Dynamic finite-strain modelling of the human left ventricle in health and disease using an immersed boundary-finite element method

Detailed models of the biomechanics of the heart are important both for developing improved interventions for patients with heart disease and also for patient risk stratification and treatment planning. For instance, stress distributions in the heart affect cardiac remodelling, but such distributions are not presently accessible in patients. Biomechanical models of the heart offer detailed three-dimensional deformation, stress and strain fields that can supplement conventional clinical data. In this work, we introduce dynamic computational models of the human left ventricle (LV) that are derived from clinical imaging data obtained from a healthy subject and from a patient with a myocardial infarction (MI). Both models incorporate a detailed invariant-based orthotropic description of the passive elasticity of the ventricular myocardium along with a detailed biophysical model of active tension generation in the ventricular muscle. These constitutive models are employed within a dynamic simulation framework that accounts for the inertia of the ventricular muscle and the blood that is based on an immersed boundary (IB) method with a finite element description of the structural mechanics. The geometry of the models is based on data obtained non-invasively by cardiac magnetic resonance (CMR). CMR imaging data are also used to estimate the parameters of the passive and active constitutive models, which are determined so that the simulated end-diastolic and end-systolic volumes agree with the corresponding volumes determined from the CMR imaging studies. Using these models, we simulate LV dynamics from end-diastole to end-systole. The results of our simulations are shown to be in good agreement with subject-specific CMR-derived strain measurements and also with earlier clinical studies on human LV strain distributions

Advances in computational modelling for personalised medicine after myocardial infarction

Myocardial infarction (MI) is a leading cause of premature morbidity and mortality worldwide. Determining which patients will experience heart failure and sudden cardiac death after an acute MI is notoriously difficult for clinicians. The extent of heart damage after an acute MI is informed by cardiac imaging, typically using echocardiography or sometimes, cardiac magnetic resonance (CMR). These scans provide complex data sets that are only partially exploited by clinicians in daily practice, implying potential for improved risk assessment. Computational modelling of left ventricular (LV) function can bridge the gap towards personalised medicine using cardiac imaging in patients with post-MI. Several novel biomechanical parameters have theoretical prognostic value and may be useful to reflect the biomechanical effects of novel preventive therapy for adverse remodelling post-MI. These parameters include myocardial contractility (regional and global), stiffness and stress. Further, the parameters can be delineated spatially to correspond with infarct pathology and the remote zone. While these parameters hold promise, there are challenges for translating MI modelling into clinical practice, including model uncertainty, validation and verification, as well as time-efficient processing. More research is needed to (1) simplify imaging with CMR in patients with post-MI, while preserving diagnostic accuracy and patient tolerance (2) to assess and validate novel biomechanical parameters against established prognostic biomarkers, such as LV ejection fraction and infarct size. Accessible software packages with minimal user interaction are also needed. Translating benefits to patients will be achieved through a multidisciplinary approach including clinicians, mathematicians, statisticians and industry partners

The isolated muscle fibre as a model of disuse atrophy: characterization using PhAct, a method to quantify f-actin

Research into muscle atrophy and hypertrophy is hampered by limitations of the available experimental models. Interpretation of in vivo experiments is confounded by the complexity of the environment while in vitro models are subject to the marked disparities between cultured myotubes and the mature myofibres of living tissues. Here we develop a method (PhAct) based on ex vivo maintenance of the isolated myofibre as a model of disuse atrophy, using standard microscopy equipment and widely available analysis software, to measure f-actin content per myofibre and per nucleus over two weeks of ex vivo maintenance. We characterize the 35% per week atrophy of the isolated myofibre in terms of early changes in gene expression and investigate the effects on loss of muscle mass of modulatory agents, including Myostatin and Follistatin. By tracing the incorporation of a nucleotide analogue we show that the observed atrophy is not associated with loss or replacement of myonuclei. Such a completely controlled investigation can be conducted with the myofibres of a single muscle. With this novel method we can identify those features and mechanisms of atrophy and hypertrophy that are intrinsic to the muscle fibre from those that include activities of other tissues and systemic agents

Lack of Tgfbr1 and Acvr1b synergistically stimulates myofibre hypertrophy and accelerates muscle regeneration

