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

Ameliorating impaired cardiac function in myocardial infarction using exosome-loaded gallic-acid-containing polyurethane scaffolds

Myocardial infarction (MI) can be tackled by implanting cardiac patches which provide mechanical support to the heart. However, most tissue-engineered scaffolds face difficulty in attenuating oxidative stress, maintaining mechanical stability, and regenerating damaged cardiomyocytes. Here, we fabricated elastic cryogels using polyurethane modified with antioxidant gallic acid in its backbone (PUGA) and further coated them with decellularized extracellular matrix (dECM) to improve adhesiveness, biocompatibility and hemocompatibility. The scaffold was functionalized with exosomes (EXO) isolated from adipose-derived stem cells having regenerative potential. PUGA-dECM + EXO was tested in a rat model with induced MI where echocardiography after 8 weeks of implantation showed significant recovery in treatment group. Histological analysis revealed a decrease in fibrosis after application of patch and promotion of angiogenesis with reduced oxidative stress was shown by immunostaining. Expression of cardiac tissue contractile function marker was also observed in treatment groups. Thus, the proposed biomaterial has a promising application to be utilized as a patch for cardiac regeneration. More detailed studies with larger animal species are needed for using these observations for specific applications.

Novel hydrogels: are they poised to transform 3D cell-based assay systems in early drug discovery?

INTRODUCTION

Success in drug discovery remains unpredictable. However, more predictive and relevant disease models are becoming pivotal to demonstrating the clinical benefits of new drugs earlier in the lengthy drug discovery process. Novel hydrogel scaffolds are being developed to transform the relevance of such 3D cell-based in vitro assay systems.

AREAS COVERED

Most traditional hydrogels are still of unknown composition and suffer significant batch-to-batch variations, which lead to technical constraints. This article looks at how a new generation of novel synthetic hydrogels that are based on self-assembling peptides are poised to transform 3D cell-based assay systems by improving their relevance, reproducibility and scalability.

EXPERT OPINION

The emerging advantages of using these novel hydrogels for human 3D screening assays should enable the discovery of more cost-effective drugs, leading to improved patient benefits. Such a disruptive change could also reduce the considerable time lag from obtaining in vitro assay data to initiating clinical trials. There is now a sufficient body of data available in the literature to enable this ambition to become a reality by significantly improving the predictive validity of 3D cell-based assays in early drug discovery. Novel hydrogels are key to unlocking the full potential of these assay systems.

Myocardial infarction from a tissue engineering and regenerative medicine point of view: A comprehensive review on models and treatments

In the modern world, myocardial infarction is one of the most common cardiovascular diseases, which are responsible for around 18 million deaths every year or almost 32% of all deaths. Due to the detrimental effects of COVID-19 on the cardiovascular system, this rate is expected to increase in the coming years. Although there has been some progress in myocardial infarction treatment, translating pre-clinical findings to the clinic remains a major challenge. One reason for this is the lack of reliable and human representative healthy and fibrotic cardiac tissue models that can be used to understand the fundamentals of ischemic/reperfusion injury caused by myocardial infarction and to test new drugs and therapeutic strategies. In this review, we first present an overview of the anatomy of the heart and the pathophysiology of myocardial infarction, and then discuss the recent developments on pre-clinical infarct models, focusing mainly on the engineered three-dimensional cardiac ischemic/reperfusion injury and fibrosis models developed using different engineering methods such as organoids, microfluidic devices, and bioprinted constructs. We also present the benefits and limitations of emerging and promising regenerative therapy treatments for myocardial infarction such as cell therapies, extracellular vesicles, and cardiac patches. This review aims to overview recent advances in three-dimensional engineered infarct models and current regenerative therapeutic options, which can be used as a guide for developing new models and treatment strategies.

Dextran-based scaffolds for in-situ hydrogelation: Use for next generation of bioartificial cardiac tissues

In pursuit of a chemically-defined matrix for in vitro cardiac tissue generation, we present dextran (Dex)-derived hydrogels as matrices suitable for bioartificial cardiac tissues (BCT). The dextran hydrogels were generated in situ by using hydrazone formation as the crosslinking reaction. Material properties were flexibly adjusted, by varying the degrees of derivatization and the molecular weight of dextran used. Furthermore, to modulate dextran's bioactivity, cyclic pentapeptide RGD was coupled to its backbone. BCTs were generated by using a blend of modified dextran and human collagen (hColI) in combination with induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) and fibroblasts. These hColI + Dex blends with or without RGD supported tissue formation and functional maturation of CMs. Contraction forces (hColI + Dex-RGD: 0.27 ± 0.02 mN; hColI + Dex: 0.26 ± 0.01 mN) and frequencies were comparable to published constructs. Thus, we could demonstrate that, independent of the presence of RGD, our covalently linked dextran hydrogels are a promising matrix for building cardiac grafts.

