Modulation of cardiac myocyte phenotype in vitro by the composition and orientation of the extracellular matrix

Cellular mechanotransduction in health and diseases: from molecular mechanism to therapeutic targets

Cellular mechanotransduction, a critical regulator of numerous biological processes, is the conversion from mechanical signals to biochemical signals regarding cell activities and metabolism. Typical mechanical cues in organisms include hydrostatic pressure, fluid shear stress, tensile force, extracellular matrix stiffness or tissue elasticity, and extracellular fluid viscosity. Mechanotransduction has been expected to trigger multiple biological processes, such as embryonic development, tissue repair and regeneration. However, prolonged excessive mechanical stimulation can result in pathological processes, such as multi-organ fibrosis, tumorigenesis, and cancer immunotherapy resistance. Although the associations between mechanical cues and normal tissue homeostasis or diseases have been identified, the regulatory mechanisms among different mechanical cues are not yet comprehensively illustrated, and no effective therapies are currently available targeting mechanical cue-related signaling. This review systematically summarizes the characteristics and regulatory mechanisms of typical mechanical cues in normal conditions and diseases with the updated evidence. The key effectors responding to mechanical stimulations are listed, such as Piezo channels, integrins, Yes-associated protein (YAP) /transcriptional coactivator with PDZ-binding motif (TAZ), and transient receptor potential vanilloid 4 (TRPV4). We also reviewed the key signaling pathways, therapeutic targets and cutting-edge clinical applications of diseases related to mechanical cues.

Integrins Increase Sarcoplasmic Reticulum Activity for Excitation-Contraction Coupling in Human Stem Cell-Derived Cardiomyocytes

Engagement of the sarcoplasmic reticulum (SR) Ca2+ stores for excitation-contraction (EC)-coupling is a fundamental feature of cardiac muscle cells. Extracellular matrix (ECM) proteins that form the extracellular scaffolding supporting cardiac contractile activity are thought to play an integral role in the modulation of EC-coupling. At baseline, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) show poor utilisation of SR Ca2+ stores, leading to inefficient EC-coupling, like developing or human CMs in cardiac diseases such as heart failure. We hypothesised that integrin ligand-receptor interactions between ECM proteins and CMs recruit the SR to Ca2+ cycling during EC-coupling. hiPSC-CM monolayers were cultured on fibronectin-coated glass before 24 h treatment with fibril-forming peptides containing the integrin-binding tripeptide sequence arginine-glycine-aspartic acid (2 mM). Micropipette application of 40 mM caffeine in standard or Na+/Ca2+-free Tyrode's solutions was used to assess the Ca2+ removal mechanisms. Microelectrode recordings were conducted to analyse action potentials in current-clamp. Confocal images of labelled hiPSC-CMs were analysed to investigate hiPSC-CM morphology and ultrastructural arrangements in Ca2+ release units. This study demonstrates that peptides containing the integrin-binding sequence arginine-glycine-aspartic acid (1) abbreviate hiPSC-CM Ca2+ transient and action potential duration, (2) increase co-localisation between L-type Ca2+ channels and ryanodine receptors involved in EC-coupling, and (3) increase the rate of SR-mediated Ca2+ cycling. We conclude that integrin-binding peptides induce recruitment of the SR for Ca2+ cycling in EC-coupling through functional and structural improvements and demonstrate the importance of the ECM in modulating cardiomyocyte function in physiology.

Cardiac Fibrosis: Key Role of Integrins in Cardiac Homeostasis and Remodeling

Cardiac fibrosis is a common finding that is associated with the progression of heart failure (HF) and impacts all chambers of the heart. Despite intense research, the treatment of HF has primarily focused upon strategies to prevent cardiomyocyte remodeling, and there are no targeted antifibrotic strategies available to reverse cardiac fibrosis. Cardiac fibrosis is defined as an accumulation of extracellular matrix (ECM) proteins which stiffen the myocardium resulting in the deterioration cardiac function. This occurs in response to a wide range of mechanical and biochemical signals. Integrins are transmembrane cell adhesion receptors, that integrate signaling between cardiac fibroblasts and cardiomyocytes with the ECM by the communication of mechanical stress signals. Integrins play an important role in the development of pathological ECM deposition. This review will discuss the role of integrins in mechano-transduced cardiac fibrosis in response to disease throughout the myocardium. This review will also demonstrate the important role of integrins as both initiators of the fibrotic response, and modulators of fibrosis through their effect on cardiac fibroblast physiology across the various heart chambers.

3D Co-culture of hiPSC-Derived Cardiomyocytes With Cardiac Fibroblasts Improves Tissue-Like Features of Cardiac Spheroids

Purpose: Both cardiomyocytes and cardiac fibroblasts (CF) play essential roles in cardiac development, function, and remodeling. Properties of 3D co-cultures are incompletely understood. Hence, 3D co-culture of cardiomyocytes and CF was characterized, and selected features compared with single-type and 2D culture conditions. Methods: Human cardiomyocytes derived from induced-pluripotent stem cells (hiPSC-CMs) were obtained from Cellular Dynamics or Ncardia, and primary human cardiac fibroblasts from ScienCell. Cardiac spheroids were investigated using cryosections and whole-mount confocal microscopy, video motion analysis, scanning-, and transmission-electron microscopy (SEM, TEM), action potential recording, and quantitative PCR (qPCR). Results: Spheroids formed in hanging drops or in non-adhesive wells showed spontaneous contractions for at least 1 month with frequent media changes. SEM of mechanically opened spheroids revealed a dense inner structure and no signs of blebbing. TEM of co-culture spheroids at 1 month showed myofibrils, intercalated disc-like structures and mitochondria. Ultrastructural features were comparable to fetal human myocardium. We then assessed immunostained 2D cultures, cryosections of spheroids, and whole-mount preparations by confocal microscopy. CF in co-culture spheroids assumed a small size and shape similar to the situation in ventricular tissue. Spheroids made only of CF and cultured for 3 weeks showed no stress fibers and strongly reduced amounts of alpha smooth muscle actin compared to early spheroids and 2D cultures as shown by confocal microscopy, western blotting, and qPCR. The addition of CF to cardiac spheroids did not lead to arrhythmogenic effects as measured by sharp-electrode electrophysiology. Video motion analysis showed a faster spontaneous contraction rate in co-culture spheroids compared to pure hiPSC-CMs, but similar contraction amplitudes and kinetics. Spontaneous contraction rates were not dependent on spheroid size. Applying increasing pacing frequencies resulted in decreasing contraction amplitudes without positive staircase effect. Gene expression analysis of selected cytoskeleton and myofibrillar proteins showed more tissue-like expression patterns in co-culture spheroids than with cardiomyocytes alone or in 2D culture. Conclusion: We demonstrate that the use of 3D co-culture of hiPSC-CMs and CF is superior over 2D culture conditions for co-culture models and more closely mimicking the native state of the myocardium with relevance to drug development as well as for personalized medicine.

