Immobilized DNA aptamers used as potent attractors for porcine endothelial precursor cells

Translational tissue-engineered vascular grafts: From bench to bedside

Cardiovascular disease is a primary cause of mortality worldwide, and patients often require bypass surgery that utilizes autologous vessels as conduits. However, the limited availability of suitable vessels and the risk of failure and complications have driven the need for alternative solutions. Tissue-engineered vascular grafts (TEVGs) offer a promising solution to these challenges. TEVGs are artificial vascular grafts made of biomaterials and/or vascular cells that can mimic the structure and function of natural blood vessels. The ideal TEVG should possess biocompatibility, biomechanical mechanical properties, and durability for long-term success in vivo. Achieving these characteristics requires a multi-disciplinary approach involving material science, engineering, biology, and clinical translation. Recent advancements in scaffold fabrication have led to the development of TEVGs with improved functional and biomechanical properties. Innovative techniques such as electrospinning, 3D bioprinting, and multi-part microfluidic channel systems have allowed the creation of intricate and customized tubular scaffolds. Nevertheless, multiple obstacles must be overcome to apply these innovations effectively in clinical practice, including the need for standardized preclinical models and cost-effective and scalable manufacturing methods. This review highlights the fundamental approaches required to successfully fabricate functional vascular grafts and the necessary translational methodologies to advance their use in clinical practice.

Re-endothelialization of Decellularized Scaffolds With Endothelial Progenitor Cell Capturing Aptamer: A New Strategy for Tissue-Engineered Heart Valve

Tissue-engineered heart valve (TEHV) is a promising alternative to current heart valve substitute. Decellularized porcine aortic heart valves (DAVs) are the most common scaffolds of TEHV. Hard to endothelialization is one of the disadvantages of DAVs. Therefore, we aimed to immobilize endothelial progenitor cell (EPC)-aptamer onto DAVs for accelerating endothelialization. In this study, three groups of scaffolds were constructed: DAVs, aptamer-immobilized DAVs (aptamer-DAVs), and glutaraldehyde crosslinked DAVs (GA-DAVs). The results of flow cytometry revealed that EPC-aptamer was specific to EPCs and was immobilized onto DAVs. Cells adhesion experiments demonstrated that EPCs adhered more tightly onto aptamer-DAVs group than other two groups of scaffolds. And cell proliferation assay indicated that EPCs seeded onto aptamer-DAVs group grew faster than DAVs group and GA-DAVs group. Moreover, dynamic capture experiment in flow conditions revealed that the number of EPCs captured by aptamer-DAVs group was more than other two groups. In conclusion, aptamer-DAVs could specifically promote adhesion and proliferation of EPCs and had ability to capture EPCs in simulated flow condition. This could promote re-endothelialization of scaffolds.

Aptamer-enriched scaffolds for tissue regeneration: a systematic review of the literature

Introduction: Aptamers are a brand-new class of receptors that can be exploited to improve the bioactivity of tissue engineering grafts. The aim of this work was to revise the current literature on in vitro and in vivo studies in order to i) identify current strategies adopted to improve scaffold bioactivity by aptamers; ii) assess effects of aptamer functionalization on cell behavior and iii) on tissue regeneration. Methods: Using a systematic search approach original research articles published up to 30 April 2022, were considered and screened. Results: In total, 131 records were identified and 18 were included in the final analysis. Included studies showed that aptamers can improve the bioactivity of biomaterials by specific adsorption of adhesive molecules or growth factors from the surrounding environment, or by capturing specific cell types. All the studies showed that aptamers ameliorate scaffold colonization by cells without modifying the physicochemical characteristics of the bare scaffold. Additionally, aptamers seem to promote the early stages of tissue healing and to promote anatomical and functional regeneration. Discussion: Although a metanalysis could not be performed due to the limited number of studies, we believe these findings provide solid evidence supporting the use of aptamers as a suitable modification to improve the bioactivity of tissue engineering constructs.

Aptamer-based applications for cardiovascular disease

Cardiovascular disease (especially atherosclerosis) is a major cause of death worldwide, and novel diagnostic tools and treatments for this disease are urgently needed. Aptamers are single-stranded oligonucleotides that specifically recognize and bind to the targets by forming unique structures in vivo, enabling them to rival antibodies in cardiac applications. Chemically synthesized aptamers can be readily modified in a site-specific way, so they have been engineered in the diagnosis of cardiac diseases and anti-thrombosis therapeutics. Von Willebrand Factor plays a unique role in the formation of thrombus, and as an aptamer targeting molecule, has shown initial success in antithrombotic treatment. A combination of von Willebrand Factor and nucleic acid aptamers can effectively inhibit the progression of blood clots, presenting a positive diagnosis and therapeutic effect, as well as laying a novel theory and strategy to improve biocompatibility paclitaxel drug balloon or implanted stent in the future. This review summarizes aptamer-based applications in cardiovascular disease, including biomarker discovery and future management strategy. Although relevant applications are relatively new, the significant advancements achieved have demonstrated that aptamers can be promising agents to realize the integration of diagnosis and therapy in cardiac research.