Heart Slices to Model Cardiac Physiology

Translational research in the cardiovascular field is hampered by the unavailability of cardiac models that can recapitulate organ-level physiology of the myocardium. Outside the body, cardiac tissue undergoes rapid dedifferentiation and maladaptation in culture. There is an ever-growing demand for preclinical platforms that allow for accurate, standardized, long-term, and rapid drug testing. Heart slices is an emerging technology that solves many of the problems with conventional myocardial culture systems. Heart slices are thin (<400 µm) slices of heart tissue from the adult ventricle. Several recent studies using heart slices have shown their ability to maintain the adult phenotype for prolonged periods in a multi cell-type environment. Here, we review the current status of cardiac culture systems and highlight the unique advantages offered by heart slices in the light of recent efforts in developing physiologically relevant heart slice culture systems.

Maladaptive Contractility of 3D Human Cardiac Microtissues to Mechanical Nonuniformity

Cardiac tissues are able to adjust their contractile behavior to adapt to the local mechanical environment. Nonuniformity of the native tissue mechanical properties contributes to the development of heart dysfunctions, yet the current in vitro cardiac tissue models often fail to recapitulate the mechanical nonuniformity. To address this issue, a 3D cardiac microtissue model is developed with engineered mechanical nonuniformity, enabled by 3D-printed hybrid matrices composed of fibers with different diameters. When escalating the complexity of tissue mechanical environments, cardiac microtissues start to develop maladaptive hypercontractile phenotypes, demonstrated in both contractile motion analysis and force-power analysis. This novel hybrid system could potentially facilitate the establishment of "pathologically-inspired" cardiac microtissue models for deeper understanding of heart pathology due to nonuniformity of the tissue mechanical environment.

Decellularized Human Dermal Matrix as a Biological Scaffold for Cardiac Repair and Regeneration

The complex and highly organized environment in which cells reside consists primarily of the extracellular matrix (ECM) that delivers biological signals and physical stimuli to resident cells. In the native myocardium, the ECM contributes to both heart compliance and cardiomyocyte maturation and function. Thus, myocardium regeneration cannot be accomplished if cardiac ECM is not restored. We hypothesize that decellularized human skin might make an easily accessible and viable alternate biological scaffold for cardiac tissue engineering (CTE). To test our hypothesis, we decellularized specimens of both human skin and human myocardium and analyzed and compared their composition by histological methods and quantitative assays. Decellularized dermal matrix was then cut into 600-μm-thick sections and either tested by uniaxial tensile stretching to characterize its mechanical behavior or used as three-dimensional scaffold to assess its capability to support regeneration by resident cardiac progenitor cells (hCPCs) in vitro. Histological and quantitative analyses of the dermal matrix provided evidence of both effective decellularization with preserved tissue architecture and retention of ECM proteins and growth factors typical of cardiac matrix. Further, the elastic modulus of the dermal matrix resulted comparable with that reported in literature for the human myocardium and, when tested in vitro, dermal matrix resulted a comfortable and protective substrate promoting and supporting hCPC engraftment, survival and cardiomyogenic potential. Our study provides compelling evidence that dermal matrix holds promise as a fully autologous and cost-effective biological scaffold for CTE.

Cardiomyocyte dedifferentiation and remodeling in 3D scaffolds to generate the cellular diversity of engineering cardiac tissues

The use of engineered cardiac tissues (ECTs) is a new strategy for the repair and replacement of cardiac tissues in patients with myocardial infarction, particularly at late stages. However, the mechanisms underlying the development of ECTs, including cell-scaffold interactions, are not fully understood, although they are closely related to their therapeutic effect. In the present study, we aimed to determine the cellular fate of cardiomyocytes in a 3D scaffold microenvironment, as well as their role in generating the cellular diversity of ECTs by single-cell sequencing analysis. Consistent with the observed plasticity of cardiomyocytes during cardiac regeneration, cardiomyocytes in 3D scaffolds appeared to dedifferentiate, showing an initial loss of normal cytoskeleton organization in the adaptive response to the new scaffold microenvironment. Cardiomyocytes undergoing this process regained their proliferation potential and gradually developed into myocardial cells at different developmental stages, generating heterogeneous regenerative ECTs. To better characterize the remodeled ECTs, high-throughput single-cell sequencing was performed. The ECTs contained a wide diversity of cells related to endogenous classes in the heart, including myocardial cells at different developmental stages and different kinds of interstitial cells. Non-cardiac cells seemed to play important roles in cardiac reconstruction, especially Cajal-like interstitial cells and macrophages. Altogether, our results showed for the first time that cells underwent adaptive dedifferentiation for survival in a 3D scaffold microenvironment to generate heterogeneous tissues. These findings provide an important basis for an improved understanding of the development and assembly of engineered tissues.

