombined. Changes in affinity of reduced or oxidised TnC for the thin filament were evaluated via TnC binding and dissociation, using a standardized test for maximal force as an index of fibre TnC content.

RESULTS

All oxidation and reduction effects observed were reversible and led to changes in TnC content. Oxidation (H(2)O(2)) reduced TnC affinity for the filament; reduction (DTT) increased it. Reducing other fibre proteins had no effect. Binding of the Cys-less TnC mutant was not altered by DTT, nor was dissociation of wild-type TnC from reconstituted hybrids (skeletal TnC in cardiac trabeculae). Thus when Cys 98 in the D/E helix of TnC is fully reduced, its binding affinity for the thin filament of skeletal muscle is enhanced and helps to anchor it to the filament.

GENERAL SIGNIFICANCE

Signal transmission between TnC and the other proteins of the regulatory complex is sensitive to the redox state of Cys 98.

Myosin is reversibly inhibited by S-nitrosylation

Nitric oxide (NO*) is synthesized in skeletal muscle and its production increases during contractile activity. Although myosin is the most abundant protein in muscle, it is not known whether myosin is a target of NO* or NO* derivatives. In the present study, we have shown that exercise increases protein S-nitrosylation in muscle, and, among contractile proteins, myosin is the principal target of exogenous SNOs (S-nitrosothiols) in both skinned skeletal muscle fibres and differentiated myotubes. The reaction of isolated myosin with S-nitrosoglutathione results in S-nitrosylation at multiple cysteine thiols and produces two populations of protein-bound SNOs with different stabilities. The less-stable population inhibits the physiological ATPase activity, without affecting the affinity of myosin for actin. However, myosin is neither inhibited nor S-nitrosylated by the NO* donor diethylamine NONOate, indicating a requirement for transnitrosylation between low-mass SNO and myosin cysteine thiols rather than a direct reaction of myosin with NO* or its auto-oxidation products. Interestingly, alkylation of the most reactive thiols of myosin by N-ethylmaleimide does not inhibit formation of a stable population of protein-SNOs, suggesting that these sites are located in less accessible regions of the protein than those that affect activity. The present study reveals a new link between exercise and S-nitrosylation of skeletal muscle contractile proteins that may be important under (patho)physiological conditions.

Effect of thyroid hormone on Mg(2+) homeostasis and extrusion in cardiac cells

The present study investigated the effect of alteration in thyroid hormone level on Mg(2+) homeostasis in cardiac ventricular myocytes. Hyperthyroid conditions increased cardiac myocytes total Mg(2+) content by ~14% as compared to cells from eu-thyroid animals. The excess Mg(2+) was localized predominantly within cytoplasm and mitochondria, and was mobilized into the extracellular compartment by addition of isoproterenol (ISO) or cAMP but not phenylephrine (PHE). Hypothyroid conditions, instead, decreased cardiac myocytes total Mg(2+) content by ~10% as compared to cells from eu-thyroid animals. Also in this case, cytoplasm and mitochondria were the two cellular pools predominantly affected. Under hypothyroid conditions, administration of ISO or cAMP resulted in a decreased Mg(2+) extrusion as compared to that observed in cardiac cells from eu-thyroid animals. Similar changes in cellular Mg(2+) content and transport were observed in cardiac ventricular myocytes isolated from hyper- and hypo-thyroid animals, as well as in cultures of H9C2 cells rendered hyper- or hypo-thyroid under in vitro conditions. Supplementation of thyroid hormone to hypothyroid animals restored Mg(2+) level and transport to levels comparable to those observed in eu-thyroid animals. Taken together, these results indicate that changes in thyroid hormone level have a major effect on Mg(2+) homeostasis and compartmentation in cardiac cells. The enlarged Mg(2+) mobilization via beta- but not alpha(1)-adrenergic receptor stimulation further suggests that beta- and alpha(1)-adrenergic receptors target selectively different Mg(2+) compartments within the cardiac myocyte. These results provide a new rationale to interpret changes in cardiac function under hyper- or hypo-thyroid conditions.

