Role of conformational sampling of Ser16 and Thr17-phosphorylated phospholamban in interactions with SERCA

Myocardial infarction elevates endoplasmic reticulum stress and protein aggregation in heart as well as brain

Cardiovascular diseases, including myocardial infarction (MI), constitute the leading cause of morbidity and mortality worldwide. Protein-aggregate deposition is a hallmark of aging and neurodegeneration. Our previous study reported that aggregation is strikingly elevated in hearts of hypertensive and aged mice; however, no prior study has addressed MI effects on aggregation in heart or brain. Here, we present novel data on heart and brain aggregation in mice following experimental MI, induced by left coronary artery (LCA) ligation. Infarcted and peri-infarcted heart tissue, and whole cerebra, were isolated from mice at sacrifice, 7 days following LCA ligation. Sham-MI mice (identical surgery without ligation) served as controls. We purified detergent-insoluble aggregates from these tissues, and quantified key protein constituents by high-resolution mass spectrometry (LC-MS/MS). Infarct heart tissue had 2.5- to 10-fold more aggregates than non-infarct or sham-MI heart tissue (each P = 0.001). Protein constituents from MI cerebral aggregates overlapped substantially with those from human Alzheimer's disease brain. Prior injection of mice with mesenchymal stem cell (MSC) exosomes, shown to limit infarct size after LCA ligation, reduced cardiac aggregation ~ 60%, and attenuated markers of endoplasmic reticulum (ER) stress in heart and brain (GRP78, ATF6, P-PERK) by 50-75%. MI also elevated aggregate constituents enriched in Alzheimer's disease (AD) aggregates, such as proteasomal subunits, heat-shock proteins, complement C3, clusterin/ApoJ, and other apolipoproteins. These data provide novel evidence that aggregation is elevated in mouse hearts and brains after myocardial ischemia, leading to cognitive impairment resembling AD, but can be attenuated by exosomes or drug (CDN1163) interventions that oppose ER stress.

Resistance training promotes reduction in Visceral Adiposity without improvements in Cardiomyocyte Contractility and Calcium handling in Obese Rats

Resistance training (RT) improves the cardiomyocyte calcium (Ca²⁺) cycling during excitation-contraction coupling. However, the role of RT in cardiomyocyte contractile function associated with Ca²⁺ handling in obesity is unclear. Wistar rats were distributed into four groups: control, sedentary obese, control plus RT, and obesity plus RT. The 10-wk RT protocol was used (4-5 vertical ladder climbs, 60-second interval, 3× a week, 50-100% of maximum load). Metabolic, hormonal, cardiovascular and biochemical parameters were determined. Reduced leptin levels, epididymal, retroperitoneal and visceral fat pads, lower body fat, and adiposity index were observed in RT. Obesity promoted elevation of collagen, but RT did not promote modifications of LV collagen in ObRT. RT induced elevation in maximum rates of contraction and relaxation, and reduction of time to 50% relaxation. ObRT group did not present improvement in the cardiomyocyte contractile function in comparison to Ob group. Reduced cardiac PLB serine¹⁶ phosphorylation (pPLB Ser¹⁶) and pPLB Ser¹⁶/PLB ratio with no alterations in sarcoplasmic reticulum Ca²⁺ ATPase (SERCA2a) and phospholamban (PLB) expression were observed in Ob groups. Resistance training improved body composition reduced fat pads and plasma leptin levels but did not promote positive alterations in cardiomyocyte contractile function, Ca²⁺ handling and phospholamban phosphorylation.

Perturbations of Native Membrane Protein Structure in Alkyl Phosphocholine Detergents: A Critical Assessment of NMR and Biophysical Studies

Membrane proteins perform a host of vital cellular functions. Deciphering the molecular mechanisms whereby they fulfill these functions requires detailed biophysical and structural investigations. Detergents have proven pivotal to extract the protein from its native surroundings. Yet, they provide a milieu that departs significantly from that of the biological membrane, to the extent that the structure, the dynamics, and the interactions of membrane proteins in detergents may considerably vary, as compared to the native environment. Understanding the impact of detergents on membrane proteins is, therefore, crucial to assess the biological relevance of results obtained in detergents. Here, we review the strengths and weaknesses of alkyl phosphocholines (or foscholines), the most widely used detergent in solution-NMR studies of membrane proteins. While this class of detergents is often successful for membrane protein solubilization, a growing list of examples points to destabilizing and denaturing properties, in particular for α-helical membrane proteins. Our comprehensive analysis stresses the importance of stringent controls when working with this class of detergents and when analyzing the structure and dynamics of membrane proteins in alkyl phosphocholine detergents.