Bio-inspired hemocompatible surface modifications for biomedical applications

When blood first encounters the artificial surface of a medical device, a complex series of biochemical reactions is triggered, potentially resulting in clinical complications such as embolism/occlusion, inflammation, or device failure. Preventing thrombus formation on the surface of blood-contacting devices is crucial for maintaining device functionality and patient safety. As the number of patients reliant on blood-contacting devices continues to grow, minimizing the risk associated with these devices is vital towards lowering healthcare-associated morbidity and mortality. The current standard clinical practice primarily requires the systemic administration of anticoagulants such as heparin, which can result in serious complications such as post-operative bleeding and heparin-induced thrombocytopenia (HIT). Due to these complications, the administration of antithrombotic agents remains one of the leading causes of clinical drug-related deaths. To reduce the side effects spurred by systemic anticoagulation, researchers have been inspired by the hemocompatibility exhibited by natural phenomena, and thus have begun developing medical-grade surfaces which aim to exhibit total hemocompatibility via biomimicry. This review paper aims to address different bio-inspired surface modifications that increase hemocompatibility, discuss the limitations of each method, and explore the future direction for hemocompatible surface research.

Application of aptamers in regenerative medicine

Regenerative medicine is a discipline that studies how to use biological and engineering principles and operation methods to repair and regenerate damaged tissues and organs. Until now, regenerative medicine has focused mainly on the in-depth study of the pathological mechanism of diseases, the further development and application of new drugs, and tissue engineering technology strategies. The emergence of aptamers has supplemented the development methods and types of new drugs and enriched the application elements of tissue engineering technology, injecting new vitality into regenerative medicine. The role and application status of aptamers screened in recent years in various tissue regeneration and repair are reviewed, and the prospects and challenges of aptamer technology are discussed, providing a basis for the design and application of aptamers in long-term transformation.

Endothelial colony forming cell rolling and adhesion supported by peptide-grafted hydrogels

The aim of this study was to investigate the ability of peptides and peptide combinations to support circulating endothelial colony forming cell (ECFC) rolling and adhesion under shear flow, informing biomaterial design in moving toward rapid cardiovascular device endothelialization. ECFCs have high proliferative capability and can differentiate into endothelial cells, making them a promising cell source for endothelialization. Both single peptides and peptide combinations designed to target integrins α₄β₁ and α₅β₁ were coupled to poly(ethylene glycol) hydrogels, and their performance was evaluated by monitoring velocity patterns during the ECFC rolling process, in addition to firm adhesion (capture). Tether percentage and velocity fluctuation, a parameter newly defined here, were found to be valuable in assessing cell rolling velocity patterns and when used in combination were able to predict cell capture. REDV-containing peptides binding integrin α₄β₁ have been previously shown to reduce ECFC rolling velocity but not to support firm adhesion. This study finds that the performance of REDV-containing peptides in facilitating ECFC dynamic adhesion and capture can be improved by combination with α₅β₁ integrin-binding peptides, which support ECFC static adhesion. Moreover, when similar in length, the peptide combinations may have synergistic effects in capturing ECFCs. With matching lengths, the peptide combinations including CRRETAWAC(cyclic)+REDV, P_RGDS+KSSP_REDV, and P_RGDS+P_REDV showed high values in both tether percentage and velocity fluctuation and improvement in ECFC capture compared to the single peptides at the shear rate of 20 s^-1. These newly identified peptide combinations have the potential to be used as vascular device coatings to recruit ECFCs. STATEMENT OF SIGNIFICANCE: Restoration of functional endothelium following placement of stents and vascular grafts is critical for maintaining long-term patency. Endothelial colony forming cells (ECFCs) circulating in blood flow are a valuable cell source for rapid endothelialization. Here we identify and test novel peptides and peptide combinations that can potentially be used as coatings for vascular devices to support rolling and capture of ECFCs from flow. In addition to the widely used assessment of final ECFC adhesion, we also recorded the rolling process to quantitatively evaluate the interaction between ECFCs and the peptides, obtaining detailed performance of the peptides and gaining insight into effective capture molecule design. Peptide combinations targeting both integrin α₄β₁ and integrin α₅β₁ showed the highest percentages of ECFC capture.

Recent advance in treatment of atherosclerosis: Key targets and plaque-positioned delivery strategies

Atherosclerosis, a chronic inflammatory disease of vascular wall, is a progressive pathophysiological process with lipids oxidation/depositing initiation and innate/adaptive immune responses. The coordination of multi systems covering oxidative stress, dysfunctional endothelium, diseased lipid uptake, cell apoptosis, thrombotic and pro-inflammatory responding as well as switched SMCs contributes to plaque growth. In this circumstance, inevitably, targeting these processes is considered to be effective for treating atherosclerosis. Arriving, retention and working of payload candidates mediated by targets in lesion direct ultimate therapeutic outcomes. Accumulating a series of scientific studies and clinical practice in the past decades, lesion homing delivery strategies including stent/balloon/nanoparticle-based transportation worked as the potent promotor to ensure a therapeutic effect. The objective of this review is to achieve a very brief summary about the effective therapeutic methods cooperating specifical targets and positioning-delivery strategies in atherosclerosis for better outcomes.

Tunable Biomaterials for Myocardial Tissue Regeneration: Promising New Strategies for Advanced Biointerface Control and Improved Therapeutic Outcomes

Following myocardial infarction (MI) and the natural healing process, the cardiac mechanostructure changes significantly leading to reduced contractile ability and putting additional pressure on the heart muscle thereby increasing the...