In Situ Generated Medical Devices

Medical devices play a major role in all areas of modern medicine, largely contributing to the success of clinical procedures and to the health of patients worldwide. They span from simple commodity products such as gauzes and catheters, to highly advanced implants, e.g., heart valves and vascular grafts. In situ generated devices are an important family of devices that are formed at their site of clinical function that have distinct advantages. Among them, since they are formed within the body, they only require minimally invasive procedures, avoiding the pain and risks associated with open surgery. These devices also display enhanced conformability to local tissues and can reach sites that otherwise are inaccessible. This review aims at shedding light on the unique features of in situ generated devices and to underscore leading trends in the field, as they are reflected by key developments recently in the field over the last several years. Since the uniqueness of these devices stems from their in situ generation, the way they are formed is crucial. It is because of this fact that in this review, the medical devices are classified depending on whether their in situ generation entails chemical or physical phenomena.

In Situ Gelling Scaffolds Loaded with Platelet Growth Factors to Improve Cardiomyocyte Survival after Ischemia

Myocardial infarction is caused by prolonged ischemia and it is one of the main cause that leads to heart failures. The aim of the present work was the development of in situ gelling systems, based on poloxamer 407 (P407) or sodium alginate (Alg), loaded with platelet lysate (PL) to enhance cardiomyocyte survival after ischemia. Chondroitin sulfate (CS), a negatively charged glycosaminoglycan able to interact with different positively charged bioactive molecules, such as growth factors, was also investigated with both the systems. The gelation properties of both systems (viscosity, viscoelasticity, consistency by means of penetrometry, and injectability) were characterized in a physiological environment. In vitro evaluation of biocompatibility using fetal cardiac cells (cardiomyocytes and cardiac fibroblasts) demonstrated that the PL loaded alginate/chondroitin sulfate system retained the highest number of viable cells with equal distribution of the populations of cardiomyocytes and fibroblasts. Furthermore, the ability of the systems to improve cardiomyocyte survival after ischemia was also assessed. PL allowed for the highest degree of survival of cardiomyocytes after oxidative damage (simulating ischemic conditions due to MI) and both the Alg + CS PL and, to a greater extent, the PL alone demonstrated a considerable increase in survival of cardiomyocytes. In conclusion, an in situ gelling alginate-chondroitin sulfate system, loaded with platelet lysate, was able to improve the survival of cardiomyocytes after oxidative damage resulting in a promising system to improve cardiac cell viability after ischemia.

Nanotechnology, an alternative with promising prospects and advantages for the treatment of cardiovascular diseases

Cardiovascular diseases (CVDs) are one of the most important causes of mortality and affecting the health status of patients. At the same time, CVDs cause a huge health and economic burden to the whole world. Although a variety of therapeutic drugs and measures have been produced to delay the progress of the disease and improve the quality of life of patients, most of the traditional therapeutic strategies can only cure the symptoms and cannot repair or regenerate the damaged ischemic myocardium. In addition, they may bring some unpleasant side effects. Therefore, it is vital to find and explore new technologies and drugs to solve the shortcomings of conventional treatments. Nanotechnology is a new way of using and manipulating the matter at the molecular scale, whose functional organization is measured in nanometers. Because nanoscale phenomena play an important role in cell signal transduction, enzyme action and cell cycle, nanotechnology is closely related to medical research. The application of nanotechnology in the field of medicine provides an alternative and novel direction for the treatment of CVDs, and shows excellent performance in the field of targeted drug therapy and the development of biomaterials. This review will briefly introduce the latest applications of nanotechnology in the diagnosis and treatment of common CVDs.

Injectable Hydrogels for Cardiac Tissue Engineering

In light of the limited efficacy of current treatments for cardiac regeneration, tissue engineering approaches have been explored for their potential to provide mechanical support to injured cardiac tissues, deliver cardio-protective molecules, and improve cell-based therapeutic techniques. Injectable hydrogels are a particularly appealing system as they hold promise as a minimally invasive therapeutic approach. Moreover, injectable acellular alginate-based hydrogels have been tested clinically in patients with myocardial infarction (MI) and show preservation of the left ventricular (LV) indices and left ventricular ejection fraction (LVEF). This review provides an overview of recent developments that have occurred in the design and engineering of various injectable hydrogel systems for cardiac tissue engineering efforts, including a comparison of natural versus synthetic systems with emphasis on the ideal characteristics for biomimetic cardiac materials.