Heterogeneous dielectric generalized Born model with a van der Waals term provides improved association energetics of membrane-embedded transmembrane helices

The heterogeneous dielectric generalized Born (HDGB) implicit membrane formalism is extended by the addition of a van der Waals dispersion term to better describe the nonpolar components of the free energy of solvation. The new model, termed HDGBvdW, improves the energy estimates in the hydrophobic interior of the membrane, where polar and charged species are rarely found and nonpolar interactions become significant. The implicit van der Waals term for the membrane environment extends the model from Gallicchio et al. (J. Comput. Chem. 2004, 25, 479) by combining separate contributions from each of the membrane components. The HDGBvdW model is validated with a series of test cases ranging from membrane insertion and pair association profiles of amino acid side chain analogs and transmembrane helices. Overall, the HDGBvdW model leads to increased agreement with explicit membrane simulation results and experimental data. © 2016 Wiley Periodicals, Inc.

The role of continuous versus fractionated physical training on muscle oxidative stress parameters and calcium-handling proteins in aged rats

Age-associated decline in skeletal muscle mass and strength is associated with oxidative stress and Ca(2+) homeostasis disturbance. Exercise should be considered a viable modality to combat aging of skeletal muscle. This study aimed to investigate whether continuous and fractionated training could be useful tools to attenuate oxidative damage and retain calcium-handling proteins. We conducted the study using 24-month-old male Wistar rats, divided into control, continuous, and fractionated groups. Animals ran at 13 m min(-1) for five consecutive days (except weekends) for 6 weeks, for a total period of 42 days. Each session comprised 45 min of exercise, either continuous or divided into three daily sessions of 15 min each. Metabolic and oxidative stress markers, protein levels of mitochondrial transcription factors, and calcium-handling proteins were analyzed. Continuous exercise resulted in reduced ROS production as well as showed a decrease in TBARS levels and carbonyl content. On the other hand, fractionated training increased the antioxidant enzyme activities. The ryanodine receptor and phospholamban protein were regulated by continuous training while sodium calcium exchange protein was increased by the fractionated training. These data suggest that intracellular Ca(2+) can be modulated by various training stimuli. In addition, the modulation of oxidative stress by continuous and fractionated training may play an important regulatory role in the muscular contraction mechanism of aged rats, due to changes in calcium metabolism.

Accurate Determination of Conformational Transitions in Oligomeric Membrane Proteins

The structural dynamics governing collective motions in oligomeric membrane proteins play key roles in vital biomolecular processes at cellular membranes. In this study, we present a structural refinement approach that combines solid-state NMR experiments and molecular simulations to accurately describe concerted conformational transitions identifying the overall structural, dynamical, and topological states of oligomeric membrane proteins. The accuracy of the structural ensembles generated with this method is shown to reach the statistical error limit, and is further demonstrated by correctly reproducing orthogonal NMR data. We demonstrate the accuracy of this approach by characterising the pentameric state of phospholamban, a key player in the regulation of calcium uptake in the sarcoplasmic reticulum, and by probing its dynamical activation upon phosphorylation. Our results underline the importance of using an ensemble approach to characterise the conformational transitions that are often responsible for the biological function of oligomeric membrane protein states.

Molecular dynamics simulations of biological membranes and membrane proteins using enhanced conformational sampling algorithms

This paper reviews various enhanced conformational sampling methods and explicit/implicit solvent/membrane models, as well as their recent applications to the exploration of the structure and dynamics of membranes and membrane proteins. Molecular dynamics simulations have become an essential tool to investigate biological problems, and their success relies on proper molecular models together with efficient conformational sampling methods. The implicit representation of solvent/membrane environments is reasonable approximation to the explicit all-atom models, considering the balance between computational cost and simulation accuracy. Implicit models can be easily combined with replica-exchange molecular dynamics methods to explore a wider conformational space of a protein. Other molecular models and enhanced conformational sampling methods are also briefly discussed. As application examples, we introduce recent simulation studies of glycophorin A, phospholamban, amyloid precursor protein, and mixed lipid bilayers and discuss the accuracy and efficiency of each simulation model and method. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.

