Endothelialization of Heart Valve Matrix Using a Computer-Assisted Pulsatile Bioreactor

Angiogenesis and Re-endothelialization in decellularized scaffolds: Recent advances and current challenges in tissue engineering

Decellularization of tissues and organs has recently become a promising approach in tissue engineering and regenerative medicine to circumvent the challenges of organ donation and complications of transplantations. However, one main obstacle to reaching this goal is acellular vasculature angiogenesis and endothelialization. Achieving an intact and functional vascular structure as a vital pathway for supplying oxygen and nutrients remains the decisive challenge in the decellularization/re-endothelialization procedure. In order to better understand and overcome this issue, complete and appropriate knowledge of endothelialization and its determining variables is required. Decellularization methods and their effectiveness, biological and mechanical characteristics of acellular scaffolds, artificial and biological bioreactors, and their possible applications, extracellular matrix surface modification, and different types of utilized cells are factors affecting endothelialization consequences. This review focuses on the characteristics of endothelialization and how to optimize them, as well as discussing recent developments in the process of re-endothelialization.

Future prospects in the tissue engineering of heart valves: a focus on the role of stem cells

INTRODUCTION

Heart valve disease is a growing burden on the healthcare system. Current solutions are insufficient for young patients and do not offer relief from reintervention. Tissue engineered heart valves (TEHVs) offer a solution that grows and responds to the native environment in a similar way to a healthy valve. Stem cells hold potential to populate these valves as a malleable source that can adapt to environmental cues.

AREAS COVERED

This review covers current methods of recapitulating features of native heart valves with tissue engineering through use of stem cell populations with in situ and in vitro methods.

EXPERT OPINION

In the field of TEHVs, we see a variety of approaches in cell source, biomaterial, and maturation methods. Choosing appropriate cell populations may be very patient specific; consistency and predictability will be key to long-term success. In situ methods are closer to translation but struggle with consistent cellularization. In vitro culture requires specialized methods but may recapitulate native valve cell populations with higher fidelity. Understanding how cell populations react to valve conditions and immune response is vital for success. Detrimental valve pathologies have proven to be difficult to avoid in early translation attempts.

3D Tissue-Engineered Vascular Drug Screening Platforms: Promise and Considerations

Despite the efforts devoted to drug discovery and development, the number of new drug approvals have been decreasing. Specifically, cardiovascular developments have been showing amongst the lowest levels of approvals. In addition, concerns over the adverse effects of drugs to the cardiovascular system have been increasing and resulting in failure at the preclinical level as well as withdrawal of drugs post-marketing. Besides factors such as the increased cost of clinical trials and increases in the requirements and the complexity of the regulatory processes, there is also a gap between the currently existing pre-clinical screening methods and the clinical studies in humans. This gap is mainly caused by the lack of complexity in the currently used 2D cell culture-based screening systems, which do not accurately reflect human physiological conditions. Cell-based drug screening is widely accepted and extensively used and can provide an initial indication of the drugs' therapeutic efficacy and potential cytotoxicity. However, in vitro cell-based evaluation could in many instances provide contradictory findings to the in vivo testing in animal models and clinical trials. This drawback is related to the failure of these 2D cell culture systems to recapitulate the human physiological microenvironment in which the cells reside. In the body, cells reside within a complex physiological setting, where they interact with and respond to neighboring cells, extracellular matrix, mechanical stress, blood shear stress, and many other factors. These factors in sum affect the cellular response and the specific pathways that regulate variable vital functions such as proliferation, apoptosis, and differentiation. Although pre-clinical in vivo animal models provide this level of complexity, cross species differences can also cause contradictory results from that seen when the drug enters clinical trials. Thus, there is a need to better mimic human physiological conditions in pre-clinical studies to improve the efficiency of drug screening. A novel approach is to develop 3D tissue engineered miniaturized constructs in vitro that are based on human cells. In this review, we discuss the factors that should be considered to produce a successful vascular construct that is derived from human cells and is both reliable and reproducible.

