Multiscale Finite Element Modeling of Active Contraction in Striated Muscle

Greater than one in three American adults have at least one type of cardiovascular disease, a major cause of morbidity. Computational cardiac mechanics has become an important part of the research effort to understand the heart’s response to mechanical stimuli and as an extension, disease progression and potential therapies. To this end, the present work aims to extend these efforts by implementing a cell level contractile model in which active stress generation in muscle tissue is driven by half-sarcomere mechanics. This is accomplished by enhancing the MyoSim model of actin and myosin in order to produce a multiscale model. This contraction model simulates cross-bridge dynamics and captures key components of contraction such as length-dependent activation, Ca2+ activation and sensitivity, and filament cooperativity. Embedding this physiologically motivated contraction model allows for the testing of hypotheses and predictions regarding the interplay between molecular mechanisms and organ level function, while capturing spatial heterogeneity. This multiscale approach has been used to predict an increase in the end-systolic pressure-volume relationship due to the inclusion of a recently discovered super-relaxed state in left-ventricle simulations. It has also been used to predict a decrease in stress generation and efficiency in skeletal muscles due to myofibril misalignment. Finally, the foundation for cardiac growth and remodeling simulations has been implemented

β1-adrenoceptor blockade treatment of right ventricular dysfunction caused by pulmonary hypertension

Failure of the right ventricle (or ventricular) (RV) is the leading cause of death in patients with pulmonary arterial hypertension (PAH), however no treatments specifically target the failing RV. β1-adrenoceptor blockers (β-blockers, BB) reduce mortality in left heart failure but current clinical guidelines caution against their use in PAH. Recent studies suggest β-blockers may be beneficial in PAH however the mechanisms remain unknown. The present study sought to establish whether the β1- blocker metoprolol (10 mg/kg/day) improved survival and function in a rat model of PAH induced by monocrotaline (60 mg/kg, MCT), and to elucidate the mechanisms responsible. Daily metoprolol or placebo was administered 15 days post-monocrotaline injection. PAH resulted in severe RV hypertrophy, dysfunction and heart failure by median day 23 in placebo treated rats (FAIL), whereas metoprolol extended the median survival to day 31 (MCT+BB). RV function measured by echocardiography and catheterisation was severely impaired in FAIL, but was partially restored in MCT+BB on day 23±1. Metoprolol appeared to act primarily on the myocardium and not the vasculature. Contractile abnormalities in isolated FAIL RV cardiomyocytes included increased cell volume, negative force and Ca2+ transient response to faster pacing, increased stiffness to stretch and shorter resting sarcomere length. Reduced creatine kinase activity was found in FAIL; creatine kinase inhibition reproduced characteristics of FAIL in healthy cells, whereas exogeneous creatine kinase reversed the shorter sarcomere length in FAIL cells. Contractile and Ca2+ handling properties of MCT+BB cells were partially or fully restored relative to healthy cells. Capillary density was reduced in FAIL and partially restored in MCT+BB; computer modelling indicated fewer areas of hypoxia in MCT+BB RV. Assessment of FAIL RV mitochondria revealed reduced creatine-coupled respiration but no other detectable defects. Metoprolol improved survival, Ca2+-handling, contractility, oxygen delivery and diastolic properties of PAH rats. β-blockers represent a novel myocardium-specific therapy to target the failing RV in PAH

La bioénergétique systémique moléculaire des cellules cardiaques (la relation structure-fonction dans la régulation du métabolisme énergétique compartmentalisé)