Engineering hiPSC cardiomyocyte in vitro model systems for functional and structural assessment

The study of human cardiomyopathies and the development and testing of new therapies has long been limited by the availability of appropriate in vitro model systems. Cardiomyocytes are highly specialized cells whose internal structure and contractile function are sensitive to the local microenvironment and the combination of mechanical and biochemical cues they receive. The complementary technologies of human induced pluripotent stem cell (hiPSC) derived cardiomyocytes (CMs) and microphysiological systems (MPS) allow for precise control of the genetics and microenvironment of human cells in in vitro contexts. These combined systems also enable quantitative measurement of mechanical function and intracellular organization. This review describes relevant factors in the myocardium microenvironment that affect CM structure and mechanical function and demonstrates the application of several engineered microphysiological systems for studying development, disease, and drug discovery.

Conductive Silk-Polypyrrole Composite Scaffolds with Bioinspired Nanotopographic Cues for Cardiac Tissue Engineering

We report on the development of bioinspired cardiac scaffolds made from electroconductive acid-modified silk fibroin-poly(pyrrole) (AMSF+PPy) substrates patterned with nanoscale ridges and grooves reminiscent of native myocardial extracellular matrix (ECM) topography to enhance the structural and functional properties of cultured human pluripotent stem cells (hPSC)-derived cardiomyocytes. Nanopattern fidelity was maintained throughout the fabrication and functionalization processes, and no loss in conductive behavior occurred due to the presence of the nanotopographical features. AMSF+PPy substrates were biocompatible and stable, maintaining high cell viability over a 21-day culture period while displaying no signs of PPy delamination. The presence of anisotropic topographical cues led to increased cellular organization and sarcomere development, and electroconductive cues promoted a significant improvement in the expression and polarization of connexin 43 (Cx43), a critical regulator of cell-cell electrical coupling. The combination of biomimetic topography and electroconductivity also increased the expression of genes that encode key proteins involved in regulating the contractile and electrophysiological function of mature human cardiac tissue.

Osteoglycin post-transcriptional regulation by miR-155 induces cellular architecture changes in H9c2 cardiomyoblasts

Several studies have demonstrated dysregulated cardiac microRNAs (miRNAs) following cardiac stress and development of cardiac hypertrophy and failure. miRNAs are also differentially expressed in the inflammation that occurs in heart failure and, among these inflammatory-related miRNAs, the miR-155 has been implicated in the regulation of cardiac hypertrophy. Despite these data showing the role of miRNA-155 in cardiomyocyte hypertrophy under a hypertrophic stimulus, it is also important to understand the endogenous regulation of this miRNA without a hypertrophic stimulus to fully appreciate its function in this cell type. The first aim of the present study was to determine whether, without a hypertrophic stimulus, miR-155 overexpression induces H9c2 cardiac cells hypertrophy in vitro. The second objective was to determine whether osteoglycin (Ogn), a key regulator of heart mass in rats, mice, and humans, is post-transcriptionally regulated by miR-155 with a potential role in inducing H9c2 cells hypertrophy. Here, we show that, without a hypertrophic stimulus, miR-155 significantly repressed Ogn protein levels, but induce neither alteration in morphological phenotype nor in the expression of the molecular markers that fully characterize pathological hypertrophy of H9c2 cells. However, most importantly, Ogn silencing in H9c2 cells mimicked the effects of miR-155 overexpression in inducing cellular architecture changes that were characterized by a transition of the cell shape from fusiform to rounded. This is a new role of the post-transcriptional regulation of Ogn by miR-155 in the maintenance of the cardiac cell morphology in physiological and pathological conditions.

Surface-modified polymers for cardiac tissue engineering

Cardiovascular disease (CVD), leading to myocardial infarction and heart failure, is one of the major causes of death worldwide. The physiological system cannot significantly regenerate the capabilities of a damaged heart. The current treatment involves pharmacological and surgical interventions; however, less invasive and more cost-effective approaches are sought. Such new approaches are developed to induce tissue regeneration following injury. Hence, regenerative medicine plays a key role in treating CVD. Recently, the extrinsic stimulation of cardiac regeneration has involved the use of potential polymers to stimulate stem cells toward the differentiation of cardiomyocytes as a new therapeutic intervention in cardiac tissue engineering (CTE). The therapeutic potentiality of natural or synthetic polymers and cell surface interactive factors/polymer surface modifications for cardiac repair has been demonstrated in vitro and in vivo. This review will discuss the recent advances in CTE using polymers and cell surface interactive factors that interact strongly with stem cells to trigger the molecular aspects of the differentiation or formulation of cardiomyocytes for the functional repair of heart injuries or cardiac defects.

Protein/polysaccharide-based scaffolds mimicking native extracellular matrix for cardiac tissue engineering applications

Tissue engineering has emerged as a viable approach to treat disease or repair damage in tissues and organs. One of the key elements for the success of tissue engineering is the use of a scaffold serving as artificial extracellular matrix (ECM). The ECM hosts the cells and improves their survival, proliferation, and differentiation, enabling the formation of new tissue. Here, we propose the development of a class of protein/polysaccharide-based porous scaffolds for use as ECM substitutes in cardiac tissue engineering. Scaffolds based on blends of a protein component, collagen or gelatin, with a polysaccharide component, alginate, were produced by freeze-drying and subsequent ionic and chemical crosslinking. Their morphological, physicochemical, and mechanical properties were determined and compared with those of natural porcine myocardium. We demonstrated that our scaffolds possessed highly porous and interconnected structures, and the chemical homogeneity of the natural ECM was well reproduced in both types of scaffolds. Furthermore, the alginate/gelatin (AG) scaffolds better mimicked the native tissue in terms of interactions between components and protein secondary structure, and in terms of swelling behavior. The AG scaffolds also showed superior mechanical properties for the desired application and supported better adhesion, growth, and differentiation of myoblasts under static conditions. The AG scaffolds were subsequently used for culturing neonatal rat cardiomyocytes, where high viability of the resulting cardiac constructs was observed under dynamic flow culture in a microfluidic bioreactor. We therefore propose our protein/polysaccharide scaffolds as a viable ECM substitute for applications in cardiac tissue engineering. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 106A: 769-781, 2018.