Strategies for improving endothelial cell adhesion to blood-contacting medical devices

The endothelium is a critical mediator of homeostasis on blood-contacting surfaces in the body, serving as a selective barrier to regulate processes such as clotting, immune cell adhesion, and cellular response to fluid shear stress. Implantable cardiovascular devices including stents, vascular grafts, heart valves, and left ventricular assist devices are in direct contact with circulating blood and carry a high risk for platelet activation and thrombosis without a stable endothelial cell (EC) monolayer. Development of a healthy endothelium on the blood-contacting surface of these devices would help ameliorate risks associated with thrombus formation and eliminate the need for long-term anti-platelet or anti-coagulation therapy. Although ECs have been seeded onto or recruited to these blood-contacting surfaces, most ECs are lost upon exposure to shear stress due to circulating blood. Many investigators have attempted to generate a stable EC monolayer by improving EC adhesion using surface modifications, material coatings, nanofiber topology, and modifications to the cells. Despite some success with enhanced EC retention in vitro and in animal models, no studies to date have proven efficacious for routinely creating a stable endothelium in the clinical setting. This review summarizes past and present techniques directed at improving the adhesion of ECs to blood-contacting devices.

Solid Organ Bioprinting: Strategies to Achieve Organ Function

The field of tissue engineering has advanced over the past decade, but the largest impact on human health should be achieved with the transition of engineered solid organs to the clinic. The number of patients suffering from solid organ disease continues to increase, with over 100 000 patients on the U.S. national waitlist and approximately 730 000 deaths in the United States resulting from end-stage organ disease annually. While flat, tubular, and hollow nontubular engineered organs have already been implanted in patients, in vitro formation of a fully functional solid organ at a translatable scale has not yet been achieved. Thus, one major goal is to bioengineer complex, solid organs for transplantation, composed of patient-specific cells. Among the myriad of approaches attempted to engineer solid organs, 3D bioprinting offers unmatched potential. This review highlights the structural complexity which must be engineered at nano-, micro-, and mesostructural scales to enable organ function. We showcase key advances in bioprinting solid organs with complex vascular networks and functioning microstructures, advances in biomaterials science that have enabled this progress, the regulatory hurdles the field has yet to overcome, and cutting edge technologies that bring us closer to the promise of engineered solid organs.

Vascular reconstruction: A major challenge in developing a functional whole solid organ graft from decellularized organs

Bioengineering a functional organ holds great potential to overcome the current gap between the organ need and shortage of available organs. Whole organ decellularization allows the removal of cells from large-scale organs, leaving behind extracellular matrices containing different growth factors, structural proteins, and a vascular network with a bare surface. Successful application of decellularized tissues as transplantable organs is hampered by the inability to completely reline the vasculature by endothelial cells (ECs), leading to blood coagulation, loss of vascular patency, and subsequent death of reseeded cells. Therefore, an intact, continuous layer of endothelium is essential to maintain proper functioning of the vascular system, which includes the transfer of nutrients to surrounding tissues and protecting other types of cells from shear stress. Here, we aimed to summarize the available cell sources that can be used for reendothelialization in addition to different trials performed by researchers to reconstruct vascularization of decellularized solid organs. Additionally, different techniques for enhancing reendothelialization and the methods used for evaluating reendothelialization efficiency along with the future prospective applications of this field are discussed. STATEMENT OF SIGNIFICANCE: Despite the great progress in whole organ decellularization, reconstruction of vasculature within the engineered constructs is still a major roadblock. Reconstructed endothelium acts as a multifunctional barrier of vessels, which can reduce thrombosis and help delivering of oxygen and nutrients throughout the whole organ. Successful reendothelialization can be achieved through reseeding of appropriate cell types on the naked vasculature with or without modification of its surface. Here, we present the current research milestones that so far established to reconstruct the vascular network in addition to the methods used for evaluating the efficiency of reendotheilization. Thus, this review is quite significant and will aid the researchers to know where we stand toward biofabricating a transplantable organ from decellularizd extracellular matrix.