Volume and free energy of folding for troponin C C-domain: linkage to ion binding and N-domain interaction

Troponin C (TnC) is an 18-kDa acidic protein of the EF-hand family that serves as the trigger for muscle contraction. In this study, we investigated the thermodynamic stability of the C-domain of TnC in all its occupancy states (apo, Mg (2+)-, and Ca (2+)-bound states) using a fluorescent mutant with Phe 105 replaced by Trp (F105W/C-domain, residues 88-162) and (1)H NMR spectroscopy. High hydrostatic pressure was employed as a perturbing agent, in combination with urea or without it. On the basis of changes in Trp emission, the C-domain apo state was denatured by pressure (in the range of 1-1000 bar) in the absence of urea. The fluorescence data were corroborated by following the changes in the (1)H NMR signal of Histidine 128. Addition of Ca (2+) or Mg (2+) increased the C-domain stability so that complete denaturation was attained only by the combined use of high hydrostatic pressure and either 7-8 M or 1.5-2 M urea, respectively. The (1)H NMR spectra in the presence of Ca (2+) was typical of a highly structured protein and allowed us to follow the changes in the local environment of several amino-acid residues as a function of pressure at 4 M Urea. Different residues presented different volume changes, but those that are in the hydrophobic core portrayed values very similar to that obtained for tryptophan 105 as measured by fluorescence, indicating that it is indeed a good probe for the overall tertiary structure. From these experiments, we calculated the thermodynamic parameters (Delta G degrees atm and Delta V) that govern the folding of the C-domain in all its possible physiological states and constructed a thermodynamic cycle. Furthermore, a comparison of the volume and free-energy changes of folding of isolated C-domain with those of intact TnC (F105W) revealed that the N-domain has little effect on the structure of the C-domain, even in the presence of Ca (2+). The volume and free-energy diagrams reveal a landscape of different conformations from the less structured, denatured apo form to the highly structured, Ca (2+)-bound form. The large change in folding free energy of the C-domain that takes place when Ca (2+) binds may explain the much higher Ca (2+) affinity of sites III and IV, 2 orders of magnitude higher than the affinity of sites I and II.

Modulation of troponin C affinity for the thin filament by different cross-bridge states in skinned skeletal muscle fibers

In vertebrate skeletal muscle, the C-domain of troponin C (TnC) serves as an anchor; the N-domain regulates the position of troponin-tropomyosin on the thin filament after changes in intracellular Ca2+. Another type of thin-filament regulation is provided by cross-bridges. In this study, we use skinned fibers reconstituted with chicken recombinant TnC (rTnC) to examine TnC-thin filament affinity when cross-bridges containing different ligands are formed. Dissociation and equilibrium binding of apo-TnC (i.e., lacking divalent cations) under different conditions were monitored by a standard test for maximum tension (P (o)). After 10 min in low-Mg2+ relaxing solution, rTnC dissociation (i.e., tension loss) was 80% vs only 45% in rigor. In rigor, adding myosin subfragment 1 (S1) reduced dissociation approximately twofold, whereas stretching to reduce filament overlap increased dissociation to nearly the value for relaxed fibers. Dissociation of rTnC after addition of Pi or MgADP to form A.M.Pi or A.M.ADP cross-bridges was significantly greater than with rigor (A.M) bridges. The increase in P (o) during equilibration with different concentrations of rTnC showed that the affinity for rTnC binding to the thin filament increased progressively with stronger cross-bridges: rTnC concentrations for half-maximal reconstitution (K (0.5)) were 8.1, 3.7, 2.9, and 1.1 microM for A + M.ADP.Pi, A.M.Pi, A.M, and A.M + S1. Cross-bridges containing MgADP(-) (A.M.ADP) were also less effective than rigor bridges in promoting rTnC binding. We conclude that cross-bridge state and number both modulate TnC affinity for the thin filament and that the TnC C-domain is a central element in this pathway.