Antisenescence Therapy Improves Function in a Human Model of Cardiac Fibrosis-on-a-Chip

Cardiac fibrosis is a significant contributor to heart failure and is characterized by abnormal ECM deposition and impaired contractile function. We have previously developed a model of cardiac fibrosis via TGF-β treatment of engineered microtissues using heart-on-a-chip technology which incorporates human induced pluripotent stem cell-derived cardiomyocytes and cardiac fibroblasts. Here, we describe that these cardiac fibrotic tissues expressed markers associated with cellular senescence via transcriptomic analysis. Treatment of fibrotic tissues with the senolytic drugs dasatinib and quercetin (D+Q) led to an improvement of contractile function, reduced passive tension, and downregulated senescence-related gene expression, an outcome we were previously unable to achieve using standard-of-care drugs. The improvement in functional parameters was also associated with a reduction in fibroblast density, though no changes in absolute collagen deposition were observed. This study demonstrates the benefit of senolytic treatment for cardiac fibrosis in a human-relevant model, supporting data in animal models, and will enable the further elucidation of cell-specific effects of senolytics and how they impact cardiac fibrosis and senescence.

Modeling Heart Diseases on a Chip: Advantages and Future Opportunities

Heart disease is a significant burden on global health care systems and is a leading cause of death each year. To improve our understanding of heart disease, high quality disease models are needed. These will facilitate the discovery and development of new treatments for heart disease. Traditionally, researchers have relied on 2D monolayer systems or animal models of heart disease to elucidate pathophysiology and drug responses. Heart-on-a-chip (HOC) technology is an emerging field where cardiomyocytes among other cell types in the heart can be used to generate functional, beating cardiac microtissues that recapitulate many features of the human heart. HOC models are showing great promise as disease modeling platforms and are poised to serve as important tools in the drug development pipeline. By leveraging advances in human pluripotent stem cell-derived cardiomyocyte biology and microfabrication technology, diseased HOCs are highly tuneable and can be generated via different approaches such as: using cells with defined genetic backgrounds (patient-derived cells), adding small molecules, modifying the cells' environment, altering cell ratio/composition of microtissues, among others. HOCs have been used to faithfully model aspects of arrhythmia, fibrosis, infection, cardiomyopathies, and ischemia, to name a few. In this review, we highlight recent advances in disease modeling using HOC systems, describing instances where these models outperformed other models in terms of reproducing disease phenotypes and/or led to drug development.

Isolation of ready-made rat microvessels and its applications in effective in vivo vascularization and in angiogenic studies in vitro

Despite recent advances in the differentiation of human pluripotent stem cells into multiple cell types for application in replacement therapies, tissue vascularization remains a bottleneck for regenerative medicine. Fragments of primary microvessels (MVs) harvested from adipose tissue retain endothelialized lumens and perivascular cell coverage. We have used these MVs to support the survival and engraftment of transplanted human pluripotent stem cell-derived cardiomyocytes, pancreatic progenitors or primary human islets. MVs connect with host vessels, perfuse with blood and form a hierarchal vascular network in vivo after subcutaneous or intracardiac transplantation. MVs also display the ability to remodel and form stable vascular networks with long-term retention (>3.5 months). MVs can be cultured in 3D hydrogels in vitro, where they retain vessel shape and undergo angiogenic sprouting without the need for exogenous growth factor supplementation. Therefore, MVs offer a robust vascularization strategy for regenerative medicine approaches and a platform for angiogenic studies and drug testing in vitro. Here we describe in detail the protocol for: (1) the isolation of MVs from rat epididymal fat by limited collagenase digestion, followed by size-selective sieving; (2) the incorporation of MVs into 3D collagen hydrogels; (3) the in vitro culture of MVs in 3D gels for angiogenic studies; and (4) the in vivo transplantation of 3D hydrogels containing MVs into the mouse subcutis. The isolation procedure does not require highly specific equipment and can be performed in ~3 h by researchers with experience in rodent handling and cell culture.

A three-dimensional human adipocyte model of fatty acid-induced obesity

Obesity prevalence has reached pandemic proportions, leaving individuals at high risk for the development of diseases such as cancer and type 2 diabetes. In obesity, to accommodate excess lipid storage, adipocytes become hypertrophic, which is associated with an increased pro-inflammatory cytokine secretion and dysfunction of metabolic processes such as insulin signaling and lipolysis. Targeting adipocyte dysfunction is an important strategy to prevent the development of obesity-associated disease. However, it is unclear how accurately animal models reflect human biology, and the long-term culture of human hypertrophic adipocytes in an in vitro 2D monolayer is challenging due to the buoyant nature of adipocytes. Here we describe the development of a human 3D in vitro disease model that recapitulates hallmarks of obese adipocyte dysfunction. First, primary human adipose-derived mesenchymal stromal cells are embedded in hydrogel, and infiltrated into a thin cellulose scaffold. The thin microtissue profile allows for efficient assembly and image-based analysis. After adipocyte differentiation, the scaffold is stimulated with oleic or palmitic acid to mimic caloric overload. Using functional assays, we demonstrated that this treatment induced important obese adipocyte characteristics such as a larger lipid droplet size, increased basal lipolysis, insulin resistance and a change in macrophage gene expression through adipocyte-conditioned media. This 3D disease model mimics physiologically relevant hallmarks of obese adipocytes, to enable investigations into the mechanisms by which dysfunctional adipocytes contribute to disease.

Microvascular Dysfunction in Skeletal Muscle Precedes Myocardial Vascular Changes in Diabetic Cardiomyopathy: Sex-Dependent Differences

Aim

To uncover sex-related microvascular abnormalities that underlie the early presentation of reduced perfusion in leg skeletal muscle in a type II rat model of diabetic cardiomyopathy.