Tissue Engineering Approaches in the Design of Healthy and Pathological In Vitro Tissue Models

In the tissue engineering (TE) paradigm, engineering and life sciences tools are combined to develop bioartificial substitutes for organs and tissues, which can in turn be applied in regenerative medicine, pharmaceutical, diagnostic, and basic research to elucidate fundamental aspects of cell functions in vivo or to identify mechanisms involved in aging processes and disease onset and progression. The complex three-dimensional (3D) microenvironment in which cells are organized in vivo allows the interaction between different cell types and between cells and the extracellular matrix, the composition of which varies as a function of the tissue, the degree of maturation, and health conditions. In this context, 3D in vitro models can more realistically reproduce a tissue or organ than two-dimensional (2D) models. Moreover, they can overcome the limitations of animal models and reduce the need for in vivo tests, according to the "3Rs" guiding principles for a more ethical research. The design of 3D engineered tissue models is currently in its development stage, showing high potential in overcoming the limitations of already available models. However, many issues are still opened, concerning the identification of the optimal scaffold-forming materials, cell source and biofabrication technology, and the best cell culture conditions (biochemical and physical cues) to finely replicate the native tissue and the surrounding environment. In the near future, 3D tissue-engineered models are expected to become useful tools in the preliminary testing and screening of drugs and therapies and in the investigation of the molecular mechanisms underpinning disease onset and progression. In this review, the application of TE principles to the design of in vitro 3D models will be surveyed, with a focus on the strengths and weaknesses of this emerging approach. In addition, a brief overview on the development of in vitro models of healthy and pathological bone, heart, pancreas, and liver will be presented.

Functional Tissue Engineering: A Prevascularized Cardiac Muscle Construct for Validating Human Mesenchymal Stem Cells Engraftment Potential In Vitro

The influence of somatic stem cells in the stimulation of mammalian cardiac muscle regeneration is still in its early stages, and so far, it has been difficult to determine the efficacy of the procedures that have been employed. The outstanding question remains whether stem cells derived from the bone marrow or some other location within or outside of the heart can populate a region of myocardial damage and transform into tissue-specific differentiated progenies, and also exhibit functional synchronization. Consequently, this necessitates the development of an appropriate in vitro three-dimensional (3D) model of cardiomyogenesis and prompts the development of a 3D cardiac muscle construct for tissue engineering purposes, especially using the somatic stem cell, human mesenchymal stem cells (hMSCs). To this end, we have created an in vitro 3D functional prevascularized cardiac muscle construct using embryonic cardiac myocytes (eCMs) and hMSCs. First, to generate the prevascularized scaffold, human cardiac microvascular endothelial cells (hCMVECs) and hMSCs were cocultured onto a 3D collagen cell carrier (CCC) for 7 days under vasculogenic culture conditions; hCMVECs/hMSCs underwent maturation, differentiation, and morphogenesis characteristic of microvessels, and formed dense vascular networks. Next, the eCMs and hMSCs were cocultured onto this generated prevascularized CCCs for further 7 or 14 days in myogenic culture conditions. Finally, the vascular and cardiac phenotypic inductions were characterized at the morphological, immunological, biochemical, molecular, and functional levels. Expression and functional analyses of the differentiated progenies revealed neo-cardiomyogenesis and neo-vasculogenesis. In this milieu, for instance, not only were hMSCs able to couple electromechanically with developing eCMs but were also able to contribute to the developing vasculature as mural cells, respectively. Hence, our unique 3D coculture system provides us a reproducible and quintessential in vitro 3D model of cardiomyogenesis and a functioning prevascularized 3D cardiac graft that can be utilized for personalized medicine.

Mechanical Regulation of Cardiac Aging in Model Systems

Unlike diet and exercise, which individuals can modulate according to their lifestyle, aging is unavoidable. With normal or healthy aging, the heart undergoes extensive vascular, cellular, and interstitial molecular changes that result in stiffer less compliant hearts that experience a general decline in organ function. Although these molecular changes deemed cardiac remodeling were once thought to be concomitant with advanced cardiovascular disease, they can be found in patients without manifestation of clinical disease. It is now mostly acknowledged that these age-related mechanical changes confer vulnerability of the heart to cardiovascular stresses associated with disease, such as hypertension and atherosclerosis. However, recent studies have aimed at differentiating the initial compensatory changes that occur within the heart with age to maintain contractile function from the maladaptive responses associated with disease. This work has identified new targets to improve cardiac function during aging. Spanning invertebrate to vertebrate models, we use this review to delineate some hallmarks of physiological versus pathological remodeling that occur in the cardiomyocyte and its microenvironment, focusing especially on the mechanical changes that occur within the sarcomere, intercalated disc, costamere, and extracellular matrix.

In vitro cardiac tissue models: Current status and future prospects

Cardiovascular disease is the leading cause of death worldwide. Achieving the next phase of potential treatment strategies and better prognostic tools will require a concerted effort from interdisciplinary fields. Biomaterials-based cardiac tissue models are revolutionizing the area of preclinical research and translational applications. The goal of in vitro cardiac tissue modeling is to create physiological functional models of the human myocardium, which is a difficult task due to the complex structure and function of the human heart. This review describes the advances made in area of in vitro cardiac models using biomaterials and bioinspired platforms. The field has progressed extensively in the past decade, and we envision its applications in the areas of drug screening, disease modeling, and precision medicine.

Biomimetic microstructure morphology in electrospun fiber mats is critical for maintaining healthy cardiomyocyte phenotype

Despite recent advances in biomimetic substrates, there is still only limited understanding of how the extracellular matrix (ECM) functions in the maintenance of cardiomyocyte (CM) phenotype. In this study, we designed electrospun substrates inspired by morphologic features of non-failing and failing human heart ECM, and examined how these substrates regulate phenotypes of adult and neonatal rat ventricular CMs (ARVM and NRVM, respectively). We found that poly(ε-caprolactone) fiber substrates designed to mimic the organized ECM of a non-failing human heart maintained healthy CM phenotype (evidenced by cell morphology, organized actin/myomesin bands and expression of β-MYH7 and SCN5A.1 and SCN5A.2) compared to both failing heart ECM-mimetic substrates and tissue culture plates. Moreover, culture of ARVMs and NRVMs on aligned substrates showed differences in m- and z-line alignment; with ARVMs aligning parallel to the ECM fibers and the NRVMs aligning perpendicular to the fibers. The results provide new insight into cardiac tissue engineering by illustrating the importance models that mimic the cardiac ECM microenvironment in vitro.