Endothelialization of cardiovascular devices

Blood-contacting surfaces of cardiovascular devices are not biocompatible for creating an endothelial layer on them. Numerous research studies have mainly sought to modify these surfaces through physical, chemical and biological means to ease early endothelial cell (EC) adhesion, migration and proliferation, and eventually to build an endothelial layer on the surfaces. The first priority for surface modification is inhibition of protein adsorption that leads to inhibition of platelet adhesion to the device surfaces, which may favor EC adhesion. Surface modification through surface texturing, if applicable, can bring some hopeful outcomes in this regard. Surface modifications through chemical and/or biological means may play a significant role in easy endothelialization of cardiovascular devices and inhibit smooth muscle cell proliferation. Cellular engineering of cells relevant to endothelialization can boost the positive outcomes obtained through surface engineering. This review briefly summarizes recent developments and research in early endothelialization of cardiovascular devices. STATEMENT OF SIGNIFICANCE: Endothelialization of cardiovascular implants, including heart valves, vascular stents and vascular grafts is crucial to solve many problems in our health care system. Numerous research efforts have been made to improve endothelialization on the surfaces of cardiovascular implants, mainly through surface modifications in three ways - physically, chemically and biologically. This review is intended to highlight comprehensive research studies to date on surface modifications aiming for early endothelialization on the blood-contacting surfaces of cardiovascular implants. It also discusses future perspectives to help guide endothelialization strategies and inspire further innovations.

Ectopic mineralization in heart valves: new insights from in vivo and in vitro procalcific models and promising perspectives on noncalcifiable bioengineered valves

Ectopic calcification of native and bioprosthetic heart valves represents a major public health problem causing severe morbidity and mortality worldwide. Valve procalcific degeneration is known to be caused mainly by calcium salt precipitation onto membranes of suffering non-scavenged cells and dead-cell-derived products acting as major hydroxyapatite nucleators. Although etiopathogenesis of calcification in native valves is still far from being exhaustively elucidated, it is well known that bioprosthesis mineralization may be primed by glutaraldehyde-mediated toxicity for xenografts, cryopreservation-related damage for allografts and graft immune rejection for both. Instead, mechanical valves, which are free from calcification, are extremely thrombogenic, requiring chronic anticoagulation therapies for transplanted patients. Since surgical substitution of failed valves is still the leading therapeutic option, progressive improvements in tissue engineering techniques are crucial to attain readily available valve implants with good biocompatibility, proper functionality and long-term durability in order to meet the considerable clinical demand for valve substitutes. Bioengineered valves obtained from acellular non-valvular scaffolds or decellularized native valves are proving to be a compelling alternative to mechanical and bioprosthetic valve implants, as they appear to permit repopulation by the host's own cells with associated tissue remodelling, growth and repair, besides showing less propensity to calcification and adequate hemodynamic performances. In this review, insights into valve calcification onset as revealed by in vivo and in vitro procalcific models are updated as well as advances in the field of valve bioengineering.

Electrospun polycaprolactone (PCL) scaffolds embedded with europium hydroxide nanorods (EHNs) with enhanced vascularization and cell proliferation for tissue engineering applications

PCL-EHNs scaffolds enhance endothelial cell proliferation, adhesion and blood vessel formation in a VEGFR2/Akt dependent signaling cascade.

Characterization of CD133 Antibody-Directed Recellularized Heart Valves

CD133mAb conjugation (CD133-C) hastens in vivo recellularization of decellularized porcine heart valve scaffolds when placed in the pulmonary position of sheep. We now characterize this early cellularization process 4 h, 3, 7, 14, 30, or 90 days post-implantation. Quantitative immunohistochemistry identified cell types as well as changes in cell markers and developmental cues. CD133(+)/CD31(-) cells adhered to the leaflet surface of CD133-C leaflets by 3 days and transitioned to native leaflet-like CD133(-)/CD31(+) cells by 30 days. Leaflet interstitium became increasingly populated with both alpha-smooth muscle actin (αSMA) and vimentin(+) cells from 14 to 90 days post-implantation. Wnt3a, and beta-catenin proteins were expressed at early (3-14 days) but not later (30-90 days) time points. In contrast, matrix metalloproteinase-2 and periostin proteins were increasingly expressed over 90 days. Thus, early development of CD133-C constructs includes a fairly rapid transition from a precursor cell adhesion/migration/transdifferentiation phenotype to a more mature cell/native valve-like matrix metabolism phenotype.