Methods and Results

Diabetes was induced using a non-obese, diet-based, low-dose streptozotocin model in adult female (18 diabetic, 9 control) and male rats (29 diabetic, 11 control). Time-course monitoring over 12 months following diabetes induction was performed using echocardiography, treadmill exercise, photoacoustic imaging, flow-mediated dilation (FMD), histopathology, and immunohistochemistry. Diabetic rats maintained normal weights. Hypertension appeared late in both diabetic males (7 months) and females (10 months), while only diabetic males had elevated cholesterol (7 months). On echocardiography, all diabetic animals maintained normal ejection fraction and exhibited diastolic dysfunction, mild systolic dysfunction, and a slightly enlarged left ventricle. Exercise tolerance declined progressively and early in males (4 months), later in females (8 months); FMD showed lower baseline femoral arterial flow but unchanged reactivity in both sexes (5 months); and photoacoustic imaging showed lower tissue oxygen saturation in the legs of diabetic males (4 months) and diabetic females (10 months). Myocardial perfusion was normal in both sexes. Histopathology at the final timepoint of Month 10 (males) and Month 12 (females) revealed that myocardial microvasculature was normal in both vessel density and structure, thus explaining normal perfusion on imaging. However, leg muscle microvasculature exhibited perivascular smooth muscle thickening around small arterioles in diabetic females and around large arterioles in diabetic males, explaining the depressed readings on photoacoustic and FMD. Histology also confirmed the absence of commonly reported HFpEF markers, including microvessel rarefaction, myocardial fibrosis, and left ventricular hypertrophy.

Conclusion

Exercise intolerance manifesting early in the progression of diabetic cardiomyopathy can be attributed to decreased perfusion to the leg skeletal muscle due to perivascular smooth muscle thickening around small arterioles in females and large arterioles in males. This microvascular abnormality was absent in the myocardium, where perfusion levels remained normal throughout the study. We conclude that although skeletal muscle microvascular dysfunction of the vasculature presents at different levels depending on sex, it consistently presents early in both sexes prior to overt cardiac changes such as rarefaction, fibrosis, or hypertrophy.

State of the field: cellular and exosomal therapeutic approaches in vascular regeneration

Pathologies of the vasculature including the microvasculature are often complex in nature, leading to loss of physiological homeostatic regulation of patency and adequate perfusion to match tissue metabolic demands. Microvascular dysfunction is a key underlying element in the majority of pathologies of failing organs and tissues. Contributing pathological factors to this dysfunction include oxidative stress, mitochondrial dysfunction, endoplasmic reticular (ER) stress, endothelial dysfunction, loss of angiogenic potential and vascular density, and greater senescence and apoptosis. In many clinical settings, current pharmacologic strategies use a single or narrow targeted approach to address symptoms of pathology rather than a comprehensive and multifaceted approach to address their root cause. To address this, efforts have been heavily focused on cellular therapies and cell-free therapies (e.g., exosomes) that can tackle the multifaceted etiology of vascular and microvascular dysfunction. In this review, we discuss 1) the state of the field in terms of common therapeutic cell population isolation techniques, their unique characteristics, and their advantages and disadvantages, 2) common molecular mechanisms of cell therapies to restore vascularization and/or vascular function, 3) arguments for and against allogeneic versus autologous applications of cell therapies, 4) emerging strategies to optimize and enhance cell therapies through priming and preconditioning, and, finally, 5) emerging strategies to bolster therapeutic effect. Relevant and recent clinical and animal studies using cellular therapies to restore vascular function or pathologic tissue health by way of improved vascularization are highlighted throughout these sections.

GP2-enriched pancreatic progenitors give rise to functional beta cells in vivo and eliminate the risk of teratoma formation

Human pluripotent stem cell (hPSC)-derived pancreatic progenitors (PPs) can be differentiated into beta-like cells in vitro and in vivo and therefore have therapeutic potential for type 1 diabetes (T1D) treatment. However, the purity of PPs varies across different hPSC lines, differentiation protocols, and laboratories. The uncommitted cells may give rise to non-pancreatic endodermal, mesodermal, or ectodermal derivatives in vivo, hampering the safety of hPSC-derived PPs for clinical applications and their differentiation efficiency in research settings. Recently, proteomics and transcriptomics analyses identified glycoprotein 2 (GP2) as a PP-specific cell surface marker. The GP2-enriched PPs generate higher percentages of beta-like cells in vitro, but their potential in vivo remains to be elucidated. Here, we demonstrate that the GP2-enriched-PPs give rise to all pancreatic cells in vivo, including functional beta-like cells. Remarkably, GP2 enrichment eliminates the risk of teratomas, which establishes GP2 sorting as an effective method for PP purification and safe pancreatic differentiation.

Microvessels support engraftment and functionality of human islets and hESC-derived pancreatic progenitors in diabetes models

Islet transplantation is a promising treatment for type 1 diabetes (T1D), yet the low donor pool, poor islet engraftment, and life-long immunosuppression prevent it from becoming the standard of care. Human embryonic stem cell (hESC)-derived pancreatic cells could eliminate donor shortages, but interventions to improve graft survival are needed. Here, we enhanced subcutaneous engraftment by employing a unique vascularization strategy based on ready-made microvessels (MVs) isolated from the adipose tissue. This resulted in improved cell survival and effective glucose response of both human islets and hESC-derived pancreatic cells, which ameliorated preexisting diabetes in three mouse models of T1D.