Expression of metalloproteases by cardiac myocytes and fibroblasts in vitro

Regulation of the turnover of extracellular matrix (ECM) components has been attributed in part to matrix metalloproteases (MMP). Isolated cardiac myocytes and fibroblasts from different developmental stages express different patterns of MMPs in vitro. Zymography of media and cell extracts of fibroblasts and myocytes indicated several apparent molecular weights (Mr) with gelatinolytic activity with prominent bands at 92 and 72 kDa. No caseinolytic activity was detected. These MMPs were characteristic of known MMP-2 and MMP-9. Fibroblasts predominantly expressed the latent 72-kDa MMP, whereas myocytes expressed a latent 92-kDa MMP. Expression of these MMPs was not affected by density of culture or the type of ECM substrate on which the cells were grown. Sodium dodecyl sulfate (SDS)-activated MMP-2 showed specific cleavage patterns on collagen types I and III but not on fibronectin, collagen type IV, or laminin. The reaction of SDS-activated MMP-2 produced a 140-kDa fragment from collagen types I and III. No specific substrate patterns were observed with activated MMP-9. MMP-2 from fibroblasts could also be activated by mechanical tension developed by fibroblasts within collagen gels or by cyclically stretching Silastic membranes on which the fibroblasts were grown. When mechanical tension was inhibited in collagen gels by antibodies against the β1 integrin, the 72-kDa MMP, or cytochalasin D, the activated band at 62 kDa was not detected. Immunocytochemical localization with antibodies against MMP-2 showed a weak reaction on cardiac myocytes, but intense staining around the focal adhesions of migrating fibroblasts. In collagen gels, staining was localized to the leading pseudopodia of the fibroblasts. Together, these data indicate that the rat MMP-2 is a collagenase primarily associated with cardiac fibroblasts, activated by mechanical tension, and may be important in cellular ECM interactions.

Cardiac fibroblasts form and function

The formation and structure of the extracellular matrix (ECM) that makes up the cardiac interstitum is well known yet the underlying mechanisms that regulate the interstitum are poorly known. This review focuses on the role of the cardiac fibroblast in the formation and regulation of the ECM components during cardiac development and in response to physiological and pathological stimulation. The role of ECM receptors (integrins), cellular phenotype, and chemical and mechanical signaling by cardiac fibroblasts are discussed.

Interactive relationship between basement-membrane development and sarcomerogenesis in single cardiomyocytes

The cardiac basement membrane (BM), the highly organized layer of the extracellular matrix (ECM) on the external side of the sarcolemma, is mainly composed of laminin and collagen IV, which assemble a dense, well-organized network to surround the surface of each adult cardiomyocyte. The development of the cardiac BM plays a key role in organogenesis of the myocardium through interactions between sarcomeres and integrins. Because of the complicated structure of cardiac muscle fibers and lack of a proper investigation method, the detailed interactions among BM development, sarcomeric growth, and integrin expression remain unclear. In this study, freshly isolated 3-day neonatal cardiomyocytes (CMs) were cultured on aligned collagen, which mimics the in vivo ECM structure and induces neonatal CMs to grow into rod-like shapes. Then double fluorescence-immunostained laminin and α-actinin or integrin β1 on neonatal CMs cultured 4-72 h were imaged using a confocal microscope, and the spatial relationship between laminin deposition and α-actinin expression was evaluated by colocalization analysis. At 4h, laminin was deposited around Z-bodies (dot-shaped α-actinin) and integrins; from 18-to-72 h, its gradual colocalization with Z-lines (line-shaped α-actinin) and integrins increased Pearson׳s coefficient; this indicates that development of the BM network from the neonatal stage to adulthood is closely related to sarcomeric formation via integrin-mediated interactions.

Method to form a fiber/growth factor dual-gradient along electrospun silk for nerve regeneration

Concentration gradients of guidance molecules influence cell behavior and growth in biological tissues and are therefore of interest for the design of biomedical scaffolds for regenerative medicine. We developed an electrospining method to generate a dual-gradient of bioactive molecules and fiber density along electrospun nanofibers without any post spinning treatment. Functionalization with fluorescent molecules demonstrated the efficiency of the method to generate a discontinuous concentration gradient along the aligned fibers. As a proof of concept for tissue engineering, the silk nanofibers were functionalized with increasing concentrations of nerve growth factor (NGF) and the biological activity was assessed and quantified with rat dorsal root ganglion (DRG) neurons cultures. Protein assays showed the absence of passive release of NGF from the functionalized fibers. The results demonstrated that the NGF concentration gradient led to an oriented and increased growth of DRG neurons (417.6 ± 55.7 μm) compared to a single uniform NGF concentration (264.5 ± 37.6 μm). The easy-to-use electrospinning technique combined with the multiple molecules that can be used for fiber functionalization makes this technique versatile for a broad range of applications from biosensors to regenerative medicine.

Laser cell-micropatterned pair of cardiomyocytes: the relationship between basement membrane development and gap junction maturation

The basement membrane (BM), a network of laminin and collagen IV, mechanically supports individual cells and directly mediates cell-cell and cell-extracellular matrix (ECM) interactions. For example, the BM network that tightly encloses each cardiomyocyte (CM) mediates the alignment of CMs with collagen I in the ECM. Additionally, the BM-laminin is involved in the formation of gap junctions (GJs), which regulate electrical coupling between two CMs in the myocardium. The role of BM in GJ maturation remains unclear because of the complicated in vivo structures and lack of an ideal in vitro culturing mode. In this study, our laser cell-micropatterning system was used to place two neonatal CMs (NCMs) in contact on an aligned collagen gel (ACG) to study the relationship between GJ maturation and BM development. The results of double immunofluorescence staining and confocal imaging showed that BM-laminin was deposited earlier than the formation of GJs in the intercellular space and that newly expressed connexin 43 clusters were preferentially assembled near the deposited BM structures. Eventually the BM network surrounded the GJs.

Enzyme-etching technique to fabricate micropatterns of aligned collagen fibrils

A technique to tailor-make pre-coated, pre-aligned bovine collagen fibrils, derived from neonatal cardiomyocytes, on the surface of a glass slide into a designated pattern is reported. The unwanted collagen-coated area was erased by a collagenase solution and the tailored area was retained by attaching a microfabricated polydimethylsiloxane stamp directly to the collagen-coated surface. Using this technique, collagen patterns with designated orientations and with clear pattern boundaries and defined shapes were fabricated.