Impact of REDV peptide density and its linker structure on the capture, movement, and adhesion of flowing endothelial progenitor cells in microfluidic devices

Ligand-immobilization to stents and vascular grafts is expected to promote endothelialization by capturing flowing endothelial progenitor cells (EPCs). However, the optimized ligand density and linker structure have not been fully elucidated. Here, we report that flowing EPCs were selectively captured by the REDV peptide conjugated with a short linker. The microchannel surface was modified with the REDV peptide via Gly-Gly-Gly (G3), (Gly-Gly-Gly)₃ (G9), and diethylene glycol (diEG) linkers, and the moving velocity and captured ratio were evaluated. On the unmodified microchannels, the moving velocity of the cells exhibited a unimodal distribution similar to the liquid flow. The velocity of the endothelial cells and EPCs on the peptide-immobilized surface indicated a bimodal distribution, and approximately 20 to 30% of cells moved slower than the liquid flow, suggesting that the cells were captured and rolled on the surface. When the immobilized ligand density was lower than 1 molecule/nm², selective cell capture was observed only in REDV with G3 and diEG linkers, but not in G9 linkers. An in silico study revealed that the G9 linker tends to form a bent structure, and the REDV peptide is oriented to the substrate side. These results indicated that REDV captured the flowing EPC in a sequence-specific manner, and that the short linker was more adequate.

Development of In Vitro Endothelialised Stents - Review

Endovascular treatment is prevalent as a primary treatment for coronary and peripheral arterial diseases. Although the introduction of drug-eluting stents (DES) dramatically reduced the risk of in-stent restenosis, stent thrombosis persists as an issue. Notwithstanding improvements in newer generation DES, they are yet to address the urgent clinical need to abolish the late stent complications that result from in-stent restenosis and are associated with late thrombus formation. These often lead to acute coronary syndromes with high mortality in coronary artery disease and acute limb ischemia with a high risk of limb amputation in peripheral arterial disease. Recently, a significant amount of research has focused on alternative solutions to improve stent biocompatibility by using tissue engineering. There are two types of tissue engineering endothelialisation methods: in vitro and in vivo. To date, commercially available in vivo endothelialised stents have failed to demonstrate antithrombotic or anti-stenosis efficacy in clinical trials. In contrast, the in vitro endothelialisation methods exhibit the advantage of monitoring cell type and growth prior to implantation, enabling better quality control. The present review discusses tissue-engineered candidate stents constructed by distinct in vitro endothelialisation approaches, with a particular focus on fabrication processes, including cell source selection, stent material composition, stent surface modifications, efficacy and safety evidence from in vitro and in vivo studies, and future directions.

Challenges and strategies for in situ endothelialization and long-term lumen patency of vascular grafts

Vascular diseases are the most prevalent cause of ischemic necrosis of tissue and organ, which even result in dysfunction and death. Vascular regeneration or artificial vascular graft, as the conventional treatment modality, has received keen attentions. However, small-diameter (diameter < 4 mm) vascular grafts have a high risk of thrombosis and intimal hyperplasia (IH), which makes long-term lumen patency challengeable. Endothelial cells (ECs) form the inner endothelium layer, and are crucial for anti-coagulation and thrombogenesis. Thus, promoting in situ endothelialization in vascular graft remodeling takes top priority, which requires recruitment of endothelia progenitor cells (EPCs), migration, adhesion, proliferation and activation of EPCs and ECs. Chemotaxis aimed at ligands on EPC surface can be utilized for EPC homing, while nanofibrous structure, biocompatible surface and cell-capturing molecules on graft surface can be applied for cell adhesion. Moreover, cell orientation can be regulated by topography of scaffold, and cell bioactivity can be modulated by growth factors and therapeutic genes. Additionally, surface modification can also reduce thrombogenesis, and some drug release can inhibit IH. Considering the influence of macrophages on ECs and smooth muscle cells (SMCs), scaffolds loaded with drugs that can promote M2 polarization are alternative strategies. In conclusion, the advanced strategies for enhanced long-term lumen patency of vascular grafts are summarized in this review. Strategies for recruitment of EPCs, adhesion, proliferation and activation of EPCs and ECs, anti-thrombogenesis, anti-IH, and immunomodulation are discussed. Ideal vascular grafts with appropriate surface modification, loading and fabrication strategies are required in further studies.

Endothelial progenitor cells as the target for cardiovascular disease prediction, personalized prevention, and treatments: progressing beyond the state-of-the-art

Stimulated by the leading mortalities of cardiovascular diseases (CVDs), various types of cardiovascular biomaterials have been widely investigated in the past few decades. Although great therapeutic effects can be achieved by bare metal stents (BMS) and drug-eluting stents (DES) within months or years, the long-term complications such as late thrombosis and restenosis have limited their further applications. It is well accepted that rapid endothelialization is a promising approach to eliminate these complications. Convincing evidence has shown that endothelial progenitor cells (EPCs) could be mobilized into the damaged vascular sites systemically and achieve endothelial repair in situ, which significantly contributes to the re-endothelialization process. Therefore, how to effectively capture EPCs via specific molecules immobilized on biomaterials is an important point to achieve rapid endothelialization. Further, in the context of predictive, preventive, personalized medicine (PPPM), the abnormal number alteration of EPCs in circulating blood and certain inflammation responses can also serve as important indicators for predicting and preventing early cardiovascular disease. In this contribution, we mainly focused on the following sections: the definition and classification of EPCs, the mechanisms of EPCs in treating CVDs, the potential diagnostic role of EPCs in predicting CVDs, as well as the main strategies for cardiovascular biomaterials to capture EPCs.