A 3-D human model of complex cardiac arrhythmias

Cardiac arrhythmias impact over 12 million people globally, with an increasing incidence of acquired arrhythmias. Although animal models have shed light onto fundamental arrhythmic mechanisms, species-specific differences and ethical concerns remain. Current human models using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) either lack the higher order tissue organization of the heart or implement unreliable arrhythmia induction techniques. Our goal was to develop a robust model of acquired arrhythmia by disrupting cardiomyocyte cell-cell signaling - one of the hallmarks of complex arrhythmias. Human 3D microtissues were generated by seeding hydrogel-embedded hiPSC-CMs and cardiac fibroblasts into an established microwell system designed to enable active and passive force assessment. Cell-cell signaling was disrupted using methyl-beta cyclodextrin (MBCD), previously shown to disassemble cardiac gap junctions. We demonstrate that arrhythmias were progressive and present in all microtissues within 5 days of treatment. Arrhythmic tissues exhibited reduced conduction velocity, an increased number of distinct action potentials, and reduced action potential cycle length. Arrhythmic tissues also showed significant reduction in contractile force generation, increased beating frequency, and increased passive tension and collagen deposition, in line with fibrosis. A subset of tissues with more complex arrhythmias exhibited 3D spatial differences in action potential propagation. Pharmacological and electrical defibrillation was successful. Transcriptomic data indicated an enrichment of genes consistent with cardiac arrhythmias. MBCD removal reversed the arrhythmic phenotype, resulting in synchronicity despite not resolving fibrosis. This innovative & reliable human-relevant 3D acquired arrhythmia model shows potential for improving our understanding of arrhythmic action potential conduction and furthering therapeutic development. STATEMENT OF SIGNIFICANCE: This work describes a 3D human model of cardiac arrhythmia-on-a-chip with high reproducibility, fidelity, and extensive functional applicability. To mimic in vivo conditions, human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and cardiac fibroblasts from healthy controls were combined in a biocompatible fibrin hydrogel and seeded between two deflectable polymeric rods. Using the innate functional properties of this 3D model as well as advanced optical imaging techniques we demonstrated dramatic changes in contraction rate, synchronicity, and electrophysiological conduction in arrhythmic tissues relative to controls. Taken together, these data demonstrate the distinctive potential of this new model for pathophysiological studies, and for arrhythmia drug testing applications.

Transplanted microvessels improve pluripotent stem cell-derived cardiomyocyte engraftment and cardiac function after infarction in rats

Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) offer an unprecedented opportunity to remuscularize infarcted human hearts. However, studies have shown that most hiPSC-CMs do not survive after transplantation into the ischemic myocardial environment, limiting their regenerative potential and clinical application. We established a method to improve hiPSC-CM survival by cotransplanting ready-made microvessels obtained from adipose tissue. Ready-made microvessels promoted a sixfold increase in hiPSC-CM survival and superior functional recovery when compared to hiPSC-CMs transplanted alone or cotransplanted with a suspension of dissociated endothelial cells in infarcted rat hearts. Microvessels showed unprecedented persistence and integration at both early (~80%, week 1) and late (~60%, week 4) time points, resulting in increased vessel density and graft perfusion, and improved hiPSC-CM maturation. These findings provide an approach to cell-based therapies for myocardial infarction, whereby incorporation of ready-made microvessels can improve functional outcomes in cell replacement therapies.

Generation of mature compact ventricular cardiomyocytes from human pluripotent stem cells

Compact cardiomyocytes that make up the ventricular wall of the adult heart represent an important therapeutic target population for modeling and treating cardiovascular diseases. Here, we established a differentiation strategy that promotes the specification, proliferation and maturation of compact ventricular cardiomyocytes from human pluripotent stem cells (hPSCs). The cardiomyocytes generated under these conditions display the ability to use fatty acids as an energy source, a high mitochondrial mass, well-defined sarcomere structures and enhanced contraction force. These ventricular cells undergo metabolic changes indicative of those associated with heart failure when challenged in vitro with pathological stimuli and were found to generate grafts consisting of more mature cells than those derived from immature cardiomyocytes following transplantation into infarcted rat hearts. hPSC-derived atrial cardiomyocytes also responded to the maturation cues identified in this study, indicating that the approach is broadly applicable to different subtypes of the heart. Collectively, these findings highlight the power of recapitulating key aspects of embryonic and postnatal development for generating therapeutically relevant cell types from hPSCs.

Cardiomyocytes of heart ventricles consist of subpopulations of trabecular and compact subtypes. Here the authors describe the generation of structurally, metabolically and functionally mature compact ventricular cardiomyocytes as well as mature atrial cardiomyocytes from human pluripotent stem cells.

Engineered human cardiac microtissues: The state-of-the-(he)art

Due to the integration of recent advances in stem cell biology, materials science, and engineering, the field of cardiac tissue engineering has been rapidly progressing toward developing more accurate functional 3D cardiac microtissues from human cell sources. These engineered tissues enable screening of cardiotoxic drugs, disease modeling (eg, by using cells from specific genetic backgrounds or modifying environmental conditions) and can serve as novel drug development platforms. This concise review presents the most recent advances and improvements in cardiac tissue formation, including cardiomyocyte maturation and disease modeling.