Regenerative therapy and tissue engineering for the treatment of end-stage cardiac failure: new developments and challenges

Regeneration of myocardium through regenerative therapy and tissue engineering is appearing as a prospective treatment modality for patients with end-stage heart failure. Focusing on this area, this review highlights the new developments and challenges in the regeneration of myocardial tissue. The role of various cell sources, calcium ion and cytokine on the functional performance of regenerative therapy is discussed. The evolution of tissue engineering and the role of tissue matrix/scaffold, cell adhesion and vascularisation on tissue engineering of cardiac tissue implant are also discussed.

Role of the basement membrane in regulation of cardiac electrical properties

In the heart muscle, each adult cardiomyocyte is enclosed by a basement membrane (BM). This innermost extracellular matrix is a layered assembly of laminin, collagen IV, glycoproteins, and proteoglycans. In this study, the role of the BM network in regulation of the electrical properties of neonatal cardiomyocytes (NCMs) cultured on an aligned collagen I gel was investigated using a multielectrode array (MEA). A laminin antibody was added to the culture medium for 48-120 h to conjugate newly secreted laminin. Then, morphology of the NCMs on an MEA was monitored using a phase contrast microscope, and the BM network that was immunocytostained for laminin was imaged using a fluorescence microscope. When the BM laminin was absent in this culture model, dramatic changes in NCM morphology were observed. Simultaneously, the MEA-recorded cardiac field potential showed changes compared to that from the control groups: The period of contraction shortened to 1/2 of that from the control groups, and the waveform of the calcium influx shifted from a flat plateau to a peak-like waveform, indicating that the electrical properties of the NCMs were closely related to the components and distribution of the BM network.

Dermal templates and the wound-healing paradigm: the promise of tissue regeneration

Dermal regeneration templates arguably represent the first and most clinically successful 'tissue engineering' solution designed for organ reconstruction. Wound healing in the skin normally occurs on a continuum. At one extreme of the continuum lies the promise of tissue regeneration and the complete restoration of normal structure and function. Unfortunately, in the adult, all too often, wound healing occurs at the other extreme of the continuum and the dermis is reconstituted as scar tissue. Dermal regeneration templates are designed to manage the wound-healing process and tip the scales toward regeneration. This review discusses the architecture and molecular composition of the skin and the events that mediate wound healing and scar formation. The development, evolution and commercialization of dermal templates are examined and the clinical and business considerations that drive the product-development cycle are discussed. In the near term, dermal templates cannot be expected to dramatically change in overall composition. Product development will be dominated by continued refinements of existing templates and the field of use will continue to expand as manufacturers seek to increase revenue and capture market share. Continued exploration of novel processing strategies, such as electrospinning, that can be used to fabricate nanoscale biomaterials, may provide a gateway to the next generation of dermal templates.

Natural ECM as biomaterial for scaffold based cardiac regeneration using adult bone marrow derived stem cells

Cellular therapy using stem cells for cardiac diseases has recently gained much interest in the scientific community due to its potential in regenerating damaged and even dead tissue and thereby restoring the organ function. Stem cells from various sources and origin are being currently used for regeneration studies directly or along with differentiation inducing agents. Long term survival and minimal side effects can be attained by using autologous cells and reduced use of inducing agents. Cardiomyogenic differentiation of adult derived stem cells has been previously reported using various inducing agents but the use of a potentially harmful DNA demethylating agent 5-azacytidine (5-azaC) has been found to be critical in almost all studies. Alternate inducing factors and conditions/stimulant like physical condition including electrical stimulation, chemical inducers and biological agents have been attempted by numerous groups to induce cardiac differentiation. Biomaterials were initially used as artificial scaffold in in vitro studies and later as a delivery vehicle. Natural ECM is the ideal biological scaffold since it contains all the components of the tissue from which it was derived except for the living cells. Constructive remodeling can be performed using such natural ECM scaffolds and stem cells since, the cells can be delivered to the site of infraction and once delivered the cells adhere and are not "lost". Due to the niche like conditions of ECM, stem cells tend to differentiate into tissue specific cells and attain several characteristics similar to that of functional cells even in absence of any directed differentiation using external inducers. The development of niche mimicking biomaterials and hybrid biomaterial can further advance directed differentiation without specific induction. The mechanical and electrical integration of these materials to the functional tissue is a problem to be addressed. The search for the perfect extracellular matrix for therapeutic applications including engineering cardiac tissue structures for post ischemic cardiac tissue regeneration continues.

Regeneration in heart disease-Is ECM the key?

The heart possesses a regeneration potential derived from endogenous and exogenous stem and progenitor cell populations, though baseline regeneration appears to be sub-therapeutic. This limitation was initially attributed to a lack of cells with cardiomyogenic potential following an insult to the myocardium. Rather, recent studies demonstrate increased numbers of cardiomyocyte progenitor cells in diseased hearts. Given that the limiting factor does not appear to be cell quantity but rather repletion of functional cardiomyocytes, it is crucial to understand potential mechanisms inhibiting progenitor cell differentiation. One of the extensively studied areas in heart disease is extracellular matrix (ECM) remodeling, with both the composition and mechanical properties of the ECM undergoing changes in diseased hearts. This review explores the influence of ECM properties on cardiomyogenesis and adult cardiac progenitor cells.

Desmoplakin is important for proper cardiac cell-cell interactions

Normal cardiac function is maintained through dynamic interactions of cardiac cells with each other and with the extracellular matrix. These interactions are important for remodeling during cardiac growth and pathophysiological conditions. However, the precise mechanisms of these interactions remain unclear. In this study we examined the importance of desmoplakin (DSP) in cardiac cell-cell interactions. Cell-cell communication in the heart requires the formation and preservation of cell contacts by cell adhesion junctions called desmosome-like structures. A major protein component of this complex is DSP, which plays a role in linking the cytoskeletal network to the plasma membrane. Our laboratory previously generated a polyclonal antibody (1611) against the detergent soluble fraction of cardiac fibroblast plasma membrane. In attempting to define which proteins 1611 recognizes, we performed two-dimensional electrophoresis and identified DSP as one of the major proteins recognized by 1611. Immunoprecipitation studies demonstrated that 1611 was able to directly pulldown DSP. We also demonstrate that 1611 and anti-DSP antibodies co-localize in whole heart sections. Finally, using a three-dimensional in vitro cell-cell interaction assay, we demonstrate that 1611 can inhibit cell-cell interactions. These data indicate that DSP is an important protein for cell-cell interactions and affects a variety of cellular functions, including cytokine secretion.