Biological assessment of a calcium silicate incorporated hydroxyapatite-gelatin nanocomposite: a comparison to decellularized bone matrix

Our laboratory utilized biomimicry to develop a synthetic bone scaffold based on hydroxyapatite-gelatin-calcium silicate (HGCS). Here, we evaluated the potential of HGCS scaffold in bone formation in vivo using the rat calvarial critical-sized defect (CSD). Twelve Sprague-Dawley rats were randomized to four groups: control (defect only), decellularized bone matrix (DECBM), and HGCS with and without multipotent adult progenitor cells (MAPCs). DECBM was prepared by removing all the cells using SDS and NH₄OH. After 12 weeks, the CSD specimens were harvested to evaluate radiographical, histological, and histomorphometrical outcomes. The in vitro osteogenic effects of the materials were studied by focal adhesion, MTS, and alizarin red. Micro-CT analysis indicated that the DECBM and the HGCS scaffold groups developed greater radiopaque areas than the other groups. Bone regeneration, assessed using histological analysis and fluorochrome labeling, was the highest in the HGCS scaffold seeded with MAPCs. The DECBM group showed limited osteoinductivity, causing a gap between the implant and host tissue. The group grafted with HGCS+MAPCs resulting in twice as much new bone formation seems to indicate a role for effective bone regeneration. In conclusion, the novel HGCS scaffold could improve bone regeneration and is a promising carrier for stem cell-mediated bone regeneration.

Tissue engineering of autologous heart valves: a focused update

The prevalence of valvular heart disease is expected to increase in the coming decades, with an associated rise in valve-related surgeries. Current options for valve prostheses remain limited, essentially confined to mechanical or biological valves. Neither selection provides an optimal balance between structural integrity and associated morbidity. Mechanical valves offer exceptional durability coupled with a considerable risk of thrombogenesis. Conversely, a biological prosthesis affords freedom from anticoagulation, but with a truncated valve lifespan. Tissue-engineered heart valves have been touted as a solution to this dilemma, by offering an immunopriviledged prosthesis combined with resistance from degeneration and the potential to grow. Although the reality of commercially available tissue-engineered heart valves remains distant, this article will highlight the cellular and clinical advancements in recent years.

A Pulsatile Bioreactor for Conditioning of Tissue-Engineered Cardiovascular Constructs under Endoscopic Visualization

Heart valve disease (HVD) is a globally increasing problem and accounts for thousands of deaths yearly. Currently end-stage HVD can only be treated by total valve replacement, however with major drawbacks. To overcome the limitations of conventional substitutes, a new clinical approach based on cell colonization of artificially manufactured heart valves has been developed. Even though this attempt seems promising, a confluent and stable cell layer has not yet been achieved due to the high stresses present in this area of the human heart. This study describes a bioreactor with a new approach to cell conditioning of tissue engineered heart valves. The bioreactor provides a low pulsatile flow that grants the correct opening and closing of the valve without high shear stresses. The flow rate can be regulated allowing a steady and sensitive conditioning process. Furthermore, the correct functioning of the valve can be monitored by endoscope surveillance in real-time. The tubeless and modular design allows an accurate, simple and faultless assembly of the reactor in a laminar flow chamber. It can be concluded that the bioreactor provides a strong tool for dynamic pre-conditioning and monitoring of colonized heart valve prostheses physiologically exposed to shear stress.

A novel culture device for the evaluation of three-dimensional extracellular matrix materials

Cell-matrix interactions in a three-dimensional (3D) extracellular matrix (ECM) are of fundamental importance in living tissue, and their in vitro reconstruction in bioartificial structures represents a core target of contemporary tissue engineering concepts. For a detailed analysis of cell-matrix interaction under highly controlled conditions, we developed a novel ECM evaluation culture device (EECD) that allows for a precisely defined surface-seeding of 3D ECM scaffolds, irrespective of their natural geometry. The effectiveness of EECD was evaluated in the context of heart valve tissue engineering. Detergent decellularized pulmonary cusps were mounted in EECD and seeded with endothelial cells (ECs) to study EC adhesion, morphology and function on a 3D ECM after 3, 24, 48 and 96 h. Standard EC monolayers served as controls. Exclusive top-surface-seeding of 3D ECM by viable ECs was demonstrated by laser scanning microscopy (LSM), resulting in a confluent re-endothelialization of the ECM after 96 h. Cell viability and protein expression, as demonstrated by MTS assay and western blot analysis (endothelial nitric oxide synthase, von Willebrand factor), were preserved at maintained levels over time. In conclusion, EECD proves as a highly effective system for a controlled repopulation and in vitro analysis of cell-ECM interactions in 3D ECM.

