Agonistic and antagonistic roles of fibroblasts and cardiomyocytes on viscoelastic stiffening of engineered human myocardium

Unexpected softening of a fibrous matrix by contracting inclusions

Cells respond to the stiffness of their surrounding environment, but quantifying the stiffness of a fibrous matrix at the scale of a cell is complicated, due to the effects of nonlinearity and complex force transmission pathways resulting from randomness in fiber density and connections. While it is known that forces produced by individual contractile cells can stiffen the matrix, it remains unclear how simultaneous contraction of multiple cells in a fibrous matrix alters the stiffness at the scale of a cell. Here, we used computational modeling and experiments to quantify the stiffness of a random fibrous matrix embedded with multiple contracting inclusions, which mimicked the contractile forces of a cell. The results showed that when the matrix was free to contract as a result of the forces produced by the inclusions, the matrix softened rather than stiffened, which was surprising given that the contracting inclusions applied tensile forces to the matrix. Using the computational model, we identified that the underlying cause of the softening was that the majority of the fibers were under a local state of axial compression, causing buckling. We verified that this buckling-induced matrix softening was sufficient for cells to sense and respond by altering their morphology and force generation. Our findings reveal that the localized forces induced by cells do not always stiffen the matrix; rather, softening can occur in instances wherein the matrix can contract in response to the cell-generated forces. This study opens up new possibilities to investigate whether cell-induced softening contributes to maintenance of homeostatic conditions or progression of disease. STATEMENT OF SIGNIFICANCE: Mechanical interactions between cells and the surrounding matrix strongly influence cellular functions. Cell-induced forces can alter matrix properties, and much prior literature in this area focused on the influence of individual contracting cells. Cells in tissues are rarely solitary; rather, they are interspersed with neighboring cells throughout the matrix. As a result, the mechanics are complicated, leaving it unclear how the multiple contracting cells affect matrix stiffness. Here, we show that multiple contracting inclusions within a fibrous matrix can cause softening that in turn affects cell sensing and response. Our findings provide new directions to determine impacts of cell-induced softening on maintenance of tissue or progression of disease.

Advances in the design, generation, and application of tissue-engineered myocardial equivalents

Due to the limited regenerative ability of cardiomyocytes, the disabling irreversible condition of myocardial failure can only be treated with conservative and temporary therapeutic approaches, not able to repair the damage directly, or with organ transplantation. Among the regenerative strategies, intramyocardial cell injection or intravascular cell infusion should attenuate damage to the myocardium and reduce the risk of heart failure. However, these cell delivery-based therapies suffer from significant drawbacks and have a low success rate. Indeed, cardiac tissue engineering efforts are directed to repair, replace, and regenerate native myocardial tissue function. In a regenerative strategy, biomaterials and biomimetic stimuli play a key role in promoting cell adhesion, proliferation, differentiation, and neo-tissue formation. Thus, appropriate biochemical and biophysical cues should be combined with scaffolds emulating extracellular matrix in order to support cell growth and prompt favorable cardiac microenvironment and tissue regeneration. In this review, we provide an overview of recent developments that occurred in the biomimetic design and fabrication of cardiac scaffolds and patches. Furthermore, we sift in vitro and in situ strategies in several preclinical and clinical applications. Finally, we evaluate the possible use of bioengineered cardiac tissue equivalents as in vitro models for disease studies and drug tests.