Strategies for tissue engineering cardiac constructs to affect functional repair following myocardial infarction

Tissue-engineered cardiac constructs are a high potential therapy for treating myocardial infarction. These therapies have the ability to regenerate or recreate functional myocardium following the infarction, restoring some of the lost function of the heart and thereby preventing congestive heart failure. Three key factors to consider when developing engineered myocardial tissue include the cell source, the choice of scaffold, and the use of biomimetic culture conditions. This review details the various biomaterials and scaffold types that have been used to generate engineered myocardial tissues as well as a number of different methods used for the fabrication and culture of these constructs. Specific bioreactor design considerations for creating myocardial tissue equivalents in vitro, such as oxygen and nutrient delivery as well as physical stimulation, are also discussed. Lastly, a brief overview of some of the in vivo studies that have been conducted to date and their assessment of the functional benefit in repairing the injured heart with engineered myocardial tissue is provided.

Mechanotransduction: the role of mechanical stress, myocyte shape, and cytoskeletal architecture on cardiac function

Mechanotransduction refers to the conversion of mechanical forces into biochemical or electrical signals that initiate structural and functional remodeling in cells and tissues. The heart is a kinetic organ whose form changes considerably during development and disease, requiring cardiac myocytes to be mechanically durable and capable of fusing a variety of environmental signals on different time scales. During physiological growth, myocytes adaptively remodel to mechanical loads. Pathological stimuli can induce maladaptive remodeling. In both of these conditions, the cytoskeleton plays a pivotal role in both sensing mechanical stress and mediating structural remodeling and functional responses within the myocyte.

Collagen Matrix Alignment Using Inkjet Printer Technology

Collagen fiber orientation plays an important role in many cell properties and actions in vivo. Collagen and other matrix proteins are aligned in many tissues during normal functioning. For example, cardiomyocytes align in the heart to produce a synchronously beating tissue. The extra-cellular matrix environment, including collagen, is aligned along the cells. This matrix helps with cell adhesion and the alignment of the fibers also contributes to the anisotropic mechanical property of the tissue. While it is easy to replicate randomly oriented collagen in vitro, it is much more difficult to create aligned collagen matrices for cell culture. In this work, a novel inkjet printer-based collagen alignment technique was established. A 1 mg/ml rat tail collagen type I solution was printed, using a modified HP DeskJet 500 printer, onto plasma cleaned and UV sterilized glass slides. The collagen was printed in an eight line pattern, designed in Microsoft Word with 87.5 μm by 23.1 mm lines. The pattern was printed three successive times on each slide to complete the alignment. Immunofluorescence imaging of primary antibodies specific to collagen type I indicated that the heat involved in the printing process was not great enough to denature the collagen. The extent of collagen alignment was quantified using atomic force microscopy and compared to random collagen films and collagen films aligned using a mechanical scraping method. Additionally, neonatal rat cardiomyocytes were cultured on the aligned matrices. These cells require extracellular matrix alignment to maintain their in vivo-like phenotype during in vitro culture. The cells grew along the lines of collagen and coordinated beating, indicating the success of the aligned matrix. This collagen alignment technique is cheap, fast, precise, and easy to use in comparison to other current techniques. It can be used to align collagen on any type of substrate, such as a gel, which makes it a useful tool in many applications. This technique may also be used to align other extra-cellular matrix proteins and could even be used to create a three dimensional construct with varying fiber orientations.

The mechanical coupling of adult marrow stromal stem cells during cardiac regeneration assessed in a 2-D co-culture model

Postnatal cardiomyocytes undergo terminal differentiation and a restricted number of human cardiomyocytes retain the ability to divide and regenerate in response to ischemic injury. However, whether these neo-cardiomyocytes are derived from endogenous population of resident cardiac stem cells or from the exogenous double assurance population of resident bone marrow-derived stem cells that populate the damaged myocardium is unresolved and under intense investigation. The vital challenge is to ameliorate and/or regenerate the damaged myocardium. This can be achieved by stimulating proliferation of native quiescent cardiomyocytes and/or cardiac stem cell, or by recruiting exogenous autologous or allogeneic cells such as fetal or embryonic cardiomyocyte progenitors or bone marrow-derived stromal stem cells. The prerequisites are that these neo-cardiomyocytes must have the ability to integrate well within the native myocardium and must exhibit functional synchronization. Adult bone marrow stromal cells (BMSCs) have been shown to differentiate into cardiomyocyte-like cells both in vitro and in vivo. As a result, BMSCs may potentially play an essential role in cardiac repair and regeneration, but this concept requires further validation. In this report, we have provided compelling evidence that functioning cardiac tissue can be generated by the interaction of multipotent BMSCs with embryonic cardiac myocytes (ECMs) in two-dimensional (2-D) co-cultures. The differentiating BMSCs were induced to undergo cardiomyogenic differentiation pathway and were able to express unequivocal electromechanical coupling and functional synchronization with ECMs. Our 2-D co-culture system provides a useful in vitro model to elucidate various molecular mechanisms underpinning the integration and orderly maturation and differentiation of BMSCs into neo-cardiomyocytes during myocardial repair and regeneration.

ZO-1 determines adherens and gap junction localization at intercalated disks

The disruption of the spatial order of electromechanical junctions at myocyte-intercalated disks (ICDs) is a poorly understood characteristic of many cardiac disease states. Here, in vitro and in vivo evidence is provided that zonula occludens-1 (ZO-1) regulates the organization of gap junctions (GJs) and adherens junctions (AJs) at ICDs. We investigated the contribution of ZO-1 to cell-cell junction localization by expressing a dominant-negative ZO-1 construct (DN-ZO-1) in rat ventricular myocytes (VMs). The expression of DN-ZO-1 in cultured neonatal VMs for 72 h reduced the interaction of ZO-1 and N-cadherin, as assayed by colocalization and coimmunoprecipitation, prompting cytoplasmic internalization of AJ and GJ proteins. DN-ZO-1 expression in adult VMs in vivo also reduced N-cadherin colocalization with ZO-1, a phenomenon not observed when the connexin-43 (Cx43)-ZO-1 interaction was disrupted using a mimetic of the ZO-1-binding ligand from Cx43. DN-ZO-1-infected VMs demonstrated large GJs at the ICD periphery and showed a loss of focal ZO-1 concentrations along plaque edges facing the disk interior. Additionally, there was breakdown of the characteristic ICD pattern of small interior and large peripheral GJs. Continuous DN-ZO-1 expression in VMs over postnatal development reduced ICD-associated Cx43 GJs and increased lateralized and cytoplasmic Cx43. We conclude that ZO-1 regulation of GJ localization is via an association with the N-cadherin multiprotein complex and that this is a key determinant of stable localization of both AJs and GJs at the ICD.