Human cardiac fibrosis-on-a-chip model recapitulates disease hallmarks and can serve as a platform for drug testing

While interstitial fibrosis plays a significant role in heart failure, our understanding of disease progression in humans is limited. To address this limitation, we have engineered a cardiac-fibrosis-on-a-chip model consisting of a microfabricated device with live force measurement capabilities using co-cultured human cardiac fibroblasts and pluripotent stem cell-derived cardiomyocytes. Transforming growth factor-β was used as a trigger for fibrosis. Here, we have reproduced the classic hallmarks of fibrosis-induced heart failure including high collagen deposition, increased tissue stiffness, BNP secretion, and passive tension. Force of contraction was significantly decreased in fibrotic tissues that displayed a transcriptomic signature consistent with human cardiac fibrosis/heart failure. Treatment with an anti-fibrotic drug decreased tissue stiffness and BNP secretion, with corresponding changes in the transcriptomic signature. This model represents an accessible approach to study human heart failure in vitro, and allows for testing anti-fibrotic drugs while facilitating the real-time assessment of cardiomyocyte function.

Endothelium-mediated contributions to fibrosis

Fibrosis, characterized by abnormal and excessive deposition of extracellular matrix, results in compromised tissue and organ structure. This can lead to reduced organ function and eventual failure. Although activated fibroblasts, called myofibroblasts, are considered the central players in fibrosis, the contribution of endothelial cells to the inception and progression of fibrosis has become increasingly recognized. Endothelial cells can contribute to fibrosis by acting as a source of myofibroblasts via endothelial-mesenchymal transition (EndoMT), or by becoming senescent, by secretion of profibrotic mediators and pro-inflammatory cytokines, chemokines and exosomes, promoting the recruitment of immune cells, and by participating in vascular rarefaction and decreased angiogenesis. In this review, we provide an overview of the different aspects of fibrosis in which endothelial cells have been implicated.

Correction to: Type I Diabetes Delays Perfusion and Engraftment of 3D Constructs by Impinging on Angiogenesis; Which can be Rescued by Hepatocyte Growth Factor Supplementation

[This corrects the article DOI: 10.1007/s12195-019-00574-3.].

Type I Diabetes Delays Perfusion and Engraftment of 3D Constructs by Impinging on Angiogenesis; Which can be Rescued by Hepatocyte Growth Factor Supplementation

Introduction

The biggest bottleneck for cell-based regenerative therapy is the lack of a functional vasculature to support the grafts. This problem is exacerbated in diabetic patients, where vessel growth is inhibited. To address this issue, we aim to identify the causes of poor vascularization in 3D engineered tissues in diabetes and to reverse its negative effects.

Methods

We used 3D vascularized constructs composed of microvessel fragments containing all cells present in the microcirculation, embedded in collagen type I hydrogels. Constructs were either cultured in vitro or implanted subcutaneously in non-diabetic or in a type I diabetic (streptozotocin-injected) mouse model. We used qPCR, ELISA, immunostaining, FACs and co-culture assays to characterize the effect of diabetes in engineered constructs.

Results

We demonstrated in 3D vascularized constructs that perivascular cells secrete hepatocyte growth factor (HGF), driving microvessel sprouting. Blockage of HGF or HGF receptor signaling in 3D constructs prevented vessel sprouting. Moreover, HGF expression in 3D constructs in vivo is downregulated in diabetes; while no differences were found in HGF receptor, VEGF or VEGF receptor expression. Low HGF expression in diabetes delayed the inosculation of graft and host vessels, decreasing blood perfusion and preventing tissue engraftment. Supplementation of HGF in 3D constructs, restored vessel sprouting in a diabetic milieu.

Conclusion

We show for the first time that diabetes affects HGF secretion in microvessels, which in turn prevents the engraftment of engineered tissues. Exogenous supplementation of HGF, restores angiogenic growth in 3D constructs showing promise for application in cell-based regenerative therapies.

Blood Vessel Maturation in Health and Disease and its Implications for Vascularization of Engineered Tissues

Engineered blood vessels have often been found to be immature and unstable. Similarly, numerous pathologies such as diabetic retinopathy and cancer are characterized by highly abnormal, defective, hypervascular networks, consisting of immature, leaky, and irregular vessels with a marked loss of perivascular cell coverage. An emerging therapeutic concept in treatment of such vascular diseases and their management is the potential to normalize blood vessels by strengthening the cellular components that form the vascular network. Vessel normalization is characterized by the reduction in the number and size of immature vessels, a decrease in interstitial fluid pressure, and increase in perivascular cell coverage. Understanding the molecular and cellular defects associated with abnormal blood vessels will allow us to find appropriate treatment options that can promote normal blood vessel development. These, in turn, can be applied to improve vessel maturation in engineered tissues. In this review, we describe the major perivascular abnormalities associated with various human diseases and engineered vasculatures and the major advances in obtaining mature vasculatures for translational applications.

Maturation of Human Stem Cell-derived Cardiomyocytes in Biowires Using Electrical Stimulation

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) have been a promising cell source and have thus encouraged the investigation of their potential applications in cardiac research, including drug discovery, disease modeling, tissue engineering, and regenerative medicine. However, cells produced by existing protocols display a range of immaturity compared with native adult ventricular cardiomyocytes. Many efforts have been made to mature hPSC-CMs, with only moderate maturation attained thus far. Therefore, an engineered system, called biowire, has been devised by providing both physical and electrical cues to lead hPSC-CMs to a more mature state in vitro. The system uses a microfabricated platform to seed hPSC-CMs in collagen type I gel along a rigid template suture to assemble into aligned cardiac tissue (biowire), which is subjected to electrical field stimulation with a progressively increasing frequency. Compared to nonstimulated controls, stimulated biowired cardiomyocytes exhibit an enhanced degree of structural and electrophysiological maturation. Such changes are dependent upon the stimulation rate. This manuscript describes in detail the design and creation of biowires.