Applications of Cardiac Extracellular Matrix in Tissue Engineering and Regenerative Medicine

The role of the cardiac extracellular matrix (cECM) in providing biophysical and biochemical cues to the cells housed within during disease and development has become increasingly apparent. These signals have been shown to influence many fundamental cardiac cell behaviors including contractility, proliferation, migration, and differentiation. Consequently, alterations to cell phenotype result in directed remodeling of the cECM. This bidirectional communication means that the cECM can be envisioned as a medium for information storage. As a result, the reprogramming of the cECM is increasingly being employed in tissue engineering and regenerative medicine as a method with which to treat disease. In this chapter, an overview of the composition and structure of the cECM as well as its role in cardiac development and disease will be provided. Additionally, therapeutic modulation of cECM for cardiac regeneration as well as bottom-up and top-down approaches to ECM-based cardiac tissue engineering is discussed. Finally, lingering questions regarding the role of cECM in tissue engineering and regenerative medicine are offered as a catalyst for future research.

Zwitterionic starch-based hydrogel for the expansion and "stemness" maintenance of brown adipose derived stem cells

Brown adipose derived stem cells (BADSCs) have become a promising stem cell treatment candidate for myocardial infarction because of their efficiently spontaneous differentiation capacity towards cardiomyocytes. The lack of existing cell passage protocols motivates us to develop a neotype 3D cell expansion technique for BADSCs. In this study, "clickable" zwitterionic starch based hydrogels are developed using methacrylate modified sulfobetaine derived starch with dithiol-functionalized poly (ethylene glycol) as crosslinker via the "thiol-ene" Michael addition reaction. Moreover, CGRGDS peptide is immobilized into the hydrogel via a similar "clickable" approach. Their Young's moduli range from 22.28 to 74.81 kPa depending on the concentration of precursor solutions. Excellent anti-fouling property is also presented owing to the introduction of zwitterionic moieties. BADSCs are homogeneously encapsulated in the hydrogels and then routinely cultured for 10 days. Results suggest a capacious cell proliferation and the extent increases with either the decrease of mechanical strength or the introduction of CGRGDS. More excitingly, the cell "stemness" is well maintained during this period and the expanded cells released from the hydrogels well keep the efficiently spontaneous cardiomyogenic differentiation capacity. Therefore, it is suggested that zwitterionic starch based hydrogel is able for the expansion and "stemness " maintenance of BADSCs.

Stem cell-based therapy: Improving myocardial cell delivery

Stem cell-based therapies form an exciting new class of medicine that attempt to provide the body with the building blocks required for the reconstruction of damaged organs. However, delivering cells to the correct location, while preserving their integrity and functional properties, is a complex undertaking. These challenges have led to the development of a highly dynamic interdisciplinary research field, wherein medical, biological, and chemical sciences have collaborated to develop strategies to overcome the physiological barriers imposed on the cellular therapeutics. In this respect, improving the acute retention and subsequent survival of stem cells is key to effectively increase the effect of the therapy, while proper tissue integration is imperative for stem cells to functionally replace lost cells in damaged organs. In this review, we will use the heart as an example to highlight the current knowledge of therapeutic stem cell utilization, the existing pitfalls and limitations, and the approaches that have been developed to overcome them.

An injectable silk sericin hydrogel promotes cardiac functional recovery after ischemic myocardial infarction

Acute myocardial infarction (MI) leads to morbidity and mortality due to cardiac dysfunction. Here we identify sericin, a silk-derived protein, as an injectable therapeutic biomaterial for the minimally invasive MI repair. For the first time, sericin prepared in the form of an injectable hydrogel has been utilized for cardiac tissue engineering and its therapeutical outcomes evaluated in a mouse MI model. The injection of this sericin hydrogel into MI area reduces scar formation and infarct size, increases wall thickness and neovascularization, and inhibits the MI-induced inflammatory responses and apoptosis, thereby leading to a significant functional improvement. The potential therapeutical mechanisms have been further analyzed in vitro. Our results indicate that sericin downregulates pro-inflammatory cytokines (TNF-α and IL-18) and chemokine (CCL2) and reduces TNF-α expression by suppressing the TLR4-MAPK/NF-κB pathways. Moreover, sericin exhibits angiogenic activity by promoting migration and tubular formation of human umbilical vessel endothelial cells (HUVECs). Also, sericin stimulates VEGFa expression via activating ERK phosphorylation. Further, sericin protects endothelial cells and cardiomyocytes from apoptosis by inhibiting the activation of caspase 3. Together, these diverse biochemical activities of sericin protein lead to a significant recovery of cardiac function. This work represents the first study reporting sericin as an effective therapeutic biomaterial for ischemic myocardial repair in vivo.