In skeletal muscle, transforming growth factor-β (TGF-β) family growth factors, TGF-β1 and myostatin, are involved in atrophy and muscle wasting disorders. Simultaneous interference with their signalling pathways may improve muscle function; however, little is known about their individual and combined receptor signalling. Here, we show that inhibition of TGF-β signalling by simultaneous muscle-specific knockout of TGF-β type I receptors Tgfbr1 and Acvr1b in mice, induces substantial hypertrophy, while such effect does not occur by single receptor knockout. Hypertrophy is induced by increased phosphorylation of Akt and p70S6K and reduced E3 ligases expression, while myonuclear number remains unaltered. Combined knockout of both TGF-β type I receptors increases the number of satellite cells, macrophages and improves regeneration post cardiotoxin-induced injury by stimulating myogenic differentiation. Extra cellular matrix gene expression is exclusively elevated in muscle with combined receptor knockout. Tgfbr1 and Acvr1b are synergistically involved in regulation of myofibre size, regeneration, and collagen deposition

Myocardial segment-specific model generation for simulating the electrical action of the heart

<p>Abstract</p> <p>Background</p> <p>Computer models of the electrical and mechanical actions of the heart, solved on geometrically realistic domains, are becoming an increasingly useful scientific tool. Construction of these models requires detailed measurement of the microstructural features which impact on the function of the heart. Currently a few generic cardiac models are in use for a wide range of simulation problems, and contributions to publicly accessible databases of cardiac structures, on which models can be solved, remain rare. This paper presents to-date the largest database of porcine left ventricular segment microstructural architecture, for use in both electrical and mechanical simulation.</p> <p>Methods</p> <p>Cryosectioning techniques were used to reconstruct the myofibre and myosheet orientations in tissue blocks of size ~15 × 15 × 15 mm, taken from the mid-anterior left ventricular freewall, of seven hearts. Tissue sections were gathered on orthogonal planes, and the angles of intersection of myofibres and myosheets with these planes determined automatically with a gradient intensity based algorithm. These angles were then combined to provide a description of myofibre and myosheet variation throughout the tissue, in a form able to be input to biophysically based computational models of the heart.</p> <p>Results</p> <p>Several microstructural features were common across all hearts. Myofibres rotated through 141 ± 18° (mean ± SD) from epicardium to endocardium, in near linear fashion. In the outer two-thirds of the wall sheet angles were predominantly negative, however, in the inner one-third an abrupt change in sheet angle, with reversal in sign, was seen in six of the seven hearts. Two distinct populations of sheets with orthogonal orientations often co-existed, usually with one population dominating. The utility of the tissue structures was demonstrated by simulating the passive and active electrical responses of two of the tissue blocks to current injection. Distinct patterns of electrical response were obtained in the two tissue blocks, illustrating the importance of testing model based predictions on a variety of tissue architectures.</p> <p>Conclusion</p> <p>This study significantly expands the set of geometries on which models of cardiac function can be solved.</p

Computational biomechanics of acute myocardial infarction and its treatment

The intramyocardial injection of biomaterials is an emerging therapy for myocardial infarction. Computational methods can help to study the mechanical effect s of biomaterial injectates on the infarcted heart s and can contribute to advance and optimise the concept of this therapy. The distribution of polyethylene glycol hydrogel injectate delivered immediately after the infarct induction was studied using rat infarct model. A micro-structural three-dimensional geometrical model of the entire injectate was reconstructed from histological micro graphs. The model provides a realistic representation of biomaterial injectates in computational models at macroscopic and microscopic level. Biaxial and compression mechanical testing was conducted for healing rat myocardial infarcted tissue at immediate (0 day), 7, 14 and 28 days after infarction onset. Infarcts were found to be mechanically anisotropic with the tissue being stiffer in circumferential direction than in longitudinal direction. The 0, 7, 14 and 28 days infarcts showed 443, 670, 857 and 1218 kPa circumferential tensile moduli. The 28 day infarct group showed a significantly higher compressive modulus compared to the other infarct groups (p= 0.0055, 0.028, and 0.018 for 0, 7 and 14 days groups). The biaxial mechanical data were utilized to establish material constitutive models of rat healing infarcts. Finite element model s and genetic algorithms were employed to identify the parameters of Fung orthotropic hyperelastic strain energy function for the healing infarcts. The provided infarct mechanical data and the identified constitutive parameters offer a platform for investigations of mechanical aspects of myocardial infarction and therapies in the rat, an experimental model extensively used in the development of infarct therapies. Micro-structurally detailed finite element model of a hydrogel injectate in an infarct was developed to provide an insight into the micromechanics of a hydrogel injectate and infarct during the diastolic filling. The injectate caused the end-diastolic fibre stresses in the infarct zone to decrease from 22.1 to 7.7 kPa in the 7 day infarct and from 35.7 to 9.7 kPa in the 28 day infarct. This stress reduction effect declined as the stiffness of the biomaterial increased. It is suggested that the gel works as a force attenuating system through micromechanical mechanisms reducing the force acting on tissue layers during the passive diastolic dilation of the left ventricle and thus reducing the stress induced in these tissue layers