Development of a growing rat model for the in vivo assessment of engineered aortic conduits

BACKGROUND

Numerous limitations of aortic valve grafts currently used in pediatric patients cause the need for alternative prostheses. For the purpose of in vivo evaluation of novel engineered aortic conduit grafts, we aimed at downsizing a previously described model to create a growing rodent model.

MATERIALS AND METHODS

U-shaped aortic conduits were sutured to the infrarenal aorta of young Wistar rats (70-80 g, n = 10) in an end-to-side manner. Functional assessment was performed by Doppler sonography and high resolution rodent MRI. Histology and immunohistochemistry followed after 8 wk.

RESULTS

Postoperative recovery rate was 80%. Conforming to clinical observations, postoperative MRI (d 5) and Doppler sonography (wk 8) revealed unimpaired conduit perfusion. Explanted implants were luminally completely covered by an endothelial cell layer with local hyperplasia and accumulation of α-smooth muscle actin (+) cells. Moreover microcalcification of the decellularized scaffolds was observed.

CONCLUSIONS

Our downsized model of aortic conduit transplantation enables overall characterization with detailed analysis of maturation of engineered aortic grafts in a growing organism.

Bioengineered self-seeding heart valves

OBJECTIVE

Mechanical and biological prostheses are used to replace damaged heart valves but are associated with significant morbidities. Although there is increased interest in bioengineering cell-seeded heart valve scaffolds, it is a time-consuming and technically difficult process. The goal of this project was to engineer self-seeding heart valves that mature quickly in vivo and have a shorter preparation time.

METHODS

Porcine pulmonary valves were decellularized using detergent methods and then either (1) left untreated (unconjugated, n = 6), (2) reseeded with autologous endothelial progenitor cell-derived endothelial cells (cell-seeded, n = 4), or (3) conjugated with CD133 antibodies (conjugated, n = 8). The valve constructs were transplanted into the pulmonary position of sheep using standard surgical techniques. After 1 or 3 months, the implants were removed and assessed for cell and matrix content as well as biomechanical properties.

RESULTS

Endothelial cells expressing von Willebrand factor lined the entire length of both ventricular and arterial surfaces of conjugated valves by 1 month after implantation. Interstitial cell and structural protein content of conjugated valves increased from 1 month to 3 months with interstitial expression of metalloproteinase-9 and new collagen formation. In contrast, there were few endothelial or interstitial cells associated with unconjugated, or cell-seeded valves at any time point. No calcification or thrombi were noted on any of the valves. Young's modulus and tensile strength was greater in the conjugated valves versus unconjugated or cell-seeded valves.

CONCLUSIONS

Results indicate that tissue-engineered heart valve replacement constructs can be made quickly and therefore may be a clinically relevant option for patients needing heart valve surgery in a timely fashion.

Heart valve tissue engineering: quo vadis?

Surgical replacement of diseased heart valves by mechanical and tissue valve substitutes is now commonplace and generally enhances survival and quality of life. However, a fundamental problem inherent to the use of existing mechanical and biological prostheses in the pediatric population is their failure to grow, repair, and remodel. A tissue engineered heart valve could, in principle, accommodate these requirements, especially somatic growth. This review provides a brief overview of the field of heart valve tissue engineering, with emphasis on recent studies and evolving concepts, especially those that establish design criteria and key hurdles that must be surmounted before clinical implementation.