STATEMENT OF SIGNIFICANCE

Intramyocardial biomaterial injection is thought to be a potential therapeutic approach to improve cardiac performance after ischemic myocardial infarction. In this study, we report the successful fabrication and in vivo application of an injectable sericin hydrogel for ischemic heart disease. We for the first time show that the injection of in situ forming crosslinked sericin hydrogel promotes heart functional recovery accompanied with reduced inflammatory responses, attenuated apoptosis and increased microvessel density in the infarcted hearts. Further, we reveal that the improvement in those aspects is ascribed to sericin protein's functional bioactivities that are comprehensively uncovered in this study. Thus, we identify sericin, a natural protein, as a biomaterial suitable for myocardial repair and demonstrate that the in vivo application of this injectable sericin hydrogel can be an effective strategy for treating MI.

Electrical and mechanical stimulation of cardiac cells and tissue constructs

The field of cardiac tissue engineering has made significant strides over the last few decades, highlighted by the development of human cell derived constructs that have shown increasing functional maturity over time, particularly using bioreactor systems to stimulate the constructs. However, the functionality of these tissues is still unable to match that of native cardiac tissue and many of the stem-cell derived cardiomyocytes display an immature, fetal like phenotype. In this review, we seek to elucidate the biological underpinnings of both mechanical and electrical signaling, as identified via studies related to cardiac development and those related to an evaluation of cardiac disease progression. Next, we review the different types of bioreactors developed to individually deliver electrical and mechanical stimulation to cardiomyocytes in vitro in both two and three-dimensional tissue platforms. Reactors and culture conditions that promote functional cardiomyogenesis in vitro are also highlighted. We then cover the more recent work in the development of bioreactors that combine electrical and mechanical stimulation in order to mimic the complex signaling environment present in vivo. We conclude by offering our impressions on the important next steps for physiologically relevant mechanical and electrical stimulation of cardiac cells and engineered tissue in vitro.

Cardiac-Derived Extracellular Matrix Enhances Cardiogenic Properties of Human Cardiac Progenitor Cells

The use of biomaterials has been demonstrated as a viable strategy to promote cell survival and cardiac repair. However, limitations on combinational cell-biomaterial therapies exist, as cellular behavior is influenced by the microenvironment and physical characteristics of the material. Among the different scaffolds employed for cardiac tissue engineering, a myocardial matrix hydrogel has been shown to promote cardiogenesis in murine cardiac progenitor cells (mCPCs) in vitro. In this study, we investigated the influence of the hydrogel on Sca-1-like human fetal and adult CPCs (fCPCs and aCPCs) when encapsulated in three-dimensional (3D) material in vitro. fCPCs encapsulated in the myocardial matrix showed an increase in the gene expression level of cardiac markers GATA-4 and MLC2v and the vascular marker vascular endothelial growth factor receptor 2 (VEGFR2) after 4 days in culture, and a significant increase in GATA-4 up to 1 week. Increased gene expression levels of Nkx2.5, MEF2c, VEGFR2, and CD31 were also observed when aCPCs were cultured in the matrix compared to collagen. Cell survival was sustained in both hydrogels up to 1 week in culture with the myocardial matrix capable of enhancing the expression of the proliferation marker Ki-67 after 4 days in culture. When encapsulated CPCs were treated with H₂O₂, an improved survival of the cells cultured in the myocardial matrix was observed. Finally, we evaluated the use of the myocardial matrix as hydrogel for in vivo cell transplantation and demonstrated that the gelation properties of the hydrogel are not influenced by the cells. In summary, we showed that the myocardial matrix hydrogel promotes human CPC cardiogenic potential, proliferation, and survival and is a favorable hydrogel for 3D in vitro culture. Furthermore, we demonstrated the in vivo applicability of the matrix as a potential vehicle for cell transplantation.

Partially Digested Adult Cardiac Extracellular Matrix Promotes Cardiomyocyte Proliferation In Vitro

Stimulating or maintaining the proliferative capacity of postnatal mammalian cardiomyocytes is a major challenge to cardiac regeneration. Previously, it is found that fetal cardiac extracellular matrix (ECM) can promote neonatal rat cardiomyocyte proliferation in vitro better than neonatal or adult ECM. It is hypothesized that partial digestion of adult ECM (PD-ECM) would liberate less crosslinked components that promote cardiomyocyte proliferation, similar to fetal ECM. Neonatal rat cardiac cells are seeded onto substrates coated with adult rat cardiac ECM that has been solubilized in pepsin-HCl for 1, 3, 6, 12, 24, or 48 h. Cardiomyocyte proliferation and fold-change in numbers from 1 to 5 d are highest on 1 and 3 h PD-ECM compared to other conditions. Sarcomeres tend to mature on 24 and 48 h PD-ECM where low proliferation is observed. 3 h PD-ECM is primarily composed of Fibrillin-1, Fibrinogen, and Laminins while 48 h PD-ECM is dominated by Collagen I. Our results suggest that adult ECM retains regenerative cues that may be masked by more abundant, mature ECM components. PD-ECM provides a simple yet powerful approach to promoting cardiomyocyte proliferation.