An important element of metabolic regulation of cardiac and skeletal muscle energetics is the interaction of mitochondria with cytoskeleton. Mitochondria are in charge of supplying the cells with energy, adjusting its functional activity under conditions of stress or other aspects of life. Mitochondria display a tissue-specific distribution. In adult rat cardiomyocytes, mitochondria are arranged regularly in a longitudinal lattice at the level of A band between the myofibrils and located within the limits of the sarcomeres. In interaction with cytoskeleton, sarcomeres and sarcoplasmic reticulum they form the functional complexes, the intracellular energetic units (ICEUs). The ICEUs have specialized pathways of energy transfer and metabolic feedback regulation between mitochondria and ATPases, mediated by CK and AK. The central structure of ICEUs is the mitochondrial interactosome (MI) containing ATP Synthasome, respiratory chain, mitochondrial creatine kinase and VDAC, regulated by tubulins. The main role of MI is the regulation of respiration and the intracellular energy fluxes via phosophotransfer networks. The regulation of ICEUs is associated with structural proteins. The association of mitochondria with several cytoskeletal proteins described by several groups has brought to light the importance of structure-function relationship in the metabolic regulation of adult rat cardiomyocytes. To purvey a better understanding of these findings, the present work investigated the mechanism of energy fluxes control and the role of structure-function relationship in the metabolic regulation of adult rat cardiomyocytes. To show these complex associations in adult cardiac cells several proteins were visualized by confocal microscopy: a-actinin and b-tubulin isotypes. For the first time, it was showed the existence of the specific distribution of b-tubulin isotypes in adult cardiac cells. Respiratory measurements were performed to study the role of tubulins in the regulation of oxygen consumption. These results together confirmed the crucial role of cytoskeletal proteins -i.e. tubulins, a-actinin, plectin, desmin, and others- for the normal shape of cardiac cells as well as mitochondrial arrangement and regulation. In addition, in vivo - in situ mitochondrial dynamics were studied by the transfection of GFP-a-actinin, finding that fusion phenomenon does not occur as often as it is believed in healthy adult cardiac cells.Un élément important de la régulation du métabolisme énergétique des muscles cardiaque et squelettiques est l'interaction des mitochondries avec le cytosquelette. Les mitochondries sont responsables de l'approvisionnement des cellules en énergie, elles sont capables d'ajuster leur activité fonctionnelle en fonction des conditions de stress ou d'autres aspects de la vie. Les mitochondries ont une distribution spécifique selon les tissus. Dans les cardiomyocytes de rats adultes, les mitochondries sont disposées régulièrement dans un entrelacement longitudinal au niveau des bandes A, entre les myofibrilles et dans les limites des sarcomères. En interaction avec le cytosquelette, le sarcomère et le réticulum sarcoplasmique, elles forment des complexes fonctionnels appelés unités énergétiques intracellulaires (ICEUs). Les ICEUs ont des voies spécialisées de transfert d'énergie et de régulation des feedback métaboliques entre les mitochondries et les ATPases, médiée par la CK et l'AK. La structure centrale des ICEUs est l'interactosome mitochondrial (MI) qui confient l'ATP synthasome, la chaîne respiratoire, la créatine kinase mitochondriale et VDAC, qui pourrait être régulé par les tubulines. Le rôle principal du MI est la régulation de la respiration et des flux d'énergie intracellulaires via les réseaux de phosphotransfert. La régulation des ICEUs est liée aux protéines structurales. L'association des mitochondries avec plusieurs protéines du cytosquelette, décrite par plusieurs groupes, a mis en évidence l'importance de la relation structure-fonction dans la régulation métabolique des cardiomyocytes de rats adultes. Pour fournir une meilleure compréhension de ces résultats, le présent travail étudie le mécanisme de contrôle des flux d'énergie et le rôle des relations structure-fonction dans la régulation métabolique de cardiomyocytes de rats adultes. Pour montrer ces associations complexes dans les cellules cardiaques adultes, plusieurs protéines ont été visualisées par microscopie confocale: l'a-actinine et les isoformes des b-tubulines. Pour la première fois, l'existence d'une distribution spécifique des isoformes de b-tubuline dans les cellules cardiaques adultes a été montré. Des mesures respiratoires ont été réalisées pour étudier le rôle des tubulines dans la régulation de la consommation d'oxygène. Ces résultats ont confirmé le rôle déterminant des protéines du cytosquelette -tubulines, a-actinine, plectine, desmine, et autres- pour le maintien de la forme normale des cellules cardiaques, ainsi que de l'arrangement et de la régulation mitochondrial. En outre, la dynamique mitochondriale a été étudiée in vivo et in situ par la transfection de la GFP-a-actinine, ceci permettant la mise en évidence du fait que le phénomène de fusion ne se produit pas aussi souvent qu'on ne le croit pour des cellules cardiaques adultes en bonne santé.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

A Spatially Detailed Model of Isometric Contraction Based on Competitive Binding of Troponin I Explains Cooperative Interactions between Tropomyosin and Crossbridges

Biophysical models of cardiac tension development provide a succinct representation of our understanding of force generation in the heart. The link between protein kinetics and interactions that gives rise to high cooperativity is not yet fully explained from experiments or previous biophysical models. We propose a biophysical ODE-based representation of cross-bridge (XB), tropomyosin and troponin within a contractile regulatory unit (RU) to investigate the mechanisms behind cooperative activation, as well as the role of cooperativity in dynamic tension generation across different species. The model includes cooperative interactions between regulatory units (RU-RU), between crossbridges (XB-XB), as well more complex interactions between crossbridges and regulatory units (XB-RU interactions). For the steady-state force-calcium relationship, our framework predicts that: (1) XB-RU effects are key in shifting the half-activation value of the force-calcium relationship towards lower [Ca(2+)], but have only small effects on cooperativity. (2) XB-XB effects approximately double the duty ratio of myosin, but do not significantly affect cooperativity. (3) RU-RU effects derived from the long-range action of tropomyosin are a major factor in cooperative activation, with each additional unblocked RU increasing the rate of additional RU's unblocking. (4) Myosin affinity for short (1-4 RU) unblocked stretches of actin of is very low, and the resulting suppression of force at low [Ca(2+)] is a major contributor in the biphasic force-calcium relationship. We also reproduce isometric tension development across mouse, rat and human at physiological temperature and pacing rate, and conclude that species differences require only changes in myosin affinity and troponin I/troponin C affinity. Furthermore, we show that the calcium dependence of the rate of tension redevelopment k(tr) is explained by transient blocking of RU's by a temporary decrease in XB-RU effects

Protein Kinases

Proteins are the work horses of the cell. As regulators of protein function, protein kinases are involved in the control of cellular functions via intricate signalling pathways, allowing for fine tuning of physiological functions. This book is a collaborative effort, with contribution from experts in their respective fields, reflecting the spirit of collaboration - across disciplines and borders - that exists in modern science. Here, we review the existing literature and, on occasions, provide novel data on the function of protein kinases in various systems. We also discuss the implications of these findings in the context of disease, treatment, and drug development