Development of in vitro endothelialized drug-eluting stent using human peripheral blood-derived endothelial progenitor cells

We propose in vitro endothelialization of drug-eluting stents (DES) to overcome late stent thrombosis by directly introducing late-outgrowth human endothelial progenitor cells (EPCs) at the target site utilizing abluminal DES. Isolated EPCs were confirmed as late-outgrowth EPCs by flow cytometric analysis. Abluminally paclitaxel-loaded stents were seeded with different cell concentrations and durations to determine optimal seeding conditions, in both uncrimped and crimped configurations. The seeding yield was determined by evaluating the percent coverage of the stent struts' area. The EPC-seeded DES were exposed to arterial shear stress to evaluate the effect of high shear stress on EPCs. To investigate how much paclitaxel elutes during the seeding procedure, a pharmacokinetic analysis was performed. Finally, to validate the proof of concept, EPC-seeded DES were placed on a fibrin matrix with and without smooth muscle cells (SMCs) and cultured for 3 days under perfusion. The seeding procedure resulted in 47% and 26% coverage of the stent surface in uncrimped and crimped conditions, respectively. After the optimal seeding, almost 99% of drug was still available. When EPC-seeded DES were placed on a fibrin matrix and cultured for 3 days, the EPCs confluently covered the stent surface and spread to the surrounding fibrin gel. When EPC-seeded DES were placed on SMC-containing fibrin layers, cells in contact with the struts died. EPCs can be successfully seeded onto DES without losing drug-eluting capability, and EPCs exhibit sufficient proliferative ability. EPC-seeded DES may combine early re-endothelialization ability with the antirestenotic effectiveness of DES.

Aptamer supported in vitro endothelialization of poly(ether imide) films

Implantation of synthetic small-diameter vascular bypass grafts is often associated with an increased risk of failure, due to thrombotic events or late intimal hyperplasia. As one of the causes an insufficient hemocompatibility of the artificial surface is discussed. Endothelialization of synthetic grafts is reported to be a promising strategy for creating a self-renewing and regulative anti-thrombotic graft surface. However, the establishment of a shear resistant cell monolayer is still challenging. In our study, cyto- and immuno-compatible poly(ether imide) (PEI) films were explored as potential biomaterial for cardiovascular applications. Recently, we reported that the initial adherence of primary human umbilical vein endothelial cells (HUVEC) was delayed on PEI-films and about 9 days were needed to establish a confluent and almost shear resistant HUVEC monolayer. To accelerate the initial adherence of HUVEC, the PEI-film surface was functionalized with an aptamer-cRGD peptide based endothelialization supporting system. With this functionalization the initial adherence as well as the shear resistance of HUVEC on PEI-films was considerable improved compared to the unmodified polymer surface. The in vitro results confirm the general applicability of aptamers for an efficient functionalization of substrate surfaces.

A glance on the role of fibronectin in controlling cell response at biomaterial interface

The bioactivity of biomaterials is closely related to cell response in contact with them. However, shortly after their insertion, materials are soon covered with proteins that constitute the biological fluids, and which render the direct surface recognition by cells almost impossible. The control of protein adsorption at the interface is therefore desirable. Extracellular matrix proteins are of particular interest in this sense, due to their well-known ability to modulate cell behavior. Particularly, fibronectin plays a leading role, being present in both healthy and injured tissues undergoing healing and regeneration. The aim of the present work is to give an overview on fibronectin and on its involvement in the control of cell behavior providing evidence of its pivotal role in the control of cell adhesion, spreading, migration, proliferation and differentiation. A deep insight into methods to enrich biomaterials surface with fibronectin will be then discussed, as well as new cues on the possibility to design tailored platforms able to specifically retain fibronectin from the surrounding extracellular milieu.

Is selective protein adsorption on biomaterials a viable option to promote periodontal regeneration?

Periodontitis is an inflammatory condition that can induce significant destruction of the periodontium, the set of specialized tissues that provide nourishment and support to the teeth. According to the guided tissue regeneration principles, the periodontium can be regenerated if the spatiotemporal control of wound healing is obtained, namely the tune control of cell response. After material implantation, protein adsorption at the interface is the first occurring biological event, which influences subsequent cell response. With the regard of this, we hypothesize that the control of selective adsorption of biological cues from the surrounding milieu may be a key-point to control selective cell colonization of scaffolds for periodontal tissue regeneration.