Bioengineering Approaches to Mature Human Pluripotent Stem Cell-Derived Cardiomyocytes

Human pluripotent stem cell-derived cardiomyocytes (hPSC-CM) represent a potential unlimited cell supply for cardiac tissue engineering and possibly regenerative medicine applications. However, hPSC-CMs produced by current protocols are not representative of native adult human cardiomyocytes as they display immature gene expression profile, structure and function. In order to improve hPSC-CM maturity and function, various approaches have been developed, including genetic manipulations to induce gene expression, delivery of biochemical factors, such as triiodothyronine and alpha-adrenergic agonist phenylephrine, induction of cell alignment in 3D tissues, mechanical stress as a mimic of cardiac load and electrical stimulation/pacing or a combination of these. In this mini review, we discuss biomimetic strategies for the maturation for hPSC-CMs with a particular focus on electromechanical conditioning methods.

Diabetes impairs arterio-venous specification in engineered vascular tissues in a perivascular cell recruitment-dependent manner

Cell-based tissue engineering is a potential treatment alternative for organ replacement. However, the lack of a robust vasculature, especially in the context of diseases such as diabetes, is a major hindrance to its success. Despite extensive research on the effects of diabetes in angiogenic sprouting, its effects on vessel arterio-venous (AV) specification have not been addressed. Using an engineered tissue that yields functional vessels with characteristic AV identities, we demonstrate that type 1 diabetes negatively affects vessel AV specification and perivascular cell (PVC) coverage. Blockage of PVC recruitment in normoglycemia does not affect blood flow parameters, but recapitulates the vascular immaturity found in diabetes, suggesting a role for PVCs in AV specification. The downregulation of Jagged1 and Notch3, key modulators of endothelial-perivascular interaction, observed in diabetes support this assertion. Co-culture assays indicate that PVCs induce arterial identity specification by inducing EphrinB2 and downregulating EphB4. This is antagonized by high glucose or blockage of endothelial Jagged1. Engineered tissues composed of microvessels from diabetic mice display normal PVC coverage and Jagged1/Notch3 gene expression when implanted into non-diabetic hosts. These indicate a lack of legacy effect and support the use of a more aggressive treatment of diabetes in patients undergoing revascularization therapies.

Biodegradable scaffold with built-in vasculature for organ-on-a-chip engineering and direct surgical anastomosis

We report the fabrication of a scaffold (hereafter referred to as AngioChip) that supports the assembly of parenchymal cells on a mechanically tunable matrix surrounding a perfusable, branched, three-dimensional microchannel network coated with endothelial cells. The design of AngioChip decouples the material choices for the engineered vessel network and for cell seeding in the parenchyma, enabling extensive remodelling while maintaining an open-vessel lumen. The incorporation of nanopores and micro-holes in the vessel walls enhances permeability, and permits intercellular crosstalk and extravasation of monocytes and endothelial cells on biomolecular stimulation. We also show that vascularized hepatic tissues and cardiac tissues engineered by using AngioChips process clinically relevant drugs delivered through the vasculature, and that millimetre-thick cardiac tissues can be engineered in a scalable manner. Moreover, we demonstrate that AngioChip cardiac tissues implanted with direct surgical anastomosis to the femoral vessels of rat hindlimbs establish immediate blood perfusion.

Vascularization strategies of engineered tissues and their application in cardiac regeneration

The primary function of vascular networks is to transport blood and deliver oxygen and nutrients to tissues, which occurs at the interface of the microvasculature. Therefore, the formation of the vessels at the microcirculatory level, or angiogenesis, is critical for tissue regeneration and repair. Current strategies for vascularization of engineered tissues have incorporated multi-disciplinary approaches including engineered biomaterials, cells and angiogenic factors. Pre-vascularization of scaffolds composed of native matrix, synthetic polymers, or other biological materials can be achieved through the use of single cells in mono or co-culture, in combination or not with angiogenic factors or by the use of isolated vessels. The advance of these methods, together with a growing understanding of the biology behind vascularization, has facilitated the development of vascularization strategies for engineered tissues with therapeutic potential for tissue regeneration and repair. Here, we review the different cell-based strategies utilized to pre-vascularize engineered tissues and in making more complex vascularized cardiac tissues for regenerative medicine applications.

Overview of hydrogel-based strategies for application in cardiac tissue regeneration

Cardiovascular diseases remain the leading cause of death globally. Since the adult heart lacks the capacity to regenerate, loss of myocardium following myocardial infarction is irreversible and ultimately leads to failure to maintain cardiac function. In order to repopulate the areas of cell loss in the damaged hearts for restoration of cardiac function, cell transplantation/replacement has been extensively investigated. Recently, biomaterials have emerged as an approach to improve delivery and viability of cells for the regeneration of the damaged heart. Here we review the most common approaches in hydrogel-based cardiac tissue regeneration with particular focus on the implementation of hydrogels to improve cell delivery.

The role of tissue engineering and biomaterials in cardiac regenerative medicine

In recent years, the development of 3-dimensional engineered heart tissue (EHT) has made large strides forward because of advances in stem cell biology, materials science, prevascularization strategies, and nanotechnology. As a result, the role of tissue engineering in cardiac regenerative medicine has become multifaceted as new applications become feasible. Cardiac tissue engineering has long been established to have the potential to partially or fully restore cardiac function after cardiac injury. However, EHTs may also serve as surrogate human cardiac tissue for drug-related toxicity screening. Cardiotoxicity remains a major cause of drug withdrawal in the pharmaceutical industry. Unsafe drugs reach the market because preclinical evaluation is insufficient to weed out cardiotoxic drugs in all their forms. Bioengineering methods could provide functional and mature human myocardial tissues, ie, physiologically relevant platforms, for screening the cardiotoxic effects of pharmaceutical agents and facilitate the discovery of new therapeutic agents. Finally, advances in induced pluripotent stem cells have made patient-specific EHTs possible, which opens up the possibility of personalized medicine. Herein, we give an overview of the present state of the art in cardiac tissue engineering, the challenges to the field, and future perspectives.