Epicardial application of cardiac progenitor cells in a 3D-printed gelatin/hyaluronic acid patch preserves cardiac function after myocardial infarction

Cardiac cell therapy suffers from limitations related to poor engraftment and significant cell death after transplantation. In this regard, ex vivo tissue engineering is a tool that has been demonstrated to increase cell retention and survival. The aim of our study was to evaluate the therapeutic potential of a 3D-printed patch composed of human cardiac-derived progenitor cells (hCMPCs) in a hyaluronic acid/gelatin (HA/gel) based matrix. hCMPCs were printed in the HA/gel matrix (30 × 10(6) cells/ml) to form a biocomplex made of six perpendicularly printed layers with a surface of 2 × 2 cm and thickness of 400 μm, in which they retained their viability, proliferation and differentiation capability. The printed biocomplex was transplanted in a mouse model of myocardial infarction (MI). The application of the patch led to a significant reduction in adverse remodeling and preservation of cardiac performance as was shown by both MRI and histology. Furthermore, the matrix supported the long-term in vivo survival and engraftment of hCMPCs, which exhibited a temporal increase in cardiac and vascular differentiation markers over the course of the 4 week follow-up period. Overall, we developed an effective and translational approach to enhance hCMPC delivery and action in the heart.

Cardiac extracellular matrix-fibrin hybrid scaffolds with tunable properties for cardiovascular tissue engineering

Solubilized cardiac extracellular matrix (ECM) is being developed as an injectable therapeutic that offers promise for promoting cardiac repair. However, the ECM alone forms a hydrogel that is very soft compared to the native myocardium. As both the stiffness and composition of the ECM are important in regulating cell behavior and can have complex synergistic effects, we sought to develop an ECM-based scaffold with tunable biochemical and mechanical properties. We used solubilized rat cardiac ECM from two developmental stages (neonatal, adult) combined with fibrin hydrogels that were cross-linked with transglutaminase. We show that ECM was retained within the gels and that the Young's modulus could be tuned to span the range of the developing and mature heart. C-kit+ cardiovascular progenitor cells from pediatric patients with congenital heart defects were seeded into the hybrid gels. Both the elastic modulus and composition of the scaffolds impacted the expression of endothelial and smooth muscle cell genes. Furthermore, we demonstrate that the hybrid gels are injectable, and thus have potential for minimally invasive therapies. ECM-fibrin hybrid scaffolds offer new opportunities for exploiting the effects of both composition and mechanical properties in directing cell behavior for tissue engineering.

Investigation into the effects of varying frequency of mechanical stimulation in a cycle-by-cycle manner on engineered cardiac construct function

Mechanical stimulation has been used extensively to improve the function of cardiac engineered tissue, as it mimics the physical environment in which the tissue is situated during normal development. However, previous mechanical stimulation has been carried out under a constant frequency that more closely resembles a diseased heart. The goal of this study was to create a bioreactor system that would allow us to control the mechanical stimulation of engineered cardiac tissue on a cycle-by-cycle basis. This unique system allows us to determine the effects on cardiac construct function of introducing variability to the mechanical stretch. To test our bioreactor system, constructs created from neonatal rat cardiomyocytes entrapped in fibrin hydrogels were stimulated under various regimes for 2 weeks and then assessed for functional outcomes. No differences were observed in the final cell number in each condition, indicating that variability in frequency did not have a negative effect on viability. The forces were higher for all mechanical stimulation groups compared to static controls, although no differences were observed between the mechanically stimulated conditions, indicating that variable frequency on a cycle-by-cycle basis has limited effects on the resulting force. Although differences in the observed twitch force were not observed, differences in the protein expression indicate that variable-frequency mechanical stimulation had an effect on cell-cell coupling and growth pathway activation in the constructs. Thus, this bioreactor system provides a valuable tool for further development and optimization of engineered myocardial tissue as a repair or replacement strategy for patients undergoing heart failure. Copyright © 2014 John Wiley & Sons, Ltd.

Mimicking isovolumic contraction with combined electromechanical stimulation improves the development of engineered cardiac constructs