The blood compatibility challenge. Part 4: Surface modification for hemocompatible materials: Passive and active approaches to guide blood-material interactions

Biomedical devices in the blood flow disturb the fine-tuned balance of pro- and anti-coagulant factors in blood and vessel wall. Numerous technologies have been suggested to reduce coagulant and inflammatory responses of the body towards the device material, ranging from camouflage effects to permanent activity and further to a responsive interaction with the host systems. However, not all types of modification are suitable for all types of medical products. This review has a focus on application-oriented considerations of hemocompatible surface fittings. Thus, passive versus bioactive modifications are discussed along with the control of protein adsorption, stability of the immobilization, and the type of bioactive substance, biological or synthetic. Further considerations are related to the target system, whether enzymes or cells should be addressed in arterial or venous system, or whether the blood vessel wall is addressed. Recent developments like feedback controlled or self-renewing systems for drug release or addressing cellular regulation pathways of blood platelets and endothelial cells are paradigms for a generation of blood contacting devices, which are hemocompatible by cooperation with the host system. STATEMENT OF SIGNIFICANCE: This paper is part 4 of a series of 4 reviews discussing the problem of biomaterial associated thrombogenicity. The objective was to highlight features of broad agreement and provide commentary on those aspects of the problem that were subject to dispute. We hope that future investigators will update these reviews as new scholarship resolves the uncertainties of today.

Fibrosis in tissue engineering and regenerative medicine: treat or trigger?

Fibrosis is a life-threatening pathological condition resulting from a dysfunctional tissue repair process. There is no efficient treatment and organ transplantation is in many cases the only therapeutic option. Here we review tissue engineering and regenerative medicine (TERM) approaches to address fibrosis in the cardiovascular system, the kidney, the lung and the liver. These strategies have great potential to achieve repair or replacement of diseased organs by cell- and material-based therapies. However, paradoxically, they might also trigger fibrosis. Cases of TERM interventions with adverse outcome are also included in this review. Furthermore, we emphasize the fact that, although organ engineering is still in its infancy, the advances in the field are leading to biomedically relevant in vitro models with tremendous potential for disease recapitulation and development of therapies. These human tissue models might have increased predictive power for human drug responses thereby reducing the need for animal testing.

Reconstruction of structure and function in tissue engineering of solid organs: Toward simulation of natural development based on decellularization

Failure of solid organs, such as the heart, liver, and kidney, remains a major cause of the world's mortality due to critical shortage of donor organs. Tissue engineering, which uses elements including cells, scaffolds, and growth factors to fabricate functional organs in vitro, is a promising strategy to mitigate the scarcity of transplantable organs. Within recent years, different construction strategies that guide the combination of tissue engineering elements have been applied in solid organ tissue engineering and have achieved much progress. Most attractively, construction strategy based on whole-organ decellularization has become a popular and promising approach, because the overall structure of extracellular matrix can be well preserved. However, despite the preservation of whole structure, the current constructs derived from decellularization-based strategy still perform partial functions of solid organs, due to several challenges, including preservation of functional extracellular matrix structure, implementation of functional recellularization, formation of functional vascular network, and realization of long-term functional integration. This review overviews the status quo of solid organ tissue engineering, including both advances and challenges. We have also put forward a few techniques with potential to solve the challenges, mainly focusing on decellularization-based construction strategy. We propose that the primary concept for constructing tissue-engineered solid organs is fabricating functional organs based on intact structure via simulating the natural development and regeneration processes.

Surface modification and endothelialization of polyurethane for vascular tissue engineering applications: a review

Cardiovascular implants, especially vascular grafts made of synthetic polymers, find wide clinical applications in the treatment of cardiovascular diseases. However, cases of failure still exist, notably caused by restenosis and thrombus formation. Aiming to solve these problems, various approaches to surface modification of synthetic vascular grafts have been used to improve both the hemocompatibility and long-term patency of artificial vascular grafts. Surface modification using hydrophilic molecules can enhance hemocompatibility, but this may limit the initial vascular endothelial cell adhesion. Therefore, the improvement of endothelialization on these grafts with specific peptides and biomolecules is now an exciting field of research. In this review, several techniques to improve surface modification and endothelialization on vascular grafts, mainly polyurethane (PU) grafts, are summarized, together with the recent development and evolution of the different strategies: from the use of PEG, zwitterions, and polysaccharides to peptides and other biomolecules and genes; from in vitro endothelialization to in vivo endothelialization; and from bio-inert and bio-active to bio-mimetic approaches.

Selection and Application of Aptamers and Intramers

Aptamers are auspicious nucleic acid ligands for targeting different molecules, such as small molecules, peptides, proteins, or even whole living cells. They are short single-stranded DNA or RNA oligonucleotides, which can fold into complex three-dimensional structures and bind selectively their targets. Using the combinatorial chemistry process SELEX (Systematic Evolution of Ligands by EXponential Enrichment), target specific aptamers can be selected. These aptamers have a variety of application possibilities and can be used as sensors, diagnostic, imaging or therapeutic agents, and in the field of regenerative medicine for tissue engineering.

Engineering Cardiovascular Implant Surfaces to Create a Vascular Endothelial Growth Microenvironment

Cardiovascular disease (CVD) is generally accepted as the leading cause of morbidity and mortality worldwide, and an increasing number of patients suffer from atherosclerosis and thrombosis annually. To treat these disorders and prolong the sufferers' life, several cardiovascular implants have been developed and applied clinically. Nevertheless, thrombosis and hyperplasia at the site of cardiovascular implants are recognized as long-term problems in the practice of interventional cardiology. Here, we start this review from the clinical requirement of the implants, such as anti-hyperplasia, anti-thrombosis, and pro-endothelialization, wherein particularly focus on the natural factors which influence functional endothelialization in situ, including the healthy smooth muscle cells (SMCs) environment, blood flow shear stress (BFSS), and the extracellular matrix (ECM) microenvironment. Then, the currently available strategies on surface modification of cardiovascular biomaterials to create vascular endothelial growth microenvironment are introduced as the main topic, e.g., BFSS effect simulation by surface micro-patterning, ECM rational construction and SMCs phenotype maintain. Finally, the prospects for extending use of the in situ construction of endothelial cells growth microenvironment are discussed and summarized in designing the next generation of vascular implants.