Current developments in the tissue engineering of autologous heart valves: moving towards clinical use

The use of tissue-engineering methods to create autologous heart valve constructs has the potential to overcome the fundamental drawbacks of more traditional valve prostheses. Traditional mechanical valves, while durable, increase the risk for endocarditis and thrombogenesis, and require the recipient to continue lifelong anticoagulant therapy. Homograft or xenograft heart valve prostheses are associated with immune reaction and progressive deterioration with limited durability. Most importantly, neither option is capable of growth and remodeling in vivo and both options place the patient at risk for valve-related complications and reoperation. These shortcomings have prompted the application of tissue-engineering techniques to create fully autologous heart valve replacements. Future clinically efficacious tissue-engineered autologous valves should be nonthrombogenic, biocompatible, capable of growth and remodeling in vivo, implantable with current surgical techniques, hemodynamically perfect, durable for the patient’s life and most importantly, significantly improve quality of life for the patient. In order to meet these expectations, the nature of the ideal biochemical milieu for conditioning an autologous heart valve will need to be elucidated. In addition, standardized criteria by which to quantitatively evaluate a tissue-engineered heart valve, as well as noninvasive analytical techniques for use in long-term animal models, will be required. This article highlights the advances, challenges and future clinical prospects in the field of tissue engineering of autologous heart valves, focusing on progress made by studies that have investigated a fully autologous, tissue-engineered pulmonary valve replacement in vivo.

Wound Healing Versus Regeneration: Role of the Tissue Environment in Regenerative Medicine

One of the major challenges in the field of regenerative medicine is how to optimize tissue regeneration in the body by therapeutically manipulating its natural ability to form scar at the time of injury or disease. It is often the balance between tissue regeneration, a process that is activated at the onset of disease, and scar formation, which develops as a result of the disease process that determines the ability of the tissue or organ to be functional. Using biomaterials as scaffolds often can provide a "bridge" for normal tissue edges to regenerate over small distances, usually up to 1 cm. Larger tissue defect gaps typically require both scaffolds and cells for normal tissue regeneration to occur without scar formation. Various strategies can help to modulate the scar response and can potentially enhance tissue regeneration. Understanding the mechanistic basis of such multivariate interactions as the scar microenvironment, the immune system, extracellular matrix, and inflammatory cytokines may enable the design of tissue engineering and wound healing strategies that directly modulate the healing response in a manner favorable to regeneration.

Design and Testing of a Pulsatile Conditioning System for Dynamic Endothelialization of Polyphenol-Stabilized Tissue Engineered Heart Valves

Heart valve tissue engineering requires biocompatible and hemocompatible scaffolds that undergo remodeling and repopulation, but that also withstand harsh mechanical forces immediately following implantation. We hypothesized that reversibly stabilized acellular porcine valves, seeded with endothelial cells and conditioned in pulsatile bioreactors would pave the way for next generations of tissue engineered heart valves (TEHVs). A novel valve conditioning system was first designed, manufactured and tested to adequately assess TEHVs. The bioreactor created proper closing and opening of valves and allowed for multiple mounting methods in sterile conditions. Porcine aortic heart valve roots were decellularized by chemical extractions and treated with penta-galloyl glucose (PGG) for stabilization. Properties of the novel scaffolds were evaluated by testing resistance to collagenase and elastase, biaxial mechanical analysis, and thermal denaturation profiles. Porcine aortic endothelial cells were seeded onto the leaflets and whole aortic roots were mounted within the dynamic pulsatile heart valve bioreactor system under physiologic pulmonary valve pressures and analyzed after 17 days for cell viability, morphology, and metabolic activity. Our tissue preparation methods effectively removed cells, including the potent α-Gal antigen, while leaving a well preserved extra-cellular matrix scaffold with adequate mechanical properties. PGG enhanced stabilization of extracellular matrix components but also showed the ability to be reversible. Engineered valve scaffolds encouraged attachment and survival of endothelial cells for extended periods and showed signs of widespread cell coverage after conditioning. Our novel approach shows promise toward development of sturdy and durable TEHVs capable of remodeling and cellular repopulation.

Current Strategies in Cardiovascular Biomaterial Functionalization

Prevention of the coagulation cascade and platelet activation is the foremost demand for biomaterials in contact with blood. In this review we describe the underlying mechanisms of these processes and offer the current state of antithrombotic strategies. We give an overview of methods to prevent protein and platelet adhesion, as well as techniques to immobilize biochemically active molecules on biomaterial surfaces. Finally, recent strategies in biofunctionalization by endothelial cell seeding as well as their possible clinical applications are discussed.