Bioreactor for modulation of cardiac microtissue phenotype by combined static stretch and electrical stimulation

We describe here a bioreactor capable of applying electrical field stimulation in conjunction with static strain and on-line force of contraction measurements. It consisted of a polydimethylsiloxane (PDMS) tissue chamber and a pneumatically driven stretch platform. The chamber contained eight tissue microwells (8.05 mm in length and 2.5 mm in width) with a pair of posts (2.78 mm in height and 0.8 mm in diameter) in each well to serve as fixation points and for measurements of contraction force. Carbon rods, stimulating electrodes, were placed into the PDMS chamber such that one pair stimulated four microwells. For feasibility studies, neonatal rat cardiomyocytes were seeded in collagen gels into the microwells. Following 3 days of gel compaction, electrical field stimulation at 3-4 V cm(-1) and 1 Hz, mechanical stimulation of 5% static strain or electromechanical stimulation (field stimulation at 3-4 V cm(-1), 1 Hz and 5% static strain) were applied for 3 days. Cardiac microtissues subjected to electromechanical stimulation exhibited elevated amplitude of contraction and improved sarcomere structure as evidenced by sarcomeric α-actinin, actin and troponin T staining compared to microtissues subjected to electrical or mechanical stimulation alone or non-stimulated controls. The expression of atrial natriuretic factor and brain natriuretic peptide was also elevated in the electromechanically stimulated group.

Design and fabrication of biological wires

Cardiac tissue engineering using human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) has facilitated the creation of in vitro diagnostic platforms to study novel small molecules and cardiac disease at the tissue level. Yet, due to the immaturity of hPSC-CMs, there is a low fidelity between tissue-engineered cardiac tissues and adult cardiac tissues. To address this challenge, we have developed a platform that combines both physical and electrical cues to guide hPSC-CMs towards a more mature state in vitro.

Generation of a functional liver tissue mimic using adipose stromal vascular fraction cell-derived vasculatures

One of the major challenges in cell implantation therapies is to promote integration of the microcirculation between the implanted cells and the host. We used adipose-derived stromal vascular fraction (SVF) cells to vascularize a human liver cell (HepG2) implant. We hypothesized that the SVF cells would form a functional microcirculation via vascular assembly and inosculation with the host vasculature. Initially, we assessed the extent and character of neovasculatures formed by freshly isolated and cultured SVF cells and found that freshly isolated cells have a higher vascularization potential. Generation of a 3D implant containing fresh SVF and HepG2 cells formed a tissue in which HepG2 cells were entwined with a network of microvessels. Implanted HepG2 cells sequestered labeled LDL delivered by systemic intravascular injection only in SVF-vascularized implants demonstrating that SVF cell-derived vasculatures can effectively integrate with host vessels and interface with parenchymal cells to form a functional tissue mimic.

Fusible core molding for the fabrication of branched, perfusable, three-dimensional microvessels for vascular tissue engineering

A novel method for fabrication of branched, tubular, perfusable microvessels for use in vascular tissue engineering is reported. A tubular, elastomeric, biodegradable scaffold is first fabricated via a new, double fusible injection molding technique that uses a ternary alloy with a low melting temperature, Field's metal, and paraffin as sacrificial components. A cylindrical core metal of 500 μm or lower dia-meter with the target branching scaffold geometry is first constructed, then the metal structure is coated with paraffin and, finally, the metal-paraffin construct is embedded in polydimethylsiloxane (PDMS). The paraffin layer is then removed by heating and replaced by a biodegradable elastomeric pre-polymer that is subsequently UV-cured inside the PDMS. Next, the metal core is melted away and the PDMS is removed to attain the branched tubular elastomeric biodegradable scaffold. Finally, it is also demonstrated that human umbilical vein endothelial cells (HUVEC) were able to spread on the surface of the scaffold and form a confluent monolayer, confirming the potential of this new technique for making engineered blood vessels.

Topological and electrical control of cardiac differentiation and assembly

Tissue engineering has developed many paradigms and techniques on how to best integrate cells and extracellular matrix to create in vitro structures that replicate native tissue. The strategy best suited for building these constructs depends mainly on the target cells, tissues, and organ of interest, and how readily their respective niches can be recapitulated in vitro with available technologies. In this review we examine engineered heart tissue and two techniques that can be used to induce tissue morphogenesis in artificial niches in vitro: engineered surface topology and electrical control of the system. For both the differentiation of stem cells into heart cells and further assembly of these cells into engineered tissues, these two techniques are effective in inducing in vivo like structure and function. Biophysical modulation through the control of topography and manipulation of the electrical microenvironment has been shown to have effects on cell growth and differentiation, expression of mature cardiac-related proteins and genes, cell alignment via cytoskeletal organization, and electrical and contractile properties. Lastly, we discuss the evolution and potential of these techniques, and bridges to regenerative therapies.