Strategies to develop endogenous stem cell-recruiting bioactive materials for tissue repair and regeneration

A leading strategy in tissue engineering is the design of biomimetic scaffolds that stimulate the body's repair mechanisms through the recruitment of endogenous stem cells to sites of injury. Approaches that employ the use of chemoattractant gradients to guide tissue regeneration without external cell sources are favored over traditional cell-based therapies that have limited potential for clinical translation. Following this concept, bioactive scaffolds can be engineered to provide a temporally and spatially controlled release of biological cues, with the possibility to mimic the complex signaling patterns of endogenous tissue regeneration. Another effective way to regulate stem cell activity is to leverage the inherent chemotactic properties of extracellular matrix (ECM)-based materials to build versatile cell-instructive platforms. This review introduces the concept of endogenous stem cell recruitment, and provides a comprehensive overview of the strategies available to achieve effective cardiovascular and bone tissue regeneration.

Nanomaterial coatings applied on stent surfaces

The advent of percutaneous coronary intervention and intravascular stents has revolutionized the field of interventional cardiology. Nonetheless, in-stent restenosis, inflammation and late-stent thrombosis are the major obstacles with currently available stents. In order to enhance the hemocompatibility of stents, advances in the field of nanotechnology allow novel designs of nanoparticles and biomaterials toward localized drug/gene carriers or stent scaffolds. The current review focuses on promising polymers used in the fabrication of newer generations of stents with a short synopsis on atherosclerosis and current commercialized stents, nanotechnology's impact on stent development and recent advancements in stent biomaterials is discussed in context.

Anti-fibronectin aptamers improve the colonization of chitosan films modified with D-(+) Raffinose by murine osteoblastic cells

The aim of the present study was to investigate how the enrichment of chitosan films with anti-fibronectin aptamers could enhance scaffold colonization by osteoblasts, by improving their adhesion and accelerating their proliferation. Chitosan discs were enriched with excess of anti-fibronectin aptamer. Aptamer adsorption on chitosan was monitored by measuring aptamer concentration in the supernatant by spectrophotometry, as well as its release, while functionalization was confirmed by labelling aptamers with a DNA intercalating dye. Chitosan samples were then characterized morphologically with atomic force microscopy and physically with contact angle measurement. Chitosan enrichment with fibronectin was then investigated by immunofluorescence and Bradford assay. 2% chitosan discs were then enriched with increasing doses of aptamers and used as culture substrates for MC3T3-E1 cells. Cell growth was monitored by optical microscopy, while cell viability and metabolic activity were assessed by chemiluminescence and by Resazurin Sodium Salt assay. Cell morphology was investigated by cytofluorescence and by scanning electron microscopy. Chitosan films efficiently bound and retained aptamers. Aptamers did not affect the amount of adsorbed fibronectin, but affected osteoblasts behavior. Cell growth was proportional to the amount of aptamer used for the functionalization, as well as aptamers influenced cell morphology and their adhesion to the substrate. Our results demonstrate that the enrichment of chitosan films with aptamers could selectively improve osteoblasts behavior. Furthermore, our results support further investigation of this type of functionalization as a suitable modification to ameliorate the biocompatibility of biomaterial for hard tissue engineering applications.

Heparin/DNA aptamer co-assembled multifunctional catecholamine coating for EPC capture and improved hemocompatibility of vascular devices

Good hemocompatibility and rapid endothelialization are two key factors in the success of stent interventional therapy. In this study, aptamers with the ability to capture endothelial progenitors and anticoagulant molecular heparin were successfully immobilized on the surface of dopamine/polyethylenimine (PDA/PEI) copolymer coating via electrostatic interaction. The results of X-ray spectroscopy (XPS), water contact angle (WCA), and immunofluorescence staining tests confirmed the successful introduction of heparin and aptamers. Platelet adhesion and whole blood experiments demonstrated that the hemocompatibility of the co-modified surface was improved. Dynamic endothelial progenitor cell (EPC) capture experiments showed that the modified surfaces could effectively capture the endothelial progenitor in dynamic conditions. More importantly, ex vivo experiments revealed that the modified surfaces could regulate the distribution of CD34/vWF-positive cells on stent surfaces, and this was beneficial for the endothelialization of vascular stents. These results suggested that heparin and aptamer co-modified stents could capture EPCs and promote endothelialization. This surface co-modification strategy has great potential for enhancing stent development.

Anti-thrombotic technologies for medical devices

Thrombosis associated with medical devices may lead to dramatic increases in morbidity, mortality and increased health care costs. Innovative strategies are being developed to reduce this complication and provide a safe biocompatible interface between device and blood. This article aims to describe the biological phenomena underlying device-associated thrombosis, and surveys the literature describing current and developing technologies designed to overcome this challenge. To reduce thrombosis, biomaterials with varying topographical properties and incorporating anti-thrombogenic substances on their surface have demonstrated potential. Overall, there is extensive literature describing technical solutions to reduce thrombosis associated with medical devices, but clinical results are required to demonstrate significant long-term benefits.