Cardiac Dysfunction Induced by Obesity Is Not Related to β-Adrenergic System Impairment at the Receptor-Signalling Pathway

Obesity has been shown to impair myocardial performance. Some factors have been suggested as responsible for possible cardiac abnormalities in models of obesity, among them beta-adrenergic (βA) system, an important mechanism of regulation of myocardial contraction and relaxation. The objective of present study was to evaluate the involvement of βA system components in myocardial dysfunction induced by obesity. Thirty-day-old male Wistar rats were distributed in control (C, n = 25) and obese (Ob, n = 25) groups. The C group was fed a standard diet and Ob group was fed four unsaturated high-fat diets for 15 weeks. Cardiac function was evaluated by isolated papillary muscle preparation and βA system evaluated by using cumulative concentrations of isoproterenol and Western blot. After 15 weeks, the Ob rats developed higher adiposity index than C rats and several comorbidities; however, were not associated with changes in systolic blood pressure. Obesity caused structural changes and the myocardial responsiveness to post-rest contraction stimulus and increased extracellular calcium (Ca²⁺) was compromised. There were no changes in cardiac function between groups after βA stimulation. The obesity was not accompanied by changes in protein expression of G protein subunit alpha (Gsα) and βA receptors (β₁AR and β₂AR). In conclusion, the myocardial dysfunction caused by unsaturated high-fat diet-induced obesity, after 15 weeks, is not related to βAR system impairment at the receptor-signalling pathway.

Structural dynamics and conformational equilibria of SERCA regulatory proteins in membranes by solid-state NMR restrained simulations

Solid-state NMR spectroscopy is emerging as a powerful approach to determine structure, topology, and conformational dynamics of membrane proteins at the atomic level. Conformational dynamics are often inferred and quantified from the motional averaging of the NMR parameters. However, the nature of these motions is difficult to envision based only on spectroscopic data. Here, we utilized restrained molecular dynamics simulations to probe the structural dynamics, topology and conformational transitions of regulatory membrane proteins of the calcium ATPase SERCA, namely sarcolipin and phospholamban, in explicit lipid bilayers. Specifically, we employed oriented solid-state NMR data, such as dipolar couplings and chemical shift anisotropy measured in lipid bicelles, to refine the conformational ensemble of these proteins in lipid membranes. The samplings accurately reproduced the orientations of transmembrane helices and showed a significant degree of convergence with all of the NMR parameters. Unlike the unrestrained simulations, the resulting sarcolipin structures are in agreement with distances and angles for hydrogen bonds in ideal helices. In the case of phospholamban, the restrained ensemble sampled the conformational interconversion between T (helical) and R (unfolded) states for the cytoplasmic region that could not be observed using standard structural refinements with the same experimental data set. This study underscores the importance of implementing NMR data in molecular dynamics protocols to better describe the conformational landscapes of membrane proteins embedded in realistic lipid membranes.

Long-term obesity promotes alterations in diastolic function induced by reduction of phospholamban phosphorylation at serine-16 without affecting calcium handling

Few studies have evaluated the relationship between the duration of obesity, cardiac function, and the proteins involved in myocardial calcium (Ca(2+)) handling. We hypothesized that long-term obesity promotes cardiac dysfunction due to a reduction of expression and/or phosphorylation of myocardial Ca(2+)-handling proteins. Thirty-day-old male Wistar rats were distributed into two groups (n = 10 each): control (C; standard diet) and obese (Ob; high-fat diet) for 30 wk. Morphological and histological analyses were assessed. Left ventricular cardiac function was assessed in vivo by echocardiographic evaluation and in vitro by papillary muscle. Cardiac protein expression of sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a), calsequestrin, L-type Ca(2+) channel, and phospholamban (PLB), as well as PLB serine-16 phosphorylation (pPLB Ser(16)) and PLB threonine-17 phosphorylation (pPLB Thr(17)) were determined by Western blot. The adiposity index was higher (82%) in Ob rats than in C rats. Obesity promoted cardiac hypertrophy without alterations in interstitial collagen levels. Ob rats had increased endocardial and midwall fractional shortening, posterior wall shortening velocity, and A-wave compared with C rats. Cardiac index, early-to-late diastolic mitral inflow ratio, and isovolumetric relaxation time were lower in Ob than in C. The Ob muscles developed similar baseline data and myocardial responsiveness to increased extracellular Ca(2+). Obesity caused a reduction in cardiac pPLB Ser(16) and the pPLB Ser(16)/PLB ratio in Ob rats. Long-term obesity promotes alterations in diastolic function, most likely due to the reduction of pPLB Ser(16), but does not impair the myocardial Ca(2+) entry and recapture to SR.