Nebulin nemaline myopathy recapitulated in a compound heterozygous mouse model with both a missense and a nonsense mutation in Neb

Nemaline myopathy (NM) caused by mutations in the gene encoding nebulin (NEB) accounts for at least 50% of all NM cases worldwide, representing a significant disease burden. Most NEB-NM patients have autosomal recessive disease due to a compound heterozygous genotype. Of the few murine models developed for NEB-NM, most are Neb knockout models rather than harbouring Neb mutations. Additionally, some models have a very severe phenotype that limits their application for evaluating disease progression and potential therapies. No existing murine models possess compound heterozygous Neb mutations that reflect the genotype and resulting phenotype present in most patients. We aimed to develop a murine model that more closely matched the underlying genetics of NEB-NM, which could assist elucidation of the pathogenetic mechanisms underlying the disease. Here, we have characterised a mouse strain with compound heterozygous Neb mutations; one missense (p.Tyr2303His), affecting a conserved actin-binding site and one nonsense mutation (p.Tyr935*), introducing a premature stop codon early in the protein. Our studies reveal that this compound heterozygous model, Neb(Y2303H, Y935X), has striking skeletal muscle pathology including nemaline bodies. In vitro whole muscle and single myofibre physiology studies also demonstrate functional perturbations. However, no reduction in lifespan was noted. Therefore, Neb(Y2303H,Y935X) mice recapitulate human NEB-NM and are a much needed addition to the NEB-NM mouse model collection. The moderate phenotype also makes this an appropriate model for studying NEB-NM pathogenesis, and could potentially be suitable for testing therapeutic applications.Peer reviewe

The in vitro engineering of craniofacial muscle constructs utilising degradable composite glass fibre-collagen scaffolds

Absent or defective craniofacial skeletal muscle can lead to loss of function and aesthetics. Current therapies are fraught with limitations: for example, the surgical transfer of tissue is associated with donor site morbidity coupled with a paucity of available tissue. The potential to engineer skeletal muscle tissue could circumvent disadvantages related to current techniques. Furthermore, the creation of a muscle testbed could provide the means to investigate the response to various manipulations. The aim of this research was to produce an in vitro human craniofacial skeletal muscle tissue suitable as a test-bed for novel therapies involving the craniofacial region. Degradable phosphate-based glass fibre scaffolds of various configurations, combined with extracellular matrix (ECM) components, were seeded with human craniofacial muscle-derived cell cultures. Myogenicity was confirmed with immunofluorescent techniques prior to seeding. The seeded scaffolds were incubated at 37°C in a humidified atmosphere of 5% CO2 in air for up to 21 days. Modulation contrast microscopy was used to analyse migration and morphology. Cell attachment and survival were assessed with the CyQUANT® and alamarBlue® assays, and cell differentiation and maturation were investigated using immunofluorescence and quantitative RT-PCR. Parallel arrays of glass fibres coated with ECM components provided the correct topology to support cell alignment and differentiation. Specifically, compared to control scaffolds, glass fibre scaffolds promoted upregulation of developmental, fast and slow myosin heavy chain genes. Further refinement of the system involved glass fibres embedded within collagen gels, created to mimic the architecture of native skeletal muscle: cells within these constructs were aligned parallel to the glass fibres, and over time, the constructs rolled along the short axis to produce a muscle ‘organoid’. Additionally, the collagen gel contracted along the long axis to reveal tufts of glass fibres analogous to tendons (mean 13.87% reduction in length). Upregulation of the myosin heavy chain genes was promoted, albeit at a later timepoint. In conclusion, degradable glass fibre-ECM scaffolds provided the correct topographical and biological cues to aid the in vitro engineering of human craniofacial skeletal muscle tissue