The dynamics of fibroblast-myocyte-capillary interactions in the heart

In the heart, electrical, mechanical, and chemical signals create an environment essential for normal cellular responses to developmental and pathologic cues. Communication between fibroblasts, myocytes, and endothelial cells, as well as the extracellular matrix, are critical to fluctuations in heart composition and function during normal development and pathology. Recent evidence suggests that cytokines play a role in cell-cell signaling in the heart. Indeed, we find that interactions between myocytes and cardiac fibroblasts results in increased interleukin-6 and tumor necrosis factor-alpha secretion. We also used confocal and transmission electron microscopy to observe close relationships and possible direct communication between these cells in vivo. Our results highlight the importance of direct cell-cell communication in the heart, and indicate that interactions between fibroblasts, myocytes, and capillary endothelium results in differential cytokine expression. Studying these cell-cell interactions has many implications for the process of cardiac remodeling and overall heart function during development and cardiopathology.

Nanotechnology in the design of soft tissue scaffolds: innovations in structure and function

Engineered scaffolds function to supplement or replace injured, missing, or compromised tissue or organs. The current direction in this research area is to create scaffolds that mimic the structure and function of the native extracellular matrix (ECM). It is believed that the fabrication of a scaffold that has both structural integrity and allows for normal cellular function and interaction will bring scaffolds closer to clinical relevance. Nanotechnology innovations have aided in the development of techniques for the production of nanofiber scaffolds. The three major processing techniques, self-assembly, phase separation, and electrospinning, produce fibers that rival the size of those found in the native ECM. However, the simplicity, versatility, and scalability of electrospinning make it an attractive processing method that can be used to reproduce aspects of the complexity that characterizes the native ECM. Novel electrospinning strategies include alterations of scaffold composition and architecture, along with the addition and encapsulation of cells, pharmaceuticals and growth factors within the scaffold. This article reviews the major nanofiber fabrication technologies as well as delves into recent significant contributions to the conception of a meaningful and practical electrospun scaffold.

Mechanical stress-induced sarcomere assembly for cardiac muscle growth in length and width

A ventricular myocyte experiences changes in length and load during every beat of the heart and has the ability to remodel cell shape to maintain cardiac performance. Specifically, myocytes elongate in response to increased diastolic strain by adding sarcomeres in series, and they thicken in response to continued systolic stress by adding filaments in parallel. Myocytes do this while still keeping the resting sarcomere length close to its optimal value at the peak of the length-tension curve. This review focuses on the little understood mechanisms by which direction of growth is matched in a physiologically appropriate direction. We propose that the direction of strain is detected by differential phosphorylation of proteins in the costamere, which then transmit signaling to the Z-disc for parallel or series addition of thin filaments regulated via the actin capping processes. In this review, we link mechanotransduction to the molecular mechanisms for regulation of myocyte length and width.

A 3-D cardiac muscle construct for exploring adult marrow stem cell based myocardial regeneration

Adult bone marrow stromal cells (BMSCs) are capable of differentiating into cardiomyocyte-like cells in vitro and contribute to myocardial regeneration in vivo. Consequently, BMSCs may potentially play a vital role in cardiac repair and regeneration. However, this concept has been limited by inadequate and inconsistent differentiation of BMSCs into cardiomyocytes along with poor survival and integration of neo-cardiomyocytes after implantation into ischemic myocardium. In order to overcome these barriers and to explore adult stem cell based myocardial regeneration, we have developed an in vitro model of three-dimensional (3-D) cardiac muscle using rat ventricular embryonic cardiomyocytes (ECMs) and BMSCs. When ECMs and BMSCs were seeded sequentially onto a 3-D tubular scaffold engineered from topographically aligned type I collagen-fibers and cultured in basal medium for 7, 14, 21, or 28 days, the maturation and co-differentiation into a cardiomyocyte lineage was observed. Phenotypic induction was characterized at morphological, immunological, biochemical and molecular levels. The observed expression of transcripts coding for cardiomyocyte phenotypic markers and the immunolocalization of cardiomyogenic lineage-associated proteins revealed typical expression patterns of neo-cardiomyogenesis. At the biochemical level differentiating cells exhibited appropriate metabolic activity and at the ultrastructural level myofibrillar and sarcomeric organization were indicative of an immature phenotype. Our 3-D co-culture system sustains the ECMs in vitro continuum of differentiation process and simultaneously induces the maturation and differentiation of BMSCs into cardiomyocyte-like cells. Thus, this novel 3-D co-culture system provides a useful in vitro model to investigate the functional role and interplay of developing ECMs and BMSCs during cardiomyogenic differentiation.

Development of a high-throughput assay for monitoring cAMP levels in cardiac ventricular myocytes

G-protein-coupled receptors (GPCRs) represent the largest family of transmembrane receptors involved in cell signal transduction. Many of these GPCRs convey their pharmacological actions by regulating intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP). Although the heart expresses more than 100 GPCRs, drug agonists for approximately one third of these GPCRs have not been identified. The goal of this project was to initiate the development of a high-throughput screening assay for monitoring cAMP in the heart. Neonatal rat cardiac ventricular myocytes were isolated and cultured on coverslips (whole-cell patch clamp recording) or in 96-well plates (fluorescent imaging plate reader measurements). Cells were infected with adenovirus expressing either beta-galactosidase (AdLacZ) or a mutant cyclic nucleotide-gated (CNG) channel containing the double mutation C460W/E583M (AdCNG). Addition of 2 microM forskolin along with 100 microM 3-isobutyl-1-methylxanthine, to increase intracellular cAMP, activated a cation current in myocytes infected with the AdCNG. In myocytes loaded with the fluorescent Ca indicator Fluo-4, stimulation with forskolin, epinephrine, norepinephrine, or the beta-adrenergic receptor agonist isoproterenol increased the fluorescent signal indicative of Ca influx through the CNG channel. In conclusion, CNG channels are readily expressed in cultured cardiac myocytes and may be utilized in high-throughput screening assays of intracellular cAMP.

Differentiation of monkey embryonic stem cells into hepatocytes and mRNA expression of cytochrome p450 enzymes responsible for drug metabolism: comparison of embryoid body formation conditions and matrices

We investigated the effects of embryoid body (EB) forming conditions on the expression of hepatocyte marker genes such as alpha-fetoprotein, albumin and CYP7A1 in cells cultured on Matrigel-coated plates for 15 d. The expression levels of hepatocyte marker genes in the cells cultured for 2 d for EB formation from cynomolgus monkey embryonic stem (cmES) cells was higher than those in cells cultured for 5 d. However, the fragment-size of cmES colonies did not markedly affect the expression levels. The expression levels of hepatocyte marker genes, and CYP1A1 and CYP2C43 in cells cultured on Matrigel were considerably higher than those on Matrigel reduced and collagen I. CYP1A1 and CYP3A8 mRNAs were significantly induced by 3-methylcholanthrene and rifampicin, respectively. However, CYP2C43 and CYP2D17 were not induced by these compounds. These results suggested that the differentiation into hepatocytes is affected by the incubation period for EB formation, and that Matrigel successfully promoted in vitro differentiation of cmES cells to hepatocytes.