Manipulating the microvasculature and its microenvironment

The microvasculature is a dynamic cellular system necessary for tissue health and function. Therapeutic strategies that target the microvasculature are expanding and evolving, including those promoting angiogenesis and microvascular expansion. When considering how to manipulate angiogenesis, either as part of a tissue construction approach or a therapy to improve tissue blood flow, it is important to know the microenvironmental factors that regulate and direct neovessel sprouting and growth. Much is known concerning both diffusible and matrix-bound angiogenic factors, which stimulate and guide angiogenic activity. How the other aspects of the extravascular microenvironment, including tissue biomechanics and structure, influence new vessel formation is less well known. Recent research, however, is providing new insights into these mechanisms and demonstrating that the extent and character of angiogenesis (and the resulting new microcirculation) is significantly affected. These observations and the resulting implications with respect to tissue construction and microvascular therapy are addressed.

Dissecting the role of human embryonic stem cell-derived mesenchymal cells in human umbilical vein endothelial cell network stabilization in three-dimensional environments

The microvasculature is principally composed of two cell types: endothelium and mural support cells. Multiple sources are available for human endothelial cells (ECs) but sources for human microvascular mural cells (MCs) are limited. We derived multipotent mesenchymal progenitor cells from human embryonic stem cells (hES-MC) that can function as an MC and stabilize human EC networks in three-dimensional (3D) collagen-fibronectin culture by paracrine mechanisms. Here, we have investigated the basis for hES-MC-mediated stabilization and identified the pleiotropic growth factor hepatocyte growth factor/scatter factor (HGF/SF) as a putative hES-MC-derived regulator of EC network stabilization in 3D in vitro culture. Pharmacological inhibition of the HGF receptor (Met) (1 μm SU11274) inhibits EC network formation in the presence of hES-MC. hES-MC produce and release HGF while human umbilical vein endothelial cells (HUVEC) do not. When HUVEC are cultured alone the networks collapse, but in the presence of recombinant human HGF or conditioned media from human HGF-transduced cells significantly more networks persist. In addition, HUVEC transduced to constitutively express human HGF also form stable networks by autocrine mechanisms. By enzyme-linked immunosorbent assay, the coculture media were enriched in both angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2), but at significantly different levels (Ang1=159±15 pg/mL vs. Ang2=30,867±2685 pg/mL) contributed by hES-MC and HUVEC, respectively. Although the coculture cells formed stabile network architectures, their morphology suggests the assembly of an immature plexus. When HUVEC and hES-MC were implanted subcutaneously in immune compromised Rag1 mice, hES-MC increased their contact with HUVEC along the axis of the vessel. This data suggests that HUVEC and hES-MC form an immature plexus mediated in part by HGF and angiopoietins that is capable of maturation under the correct environmental conditions (e.g., in vivo). Therefore, hES-MC can function as microvascular MCs and may be a useful cell source for testing EC-MC interactions.

Cardiac tissue engineering: current state and perspectives

The goal of cardiac tissue engineering is to treat cardiovascular diseases through the implantation of engineered functional tissue replacements or the injection of cells and biomaterials, as well as to provide engineered cardiac constructs that can be used as an in vitro model of healthy or diseased heart tissues. This field is rapidly advancing with the new discoveries and improvements in stem cell technologies, materials science, and bioreactor design. In this review, some of the progress made in cardiac tissue engineering in the recent years, as well as the challenges that need to be overcome in future studies, will be discussed. The topics include the advances in engineering stem cell-derived cardiac tissues, the use of natural or synthetic polymers and decellularized organs as engineering scaffolds, the scaffold-free cell sheet engineering approach, the application of perfusion and mechanical or electrical stimulation in bioreactors, the organization of cardiac cells through microfabrication techniques, and the vascularization of engineered cardiac tissues in vitro and in vivo.

Determinants of microvascular network topologies in implanted neovasculatures

OBJECTIVE

During neovascularization, the end result is a new functional microcirculation composed of a network of mature microvessels with specific topologies. Although much is known concerning the mechanisms underlying the initiation of angiogenesis, it remains unclear how the final architecture of microcirculatory beds is regulated. To begin to address this, we determined the impact of angiogenic neovessel prepatterning on the final microvascular network topology using a model of implant neovascularization.

METHODS AND RESULTS

We used 3D direct-write bioprinting or physical constraints in a manner permitting postangiogenesis vascular remodeling and adaptation to pattern angiogenic microvascular precursors (neovessels formed from isolated microvessel segments) in 3D collagen gels before implantation and subsequent network formation. Neovasculatures prepatterned into parallel arrays formed functional networks after 4 weeks postimplantation but lost the prepatterned architecture. However, maintenance of uniaxial physical constraints during postangiogenesis remodeling of the implanted neovasculatures produced networks with aligned microvessels, as well as an altered proportional distribution of arterioles, capillaries, and venules.

CONCLUSIONS

Here we show that network topology resulting from implanted microvessel precursors is independent from prepatterning of precursors but can be influenced by a patterning stimulus involving tissue deformation during postangiogenesis remodeling and maturation.

Stem cell-based cardiac tissue engineering

Cardiovascular diseases are the leading cause of death worldwide, and cell-based therapies represent a potential cure for patients with cardiac diseases such as myocardial infarction, heart failure, and congenital heart diseases. Towards this goal, cardiac tissue engineering is now being investigated as an approach to support cell-based therapies and enhance their efficacy. This review focuses on the latest research in cardiac tissue engineering based on the use of embryonic, induced pluripotent, or adult stem cells. We describe different strategies such as direct injection of cells and/or biomaterials as well as direct replacement therapies with tissue mimics. In this regard, the latest research has shown promising results demonstrating the improvement of cardiac function with different strategies. It is clear from recent studies that the most important consideration to be addressed by new therapeutic strategies is long-term functional improvement. For this goal to be realized, novel and efficient methods of cell delivery are required that enable high cell retention, followed by electrical integration and mechanical coupling of the injected cells or the engineered tissue to the host myocardium.