Electrical and mechanical stimulation have both been used extensively to improve the function of cardiac engineered tissue as each of these stimuli is present in the physical environment during normal development in vivo. However, to date, there has been no direct comparison between electrical and mechanical stimulation and current published data are difficult to compare due to the different systems used to create the engineered cardiac tissue and the different measures of functionality studied as outcomes. The goals of this study were twofold. First, we sought to directly compare the effects of mechanical and electrical stimulation on engineered cardiac tissue. Second, we aimed to determine the importance of the timing of the two stimuli in relation to each other in combined electromechanical stimulation. We hypothesized that delaying electrical stimulation after the beginning of mechanical stimulation to mimic the biophysical environment present during isovolumic contraction would improve construct function by improving proteins responsible for cell-cell communication and contractility. To test this hypothesis, we created a bioreactor system that would allow us to electromechanically stimulate engineered tissue created from neonatal rat cardiac cells entrapped in fibrin gel during 2 weeks in culture. Contraction force was higher for all stimulation groups as compared with the static controls, with the delayed combined stimulation constructs having the highest forces. Mechanical stimulation alone displayed increased final cell numbers but there were no other differences between electrical and mechanical stimulation alone. Delayed combined stimulation resulted in an increase in SERCA2a and troponin T expression levels, which did not happen with synchronous combined stimulation, indicating that the timing of combined stimulation is important to maximize the beneficial effect. Increases in Akt protein expression levels suggest that the improvements are at least in part induced by hypertrophic growth. In summary, combined electromechanical stimulation can create engineered cardiac tissue with improved functional properties over electrical or mechanical stimulation alone, and the timing of the combined stimulation greatly influences its effects on engineered cardiac tissue.

Injectable collagen implant improves survival, cardiac remodeling, and function in the early period after myocarditis in rats

AIM

Despite clear evidence of immune system involvement in the pathogenesis of myocarditis, the treatment of myocarditis remains nonspecific and supportive. We sought to test the hypothesis that injection of a collagen-based implant into the inflamed myocardium would stabilize the left ventricular (LV) wall and prevent adverse remodeling and dysfunction.

METHODS AND RESULTS

Autoimmune myocarditis was induced in 42 male Lewis rats. Development of myocarditis was evaluated and confirmed by serial echocardiography and cardiac magnetic resonance scans, LV wall thickening, global and regional LV wall motion abnormalities, and in some cases pericardial effusion. Sick animals were randomized to either injectable collagen implantation or saline injection into the anterior inflamed myocardium 14 days after immunization. Significantly, injectable collagen implantation improved 31-day survival compared with controls (85.7% vs 50%; P = .03). Furthermore, although injectable collagen significantly attenuated LV systolic and diastolic dilatation and preserved LV geometry and function, control animals developed significant LV dilatation and dysfunction. These favorable effects on LV remodeling were confirmed by postmortem morphometry. Significantly, the injectable collagen implant attenuated cardiomyocyte hypertrophy and infiltration of macrophages and lymphocytes into the myocardium.

CONCLUSIONS

The present study shows, for the first time, that injectable collagen biomaterial improves survival and attenuates cardiac inflammation, cardiomyocyte hypertrophy, LV remodeling, and dysfunction in the early period after myocarditis in rats. Our findings suggest a new biomaterial-based strategy to ameliorate the devastating effects of myocarditis.

Multistructural biomimetic substrates for controlled cellular differentiation

Multidimensional scaffolds are considered to be ideal candidates for regenerative medicine and tissue engineering based on their potential to provide an excellent microenvironment and direct the fate of the cultured cells. More recently, the use of stem cells in medicine has opened a new technological opportunity for controlled tissue formation. However, the mechanism through which the substrate directs the differentiation of stem cells is still rather unclear. Data concerning its specific surface chemistry, topology, and its signaling ability need to be further understood and analyzed. In our study, atomic force microscopy was used to study the stiffness, roughness, and topology of the collagen (Coll) and metallized collagen (MC) substrates, proposed as an excellent substrate for regenerative medicine. The importance of signaling molecules was studied by constructing a new hybrid signaling substrate that contains both collagen and laminin extracellular matrix (ECM) proteins. The cellular response-such as attachment capability, proliferation and cardiac and neuronal phenotype expression on the metallized and non-metallized hybrid substrates (collagen + laminin)-was studied using MTT viability assay and immunohistochemistry studies. Our findings indicate that such hybrid materials could play an important role in the regeneration of complex tissues.

Creation of a bioreactor for the application of variable amplitude mechanical stimulation of fibrin gel-based engineered cardiac tissue

This chapter details the creation of three-dimensional fibrin hydrogels as an engineered myocardial tissue and introduces a mechanical stretch bioreactor system that allows for the cycle-to-cycle variable amplitude mechanical stretch of the constructs as a method of conditioning the constructs to be more similar to native tissue. Though mechanical stimulation has been established as a standard method of improving construct development, most studies have been performed under constant frequency and constant amplitude, even though variability is a critical aspect of healthy cardiac physiology. The introduction of variability in other organ systems has demonstrated beneficial effects to cell function in vitro. We hypothesize that the introduction of variability in engineered cardiac tissue could have a similar effect.

Cardiac tissue engineering: renewing the arsenal for the battle against heart disease

The development of therapies that lead to the regeneration or functional repair of compromised cardiac tissue is the most important challenge facing translational cardiovascular research today.