Collagen Hydrogel Containing Polyethylenimine-Gold Nanoparticles for Drug Release and Enhanced Beating Properties of Engineered Cardiac Tissues

Cardiac tissue engineering is a promising strategy to prevent heart failure. However, several issues remain unsolved, including efficient electrical coupling and incorporating factors to enhance tissue maturation and vascularization. Herein, a biohybrid hydrogel that enhances beating properties of engineered cardiac tissues and allows drug release concurrently is developed. Gold nanoparticles (AuNPs) with different sizes (18-241 nm) and surface charges (33.9-55.4 mV) are synthesized by reducing gold (III) chloride trihydrate using branched polyethyleneimine (bPEI). These nanoparticles increase gel stiffness from ≈91 to ≈146 kPa, enhance electrical conductivity of collagen hydrogels from ≈40 to 49-68 mS cm-1 , and allow slow and steady release of loaded drugs. Engineered cardiac tissues based on bPEI-AuNP-collagen hydrogels and either primary or human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes show enhanced beating properties. hiPSC-derived cardiomyocytes exhibit more aligned and wider sarcomeres in bPEI-AuNP-collagen hydrogels compared to collagen hydrogels. Furthermore, the presence of bPEI-AuNPs result in advanced electrical coupling evidenced by synchronous and homogenous calcium flux throughout the tissue. RNA-seq analyses are in agreement with these observations. Collectively, this data demonstrate the potential of bPEI-AuNP-collagen hydrogels to improve tissue engineering approaches to prevent heart failure and possibly treat diseases of other electrically sensitive tissues.

Noncovalent functionalization of carbon nanotubes as a scaffold for tissue engineering

Tissue engineering is one of the hot topics in recent research that needs special requirements. It depends on the development of scaffolds that allow tissue formation with certain characteristics, carbon nanotubes (CNTs)-collagen composite attracted the attention of the researchers with this respect. However, CNTs suffer from low water dispersibility, which hampered their utilization. Therefore, we aim to functionalize CNTs non-covalently with pyrene moiety using an appropriate hydrophilic linker derivatized from polyethylene glycol (PEG) terminated with hydroxyl or carboxyl group to disperse them in water. The functionalization of the CNTs is successfully confirmed by TEM, absorption spectroscopy, TGA, and zeta potential analysis. 3T3 cells-based engineered connective tissues (ECTs) are generated with different concentrations of the functionalized CNTs (f-CNTs). These tissues show a significant enhancement in electrical conductivity at a concentration of 0.025%, however, the cell viability is reduced by about 10 to 20%. All ECTs containing f-CNTs show a significant reduction in tissue fibrosis and matrix porosity relative to the control tissues. Taken together, the developed constructs show great potential for further in vivo studies as engineered tissue.

Defined Engineered Human Myocardium for Disease Modeling, Drug Screening, and Heart Repair

Different engineered heart muscle formats have been developed for applications in disease modeling, drug screening, and heart repair. The advantage of 3D engineered versus 2D monolayer and 3D aggregate cardiomyocyte cultures is a clearly advanced degree of maturation, which in many aspects resembles the postnatal rather than the embryonic or fetal heart, in the most advanced 3D culture formats. According to the desired in vitro (disease modeling or drug screening) and in vivo (heart repair) application, scale and geometry of tissue engineered heart muscle must be adapted. In this updated methods paper, we report a simple and scalable (up and down) collagen-based protocol for the construction of Engineered Human Myocardium (EHM) under defined, serum-free conditions.

A guide for assessment of myocardial stiffness in health and disease

Myocardial stiffness is an intrinsic property of the myocardium that influences both diastolic and systolic cardiac function. Myocardial stiffness represents the resistance of this tissue to being deformed and depends on intracellular components of the cardiomyocyte, particularly the cytoskeleton, and on extracellular components, such as collagen fibers. Myocardial disease is associated with changes in myocardial stiffness, and its assessment is a key diagnostic marker of acute or chronic pathological myocardial disease with the potential to guide therapeutic decision-making. In this Review, we appraise the different techniques that can be used to estimate myocardial stiffness, evaluate their advantages and disadvantages, and discuss potential clinical applications.