Microvascular mural cell functionality of human embryonic stem cell-derived mesenchymal cells

Microvascular mural or perivascular cells are required for the stabilization and maturation of the remodeling vasculature. However, much less is known about their biology and function compared to large vessel smooth muscle cells. We have developed lines of multipotent mesenchymal cells from human embryonic stem cells (hES-MC); we hypothesize that these can function as perivascular mural cells. Here we show that the derived cells do not form teratomas in SCID mice and independently derived lines show similar patterns of gene expression by microarray analysis. When exposed to platelet-derived growth factor-BB, the platelet-derived growth factor receptor β is activated and hES-MC migrate in response to a gradient. We also show that in a serum-free medium, transforming growth factor β1 (TGFβ1) induces robust expression of multiple contractile proteins (α smooth muscle actin, smooth muscle myosin heavy chain, smooth muscle 22α, and calponin). TGFβ1 signaling is mediated through the TGFβR1/Alk5 pathway as demonstrated by inhibition of α smooth muscle actin expression by treatment of the Alk5-specific inhibitor SB525334 and stable retroviral expression of the Alk5 dominant negative (K232R). Coculture of human umbilical vein endothelial cell (HUVEC) with hES-MC maintains network integrity compared to HUVEC alone in three-dimensional collagen I-fibronectin by paracrine signaling. Using high-resolution laser confocal microscopy, we show that hES-MC also make direct contact with HUVEC. This demonstrates that hESC-derived mesenchymal cells possess the molecular machinery expected in a perivascular progenitor cells and can play a functional role in stabilizing EC networks in in vitro three-dimensional culture.

Angiogenic potential of microvessel fragments is independent of the tissue of origin and can be influenced by the cellular composition of the implants

We have demonstrated that MFs isolated from adipose retain angiogenic potential in vitro and form a mature, perfused network when implanted. However, adipose-derived microvessels are rich in provascularizing cells that could uniquely drive neovascularization in adipose-derived MFs implants.

OBJECTIVE

Investigate the ability of MFs from a different vascular bed to recapitulate adipose-derived microvessel angiogenesis and network formation and analyze adipose-derived vessel plasticity by assessing whether vessel function could be modulated by astrocyte-like cells.

METHODS

MFs were isolated by limited collagenase digestion from rodent brain or adipose and assembled into 3D collagen gels in the presence or absence of GRPs. The resulting neovasculatures that formed following implantation were assessed by measuring 3D vascularity and vessel permeability to small and large molecular tracers.

RESULTS

Similar to adipose-derived MFs, brain-derived MFs can sprout and form a perfused neovascular network when implanted. Furthermore, when co-implanted in the constructs, GRPs caused adipose-derived vessels to express the brain endothelial marker glucose transporter-1 and to significantly reduce microvessel permeability.

CONCLUSION

Neovascularization involving isolated microvessel elements is independent of the tissue origin and degree of vessel specialization. In addition, adipose-derived vessels have the ability to respond to environmental signals and change vessel characteristics.

Angiogenesis in a microvascular construct for transplantation depends on the method of chamber circulation

Effective tissue prevascularization depends on new vessel growth and subsequent progression of neovessels into a stable microcirculation. Isolated microvessel fragments in a collagen-based microvascular construct (MVC) spontaneously undergo angiogenesis in static conditions in vitro but form a new microcirculation only when implanted in vivo. We have designed a bioreactor, the dynamic in vitro perfusion (DIP) chamber, to culture MVCs in vitro with perfusion. By altering bioreactor circulation, microvessel fragments in the DIP chamber either maintained stable, nonsprouting, patent vessel morphologies or sprouted endothelial neovessels that extended out into the surrounding collagen matrix (i.e., angiogenesis), yielding networks of neovessels within the MVC. Neovessels formed in regions of the construct predicted by simulation models to have the steepest gradients in oxygen levels and expressed hypoxia inducible factor-1alpha. By altering circulation conditions in the DIP chamber, we can control, possibly by modulating hypoxic stress, prevascularizing activity in vitro.

Implanted microvessels progress through distinct neovascularization phenotypes

We have previously demonstrated that implanted microvessels form a new microcirculation with minimal host-derived vessel investment. Our objective was to define the vascular phenotypes present during neovascularization in these implants and identify post-angiogenesis events. Morphological, functional and transcriptional assessments identified three distinct vascular phenotypes in the implants: sprouting angiogenesis, neovascular remodeling, and network maturation. A sprouting angiogenic phenotype appeared first, characterized by high proliferation and low mural cell coverage. This was followed by a neovascular remodeling phenotype characterized by a perfused, poorly organized neovascular network, reduced proliferation, and re-associated mural cells. The last phenotype included a vascular network organized into a stereotypical tree structure containing vessels with normal perivascular cell associations. In addition, proliferation was low and was restricted to the walls of larger microvessels. The transition from angiogenesis to neovascular remodeling coincided with the appearance of blood flow in the implant neovasculature. Analysis of vascular-specific and global gene expression indicates that the intermediate, neovascular remodeling phenotype is transcriptionally distinct from the other two phenotypes. Therefore, this vascular phenotype likely is not simply a transitional phenotype but a distinct vascular phenotype involving unique cellular and vascular processes. Furthermore, this neovascular remodeling phase may be a normal aspect of the general neovascularization process. Given that this phenotype is arguably dysfunctional, many of the microvasculatures present within compromised or diseased tissues may not represent a failure to progress appropriately through a normally occurring neovascularization phenotype.