Physical properties of poly(N-isopropylacrylamide) hydrogel promote its effects on cardiac protection after myocardial infarction

OBJECTIVE

Studies have demonstrated the protective effects of biomaterials against myocardial infarction (MI), but the relationship between their physical characteristics and their function is incompletely understood. This study investigated such relationships for a hydrogel preparation.

METHODS

Two types of poly(N-isopropylacrylamide) (PNIPAAm) hydrogel with different degradation times (Gel A and Gel B) were synthesized. In vivo hydrogel formation and maintenance were observed and confirmed in mice. The solutions were also injected into the infarct area immediately after MI induction in rats.

RESULTS

PNIPAAm hydrogel exhibited a three-dimensional structure resembling native extracellular matrix (ECM). Compared with phosphate-buffered saline, Gel A and Gel B increased contractility of isolated infarcted myocardium, reduced collagen deposition, increased neovascularization, inhibited left ventricle expansion and improved cardiac function. Myocardial contractility was greater with Gel B than with Gel A.

CONCLUSIONS

Intramyocardial injection of PNIPAAm hydrogel provides structural support and some functional repair of damaged ECM, suggesting that it might be useful for cardiac tissue engineering.

Model systems for cardiovascular regenerative biology

There is an urgent clinical need to develop new therapeutic approaches to treat heart failure, but the biology of cardiovascular regeneration is complex. Model systems are required to advance our understanding of biological mechanisms of cardiac regeneration as well as to test therapeutic approaches to regenerate tissue and restore cardiac function following injury. An ideal model system should be inexpensive, easily manipulated, easily reproducible, physiologically representative of human disease, and ethically sound. In this review, we discuss computational, cell-based, tissue, and animal models that have been used to elucidate mechanisms of cardiovascular regenerative biology or to test proposed therapeutic methods to restore cardiac function following disease or injury.

Analysis of stromal cell secretomes reveals a critical role for stromal cell-derived hepatocyte growth factor and fibronectin in angiogenesis

OBJECTIVE

Angiogenesis requires tightly coordinated crosstalk between endothelial cells (ECs) and stromal cells, such as fibroblasts and smooth muscle cells. The specific molecular mechanisms moderating this process are still poorly understood.

METHODS AND RESULTS

Stromal cell-derived factors are essential for EC sprouting and lumen formation. We therefore compared the abilities of 2 primary fibroblast isolates and a primary smooth muscle cell isolate to promote in vitro angiogenesis, and analyzed their secretomes using a combination of nano liquid chromatography-mass spectrometry/mass spectrometry, quantitative PCR, and ELISA. Each isolate exhibited a different level of angiogenic ability. Using quantitative MS, we then compared the secretomes of a fibroblast isolate exhibiting low angiogenic activity, a fibroblast isolate exhibiting high angiogenic activity, and human umbilical vein ECs. High angiogenic fibroblast supernatants exhibited an overabundance of proteins associated with extracellular matrix constituents compared with low angiogenic fibroblasts or ECs. Finally, small interfering RNA technology and purified protein were used to confirm a role for stromal cell-derived hepatocyte growth factor and fibronectin in inducing EC sprouting.

CONCLUSIONS

Differences in stromal cell ability to induce angiogenesis are a result of differences in the secreted proteomes of both extracellular matrix proteins and proangiogenic growth factors.

Experimental acute myocardial infarction: telocytes involvement in neo-angiogenesis

We used rat experimental myocardial infarction to study the ultrastructural recovery, especially neo-angiogenesis in the infarction border zone. We were interested in the possible role(s) of telocytes (TCs), a novel type of interstitial cell very recently discovered in myocardim (see http://www.telocytes.com). Electron microscopy, immunocytochemistry and analysis of several proangiogenic microRNAs provided evidence for TC involvement in neo-angiogenesis after myocardial infarction. Electron microscopy showed the close spatial association of TCs with neoangiogenetic elements. Higher resolution images provided the following information: (a) the intercellular space between the abluminal face of endothelium and its surrounding TCs is frequently less than 50 nm; (b) TCs establish multiple direct nanocontacts with endothelial cells, where the extracellular space seems obliterated; such nanocontacts have a length of 0.4–1.5 μm; (c) the absence of basal membrane on the abluminal face of endothelial cell. Besides the physical contacts (either nanoscopic or microscopic) TCs presumably contribute to neo-angiognesis via paracrine secretion (as shown by immunocytochemistry for VEGF or NOS2). Last but not least, TCs contain measurable quantities of angiogenic microRNAs (e.g. let-7e, 10a, 21, 27b, 100, 126-3p, 130a, 143, 155, 503). Taken together, the direct (physical) contact of TCs with endothelial tubes, as well as the indirect (chemical) positive influence within the ‘angiogenic zones’, suggests an important participation of TCs in neo-angiogenesis during the late stage of myocardial infarction.