Growth and electrophysiological properties of rat embryonic cardiomyocytes on hydroxyl- and carboxyl-modified surfaces

Biodegradable scaffolds such as poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA) or poly(glycolic acid) (PGA) are commonly used materials in tissue engineering. The chemical composition of these scaffolds changes during degradation which provides a differential environment for the seeded cells. In this study we have developed a simple and relatively high-throughput method in order to test the physiological effects of this varying chemical environment on rat embryonic cardiac myocytes. In order to model the different degradation stages of the scaffold, glass coverslips were functionalized with 11-mercaptoundecanoic acid (MUA) and 11-mercapto-1-undecanol (MUL) as carboxyl- and hydroxyl-groups presenting surfaces, and with trimethoxysilylpropyldiethylenetriamine (DETA) and (3-aminopropyl)triethoxysilane (APTES) as controls. Embryonic cardiac myocytes formed beating islands on all tested surfaces, but the number of attached cells and beating patches was significantly lower on MUL compared to any of the other functionalized surfaces. Moreover, whole-cell patch-clamp experiments showed that the average length of action potentials generated by the beating-cardiac myocytes were significantly longer on MUL compared to the other surfaces. Our results, using our simple test system, are in basic agreement with earlier observations that utilized a complex 3D biodegradable scaffold. Thus, surface functionalization with self-assembled monolayers combined with histological/physiological testing could be a relatively high throughput method for biocompatibility studies and for the optimization of the material/tissue interface in tissue engineering.

Regulation of material properties in electrospun scaffolds: Role of cross-linking and fiber tertiary structure

We cross-linked scaffolds of electrospun collagen to varying degrees with glutaraldehyde using an ethanol-based solvent system and subsequently defined how the percentage of cross-linking impacts bulk and microscale material properties and fiber structure. At hydration, electrospun fibers underwent coiling; the extent of coiling was proportional to the percentage of cross-linking introduced into the samples and was largely suppressed as cross-linking approached saturation. These data suggest that electrospun collagen fibers are not deposited in a minimal energy state; fiber coiling may reflect a molecular reorganization. This result has functional/structural implications for protein-based electrospun scaffolds. Changes in fiber topology that develop during post-electrospinning processing may alter monomer organization, mask or unmask receptor binding sites, and/or change the biological properties of these nanomaterials. Hydrated scaffolds were mounted into a custom stretching device installed on a microscope stage and photographed after incremental changes in strain. Changes in fiber alignment were measured using the two-dimensional fast Fourier transform method. Fibers in all scaffolds underwent alignment in response to strain; however, the rate and extent of alignment that could be achieved varied as a function of cross-linking. We propose four distinct modes of scaffold response to strain: fiber uncoiling, fiber reorientation, fiber elongation and interfiber sliding. We conclude that bulk material properties and local microscale architecture must be simultaneously considered to optimize the performance of electrospun scaffolds.

Culture on electrospun polyurethane scaffolds decreases atrial natriuretic peptide expression by cardiomyocytes in vitro

The function of the mammalian heart depends on the functional alignment of cardiomyocytes, and controlling cell alignment is an important consideration in biomaterial design for cardiac tissue engineering and research. The physical cues that guide functional cell alignment in vitro and the impact of substrate-imposed alignment on cell phenotype, however, are only partially understood. In this report, primary cardiac ventricular cells were grown on electrospun, biodegradable polyurethane (ES-PU) with either aligned or unaligned microfibers. ES-PU scaffolds supported high-density cultures and cell subpopulations remained intact over two weeks in culture. ES-PU cultures contained electrically-coupled cardiomyocytes with connexin-43 localized to points of cell:cell contact. Multi-cellular organization correlated with microfiber orientation and aligned materials yielded highly oriented cardiomyocyte groupings. Atrial natriuretic peptide, a molecular marker that shows decreasing expression during ventricular cell maturation, was significantly lower in cultures grown on ES-PU scaffolds than in those grown on tissue culture polystyrene. Cells grown on aligned ES-PU had significantly lower steady state levels of ANP and constitutively released less ANP over time indicating that scaffold-imposed cell organization resulted in a shift in cell phenotype to a more mature state. We conclude that the physical organization of microfibers in ES-PU scaffolds impacts both multi-cellular architecture and cardiac cell phenotype in vitro.

Engineered cardiac organoid chambers: toward a functional biological model ventricle

A growing area in the field of tissue engineering is the development of tissue equivalents as model systems for in vitro experimentation and high-throughput screening applications. Although a variety of strategies have been developed to enhance the structure and function of engineered cardiac tissues, an inherent limitation with traditional myocardial patches is that they do not permit evaluation of the fundamental relationships between pressure and volume that characterize global contractile function of the heart. Therefore, in the following study we introduce fully biological, living engineered cardiac organoids, or simplified heart chambers, that beat spontaneously, develop pressure, eject fluid, contain residual stress, exhibit a functional Frank-Starling mechanism, and generate positive stroke work. We also demonstrate regional variations in pump function following local cryoinjury, yielding a novel engineered tissue model of myocardial infarction. With the unique ability to directly evaluate relevant pressure-volume characteristics and regulate wall stress, this organoid chamber culture system provides a flexible platform for developing a controllable biomimetic cardiac niche environment that can be adapted for a variety of high-throughput and long-term investigations of cardiac pump function.

Formation of cardiac fibers in Matrigel matrix

We report a simple in vitro model of cardiac tissue that mimics three-dimensional (3-D) environment and mechanical load conditions and, as such, may serve as a convenient method to study stem cell engraftment or address developmental questions such as cytoskeleton or intercalated disk maturation. To create in vitro cardiac fibers we used Matrigel, a commercially available basement membrane preparation. A semisolid pillow from concentrated Matrigel was overlaid with a suspension of rat neonatal cardiomyocytes in a diluted Matrigel solution. This created an environment in which the multicellular fibers continuously contracted against a mechanical load. The described approach allows continuous structural and functional monitoring of 20-300-micron-thick cardiac fibers and provides easy access to epitopes for immunostaining purposes.