Secondary structure, backbone dynamics, and structural topology of phospholamban and its phosphorylated and Arg9Cys-mutated forms in phospholipid bilayers utilizing 13C and 15N solid-state NMR spectroscopy

Phospholamban (PLB) is a membrane protein that regulates heart muscle relaxation rates via interactions with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA). When PLB is phosphorylated or Arg9Cys (R9C) is mutated, inhibition of SERCA is relieved. (13)C and (15)N solid-state NMR spectroscopy is utilized to investigate conformational changes of PLB upon phosphorylation and R9C mutation. (13)C═O NMR spectra of the cytoplasmic domain reveal two α-helical structural components with population changes upon phosphorylation and R9C mutation. The appearance of an unstructured component is observed on domain Ib. (15)N NMR spectra indicate an increase in backbone dynamics of the cytoplasmic domain. Wild-type PLB (WT-PLB), Ser16-phosphorylated PLB (P-PLB), and R9C-mutated PLB (R9C-PLB) all have a very dynamic domain Ib, and the transmembrane domain has an immobile component. (15)N NMR spectra indicate that the cytoplasmic domain of R9C-PLB adopts an orientation similar to P-PLB and shifts away from the membrane surface. Domain Ib (Leu28) of P-PLB and R9C-PLB loses the alignment. The R9C-PLB adopts a conformation similar to P-PLB with a population shift to a more extended and disordered state. The NMR data suggest the more extended and disordered forms of PLB may relate to inhibition relief.

Probing the interaction of Arg9Cys mutated phospholamban with phospholipid bilayers by solid-state NMR spectroscopy

Phospholamban (PLB) is a 52 amino acid integral membrane protein that interacts with the sarcoplasmic reticulum Ca(2+) ATPase (SERCA) and helps to regulate Ca(2+) flow. PLB inhibits SERCA impairing Ca(2+) translocation. The inhibition can be relieved upon phosphorylation of PLB. The Arg9 to Cys (R9C) mutation is a loss of function mutation with reduced inhibitory potency. The effect R9C PLB has on the membrane surface and the hydrophobic region dynamics was investigated by (31)P and (2)H solid-state NMR spectroscopy in multilamellar vesicles (MLVs). The (31)P NMR spectra indicate that, like the phosphorylated PLB (P-PLB), the mutated R9C-PLB protein has significantly less interaction with the lipid bilayer headgroup when compared to wild-type PLB (WT-PLB). Similar to P-PLB, R9C-PLB slightly decreases (31)P T1 values in the lipid headgroup region. (2)H SCD order parameters of (2)H nuclei along the lipid acyl chain decrease less dramatically for R9C-PLB and P-PLB when compared to WT-PLB. The results suggest that R9C-PLB interacts less with the membrane surface and hydrophobic region than WT-PLB. Detachment of the cytoplasmic domain of R9C-PLB from the membrane surface could be related to its loss of function.

Dynamic Heterogeneous Dielectric Generalized Born (DHDGB): An implicit membrane model with a dynamically varying bilayer thickness

An extension to the heterogeneous dielectric generalized Born (HDGB) implicit membrane formalism is presented to allow dynamic membrane deformations in response to membrane-inserted biomolecules during molecular dynamic simulations. The flexible membrane is implemented through additional degrees of freedom that represent the membrane deformation at the contact points of a membrane-inserted solute with the membrane. The extra degrees of freedom determine the dielectric and non-polar solvation free energy profiles that are used to obtain the solvation free energy in the presence of the membrane and are used to calculate membrane deformation free energies according to an elastic membrane model. With the dynamic HDGB (DHDGB) model the membrane is able to deform in response to the insertion of charged molecules thereby avoiding the overestimation of insertion free energies with static membrane models. The DHDGB model also allows the membrane to respond to the insertion of membrane-spanning solutes with hydrophobic mismatch. The model is tested with the membrane insertion of amino acid side chain analogs, arginine-containing helices, the WALP23 peptide, and the gramicidin A channel.