Control of cardiomyocyte differentiation timing by intercellular signaling pathways

Congenital Heart Disease (CHD), malformations of the heart present at birth, is the most common class of life-threatening birth defect (Hoffman (1995) [1], Gelb (2004) [2], Gelb (2014) [3]). A major research challenge is to elucidate the genetic determinants of CHD and mechanistically link CHD ontogeny to a molecular understanding of heart development. Although the embryonic origins of CHD are unclear in most cases, dysregulation of cardiovascular lineage specification, patterning, proliferation, migration or differentiation have been described (Olson (2004) [4], Olson (2006) [5], Srivastava (2006) [6], Dunwoodie (2007) [7], Bruneau (2008) [8]). Cardiac differentiation is the process whereby cells become progressively more dedicated in a trajectory through the cardiac lineage towards mature cardiomyocytes. Defects in cardiac differentiation have been linked to CHD, although how the complex control of cardiac differentiation prevents CHD is just beginning to be understood. The stages of cardiac differentiation are highly stereotyped and have been well-characterized (Kattman et al. (2011) [9], Wamstad et al. (2012) [10], Luna-Zurita et al. (2016) [11], Loh et al. (2016) [12], DeLaughter et al. (2016) [13]); however, the developmental and molecular mechanisms that promote or delay the transition of a cell through these stages have not been as deeply investigated. Tight temporal control of progenitor differentiation is critically important for normal organ size, spatial organization, and cellular physiology and homeostasis of all organ systems (Raff et al. (1985) [14], Amthor et al. (1998) [15], Kopan et al. (2014) [16]). This review will focus on the action of signaling pathways in the control of cardiomyocyte differentiation timing. Numerous signaling pathways, including the Wnt, Fibroblast Growth Factor, Hedgehog, Bone Morphogenetic Protein, Insulin-like Growth Factor, Thyroid Hormone and Hippo pathways, have all been implicated in promoting or inhibiting transitions along the cardiac differentiation trajectory. Gaining a deeper understanding of the mechanisms controlling cardiac differentiation timing promises to yield insights into the etiology of CHD and to inform approaches to restore function to damaged hearts.

Modulating the Biomechanical Properties of Engineered Connective Tissues by Chitosan-Coated Multiwall Carbon Nanotubes

Background

Under certain conditions, the physiological repair of connective tissues might fail to restore the original structure and function. Optimized engineered connective tissues (ECTs) with biophysical properties adapted to the target tissue could be used as a substitution therapy. This study aimed to investigate the effect of ECT enforcement by a complex of multiwall carbon nanotubes with chitosan (C-MWCNT) to meet in vivo demands.

Materials and Methods

ECTs were constructed from human foreskin fibroblasts (HFF-1) in collagen type I and enriched with the three different percentages 0.025, 0.05 and 0.1% of C-MWCNT. Characterization of the physical properties was performed by biomechanical studies using unidirectional strain.

Results

Supplementation with 0.025% C-MWCNT moderately increased the tissue stiffness, reflected by Young’s modulus, compared to tissues without C-MWCNT. Supplementation of ECTs with 0.1% C-MWCNT reduced tissue contraction and increased the elasticity and the extensibility, reflected by the yield point and ultimate strain, respectively. Consequently, the ECTs with 0.1% C-MWCNT showed a higher resilience and toughness as control tissues. Fluorescence tissue imaging demonstrated the longitudinal alignment of all cells independent of the condition.

Conclusion

Supplementation with C-MWCNT can enhance the biophysical properties of ECTs, which could be advantageous for applications in connective tissue repair.

Generation of Engineered Human Myocardium in a Multi-well Format

This protocol describes a robust method for the generation of engineered human myocardium (EHM) from pluripotent stem cells (PSCs) in a multi-well plate under defined, serum-free conditions. By parallel culture of up to 48 EHM in one plate, contractile heart muscle can be obtained to serve numerous applications, including drug screening and disease modelling. This protocol has been successfully applied to human embryonic stem (HES) cell- and induced PSC-derived cardiomyocytes, subtype-specific, i.e., atrial and ventricular, and commercially available cardiomyocyte preparations. For complete details on the use and execution of this protocol, please refer to Tiburcy et al. (2017).