Arrays of Individual DNA Molecules on Nanopatterned Substrates

Arrays of individual molecules can combine the advantages of microarrays and single-molecule studies. They miniaturize assays to reduce sample and reagent consumption and increase throughput, and additionally uncover static and dynamic heterogeneity usually masked in molecular ensembles. However, realizing single-DNA arrays must tackle the challenge of capturing structurally highly dynamic strands onto defined substrate positions. Here, we create single-molecule arrays by electrostatically adhering single-stranded DNA of gene-like length onto positively charged carbon nanoislands. The nanosites are so small that only one molecule can bind per island. Undesired adsorption of DNA to the surrounding non-target areas is prevented via a surface-passivating film. Of further relevance, the DNA arrays are of tunable dimensions, and fabricated on optically transparent substrates that enable singe-molecule detection with fluorescence microscopy. The arrays are hence compatible with a wide range of bioanalytical, biophysical, and cell biological studies where individual DNA strands are either examined in isolation, or interact with other molecules or cells.

Generating Aptamers Interacting with Polymeric Surfaces for Biofunctionalization

Common strategies for biofunctionalization of surfaces comprise the immobilization of bioactive molecules used as cell-binding ligands for cell recruitment. Besides covalent binding, multivalent noncovalent physical forces between substrate and ligand are an alternative way to equip surfaces with biomacromolecules. In this study, polymer binding ligands are screened by means of a DNA-based in vitro selection process. As candidate biomaterials poly(ether imide) (PEI), polystyrene, and poly[ethylene-co-(vinyl acetate)] are selected, due to their different chemical structure, but similar macroscopic interface properties, allowing physical interaction with nucleotide bases by varying valences. Multivalent interacting aptamers are successfully enriched by SELEX method and an area-wide surface functionalization is achieved, which can be used for further binding of bioactive molecules. In vitro selection against the polymers result in thymine-dominated aptamer binding motifs. The preferential interaction with thymine is attributed to its chemical structure, connected with a decreased electrostatic repulsion of the π-system and the hydrophobic character maximizing entropy. The aptamer binding stability correlates with available valences for interaction, resulting in a more stable functionalization of PEI.

Improved scaffold biocompatibility through anti-Fibronectin aptamer functionalization

Protein adsorption is the first and decisive step to define cell-biomaterial interaction. Guiding the adsorption of desired protein species may represent a viable approach to promote cell activities conducive to tissue regeneration. The aim of the present study was to investigate whether immobilized anti-Fibronectin aptamers could promote the attachment and growth of osteoblastic cells. Polyethyleneglycole diacrylate/thiolated Hyaluronic Acid hydrogels (PEGDA/tHA) were coated with anti-Fibronectin aptamers. Hydrogel loading and Fibronectin bonding were investigated, through spectrophotometry and Bradford assay. Subsequently, human osteoblasts (hOBs) were cultured on hydrogels for 10days in 2D and 3D cultures. Cells were monitored through microscopy and stained for focal adhesions, microfilaments and nuclei using fluorescence microscopy. Samples were also included in paraffin and stained with Hematoxylin-Eosin. Cell number on hydrogels was quantitated over time. Cell migration into the hydrogels was also studied through Calcein AM staining. Aptamers increased the number of adherent hOBs and their cytoplasm appeared more spread and richer in adhesion complexes than on control hydrogels. Viability assays confirmed that significantly more cells were present on hydrogels in the presence of aptamers, already after 48h of culture. When hOBs were encapsulated into hydrogels, cells were more numerous on aptamer-containing PEGDA-tHA. Cells migrated deeper in the gel in the presence of DNA aptamers, appearing on different focus planes. Our data demonstrate that anti-Fibronectin aptamers promote scaffold enrichment for this protein, thus improving cell adhesion and scaffold colonization.

STATEMENT OF SIGNIFICANCE

We believe aptamer coating of biomaterials is a useful and viable approach to improve the performance of scaffold materials for both research and possibly clinical purposes, because different medical devices could be envisaged able to capture bioactive mediators from the patients' blood and concentrate them where they are needed, on the biomaterial itself. At the same time, this technology could be used to confer 3D cell culture scaffold with the ability to store proteins, such as Fibronectin, taking it from the medium and capture what is produced by cells. This is an improvement of traditional biomaterials that can be enriched with exogenous molecules but are not able to selectively capture a desired molecule.

Adding Functions to Biomaterial Surfaces through Protein Incorporation

The concept of biomaterials has evolved from one of inert mechanical supports with a long-term, biologically inactive role in the body into complex matrices that exhibit selective cell binding, promote proliferation and matrix production, and may ultimately become replaced by newly generated tissues in vivo. Functionalization of material surfaces with biomolecules is critical to their ability to evade immunorecognition, interact productively with surrounding tissues and extracellular matrix, and avoid bacterial colonization. Antibody molecules and their derived fragments are commonly immobilized on materials to mediate coating with specific cell types in fields such as stent endothelialization and drug delivery. The incorporation of growth factors into biomaterials has found application in promoting and accelerating bone formation in osteogenerative and related applications. Peptides and extracellular matrix proteins can impart biomolecule- and cell-specificities to materials while antimicrobial peptides have found roles in preventing biofilm formation on devices and implants. In this progress report, we detail developments in the use of diverse proteins and peptides to modify the surfaces of hard biomaterials in vivo and in vitro. Chemical approaches to immobilizing active biomolecules are presented, as well as platform technologies for isolation or generation of natural or synthetic molecules suitable for biomaterial functionalization.