Nebulin nemaline myopathy recapitulated in a compound heterozygous mouse model with both a missense and a nonsense mutation in Neb

Nemaline myopathy (NM) caused by mutations in the gene encoding nebulin (NEB) accounts for at least 50% of all NM cases worldwide, representing a significant disease burden. Most NEB-NM patients have autosomal recessive disease due to a compound heterozygous genotype. Of the few murine models developed for NEB-NM, most are Neb knockout models rather than harbouring Neb mutations. Additionally, some models have a very severe phenotype that limits their application for evaluating disease progression and potential therapies. No existing murine models possess compound heterozygous Neb mutations that reflect the genotype and resulting phenotype present in most patients. We aimed to develop a murine model that more closely matched the underlying genetics of NEB-NM, which could assist elucidation of the pathogenetic mechanisms underlying the disease. Here, we have characterised a mouse strain with compound heterozygous Neb mutations; one missense (p.Tyr2303His), affecting a conserved actin-binding site and one nonsense mutation (p.Tyr935*), introducing a premature stop codon early in the protein. Our studies reveal that this compound heterozygous model, Neb(Y2303H, Y935X), has striking skeletal muscle pathology including nemaline bodies. In vitro whole muscle and single myofibre physiology studies also demonstrate functional perturbations. However, no reduction in lifespan was noted. Therefore, Neb(Y2303H,Y935X) mice recapitulate human NEB-NM and are a much needed addition to the NEB-NM mouse model collection. The moderate phenotype also makes this an appropriate model for studying NEB-NM pathogenesis, and could potentially be suitable for testing therapeutic applications.Peer reviewe

A GFPT1 deficient mouse model of congenital myasthenic syndrome

PhD ThesisCongenital myasthenic syndromes (CMS) are inherited disorders characterised by fatigable muscle weakness resulting from impaired transmission at the neuromuscular junction (NMJ). CMS occur due to mutations in genes encoding proteins responsible for maintaining the structure and function of the NMJ. Glutamine-fructose-6-phosphate transaminase 1 (GFPT1) is the rate-limiting enzyme in the hexosamine biosynthetic pathway which yields precursors required for protein and lipid glycosylation. Mutations in GFPT1 and genes downstream of this pathway are pathogenic for CMS. One hypothesis is that hypoglycosylation of NMJ proteins results in defective neurotransmission. The aim of this study is to generate and characterise a GFPT1 deficient mouse model of CMS. One of the challenges we face is the viability of Gfpt1 knockout mice. Here we generate a novel muscle-specific GFPT1 knockout mouse model using Cre/loxP technology. We demonstrate that a deficiency of GFPT1 in muscle only, is sufficient for causing a CMS phenotype. Our model recapitulates many aspects of the phenotype observed in patients with GFPT1-related CMS. Mutant mice display early changes in the morphology of postsynaptic components of the NMJ, which are accompanied by presynaptic alterations. They later develop a myopathic phenotype and formation of tubular aggregates. We further identify proteins in skeletal muscle that are differentially regulated because of GFPT1 deficiency. Our data demonstrates a critical role for GFPT1 in the development of the NMJ, neurotransmission, and skeletal muscle integrity. The muscle-specific GFPT1 deficient mouse model allows us to investigate the implications of not only GFPT1 mutations, but may also give us an insight into the pathophysiological consequences of mutations in genes downstream of GFPT1, which also result in hypoglycosylation. This model has the potential to enhance our understanding of current drug therapies, and to drive forward the development of new compounds which can be implemented in the clinic.Medical Research Council UK and The Barbour Foundation