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A robust collagen-based protocol to generate engineered human myocardium (EHM)

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EHM derived from pluripotent stem cell (PSC)-derived cardiomyocytes and fibroblasts

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Enables culture of up to 48 EHM in one plate to enable large-scale screening applications

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Usable with cardiomyocytes and fibroblasts from several sources and of different subtypes

Single-cell analysis of murine fibroblasts identifies neonatal to adult switching that regulates cardiomyocyte maturation

Cardiac maturation lays the foundation for postnatal heart development and disease, yet little is known about the contributions of the microenvironment to cardiomyocyte maturation. By integrating single-cell RNA-sequencing data of mouse hearts at multiple postnatal stages, we construct cellular interactomes and regulatory signaling networks. Here we report switching of fibroblast subtypes from a neonatal to adult state and this drives cardiomyocyte maturation. Molecular and functional maturation of neonatal mouse cardiomyocytes and human embryonic stem cell-derived cardiomyocytes are considerably enhanced upon co-culture with corresponding adult cardiac fibroblasts. Further, single-cell analysis of in vivo and in vitro cardiomyocyte maturation trajectories identify highly conserved signaling pathways, pharmacological targeting of which substantially delays cardiomyocyte maturation in postnatal hearts, and markedly enhances cardiomyocyte proliferation and improves cardiac function in infarcted hearts. Together, we identify cardiac fibroblasts as a key constituent in the microenvironment promoting cardiomyocyte maturation, providing insights into how the manipulation of cardiomyocyte maturity may impact on disease development and regeneration.

Human pluripotent stem cell-derived cardiac stromal cells and their applications in regenerative medicine

Coronary heart disease is one of the leading causes of death in the United States. Recent advances in stem cell biology have led to the development and engineering of human pluripotent stem cell (hPSC)-derived cardiac cells and tissues for application in cellular therapy and cardiotoxicity studies. Initial studies in this area have largely focused on improving differentiation efficiency and maturation states of cardiomyocytes. However, other cell types in the heart, including endothelial and stromal cells, play crucial roles in cardiac development, injury response, and cardiomyocyte function. This review discusses recent advances in differentiation of hPSCs to cardiac stromal cells, identification and classification of cardiac stromal cell types, and application of hPSC-derived cardiac stromal cells and tissues containing these cells in regenerative and drug development applications.

Human pluripotent stem cell-derived cardiomyocytes for studying energy metabolism

Cardiomyocyte energy metabolism is altered in heart failure, and primary defects of metabolic pathways can cause heart failure. Studying cardiac energetics in rodent models has principal shortcomings, raising the question to which extent human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) can provide an alternative. As metabolic maturation of CM occurs mostly after birth during developmental hypertrophy, the immaturity of hiPSC-CM is an important limitation. Here we shortly review the physiological drivers of metabolic maturation and concentrate on methods to mature hiPSC-CM with the goal to benchmark the metabolic state of hiPSC-CM against in vivo data and to see how far known abnormalities in inherited metabolic disorders can be modeled in hiPSC-CM. The current data indicate that hiPSC-CM, despite their immature, approximately mid-fetal state of energy metabolism, faithfully recapitulate some basic metabolic disease mechanisms. Efforts to improve their metabolic maturity are underway and shall improve the validity of this model.

Synchronization of viscoelastically coupled excitable oscillators

Viscoelastically coupled excitable oscillators are used to model individually beating spatially separated cardiomyocytes surrounded by an extracellular matrix (ECM). We investigate how mechanical coupling via the ECM can synchronize two such oscillators with excitation contraction coupling and electromechanical feedback and how this synchronization depends on the rheological properties of the ECM. Extending our study to a linear chain of coupled oscillators we find a transition to synchronization as the ECM becomes stiffer. In the case of purely elastic coupling we observe antiphase chimera states.