Towards a biofunctionalized vascular prosthesis: immune cell trapping via a growth factor receptor

To improve the clinical performance of vascular prostheses, which is inacceptably low for implants with small diameters (< 6 mm), biofunctionalization of synthetic implants by endothelialization has become a major, although still unreached, aim. In order to be able to recruit native endothelial progenitor cells (EPCs) to luminal implant surfaces from the blood stream, we generated monoclonal antibodies against the EPC-specific vascular endothelial growth factor receptor 2 (VEGFR-2). Employing the very efficient genetic immunization strategy, > 10 000 hybridoma clones were generated. Screening with various deletion mutants of VEGFR-2, 49 highly-specific monoclonal antibodies (mAbs) covering all seven Ig domains of VEGFR-2 were selected. mAb 9H10 was characterized in detail. Once immobilized on synthetic surfaces, mAb 9H10 allowed, within min, nearly 100-fold enrichment of VEGFR-2-expressing cells under continuous flow conditions. Cell trapping was cell-type specific and essentially not affected by competing VEGFR-2-negative cells. To exclude that the antibody would adversely modify receptor responses, four different in vitro assays were employed. Cell proliferation, angiogenic tube formation, acetylated low-density lipoprotein uptake and VEGFR-2 phosphorylation remained unaffected, suggesting that the antibody did not interfere with the receptor functioning of human umbilical vascular endothelial cells. The molecular and cellular characteristics make the selected monoclonal antibody a very promising tool for the biofunctionalization of vascular implants. Copyright © 2016 John Wiley & Sons, Ltd.

Surface biofunctionalization of β-TCP blocks using aptamer 74 for bone tissue engineering

Successful bone regeneration following oral and maxillofacial surgeries depends on efficient functionalization strategies that allow the recruitment of osteogenic progenitor cells at the tissue/implant interface. We have previously identified aptamer 74, which exhibited a binding affinity for osteogenically induced jaw periosteal cells (JPCs). In the present study, this aptamer was used for the surface biofunctionalization of β-tricalcium phosphate (β-TCP) blocks. Atomic force microscopy (AFM) measurements showed increased binding activity of aptamer 74 towards osteogenically induced JPCs compared to untreated controls. The immobilization efficiency of aptamer 74 was analyzed using the QuantiFluor ssDNA assay for 2D surfaces and by amino acid analysis for 3D β-TCP constructs. Following the successful immobilization of aptamer 74 in 2D culture wells and on 3D constructs, in vitro assays showed no significant differences in cell proliferation compared to unmodified surfaces. Interestingly, JPC mineralization was significantly higher on the 2D surfaces and higher cell adhesion was detected on the 3D constructs with immobilized aptamer. Herein, we report an established, biocompatible β-TCP matrix with surface immobilization of aptamer 74, which enhances properties such as cell adhesion on 3D constructs and mineralization on 2D surfaces. Further studies need to be performed to improve the immobilization efficiency and to develop a suitable approach for JPC mineralization growing within 3D β-TCP constructs.

Multifunctional coating based on EPC-specific peptide and phospholipid polymers for potential applications in cardiovascular implants fate

Surface biofunctional modification of cardiovascular implants via the conjugation of biomolecules to prevent thrombosis and restenosis formation and to accelerate endothelialization has attracted considerable research interest.

In situ Endothelialization: Bioengineering Considerations to Translation

Improving patency rates of current cardiovascular implants remains a major challenge. It is widely accepted that regeneration of a healthy endothelium layer on biomaterials could yield the perfect blood-contacting surface. Earlier efforts in pre-seeding endothelial cells in vitro demonstrated success in enhancing patency, but translation to the clinic is largely hampered due to its impracticality. In situ endothelialization, which aims to create biomaterial surfaces capable of self-endothelializing upon implantation, appears to be an extremely promising solution, particularly with the utilization of endothelial progenitor cells (EPCs). Nevertheless, controlling cell behavior in situ using immobilized biomolecules or physical patterning can be complex, thus warranting careful consideration. This review aims to provide valuable insight into the rationale and recent developments in biomaterial strategies to enhance in situ endothelialization. In particular, a discussion on the important bio-/nanoengineering considerations and lessons learnt from clinical trials are presented to aid the future translation of this exciting paradigm.

In vitro and in vivo cell-capture strategies using cardiac stent technology - A review

Stenosis is a symptom of coronary artery disease (CAD), and is caused by narrowing of arteries in the heart. Over the last several decades, medical implants such as cardiac stents have been developed to counter stenosis. Upon implantation of a stent to open up a restricted artery, narrowing of the artery can reoccur (restenosis), due to an immune response launched by the body towards the stent. Currently, restenosis is a major health concern for patients who have undergone heart surgery for coronary artery disease. Recently, there have been new methods developed to combat restenosis, which have shown potential signs of success. One proposed method is the use of stents to capture cells, thereby reducing immune response. This review will explore the different methods for cell capture both in vitro and in vivo. Biological modifications of the stent will be surveyed, as well as the use of surface science to immobilize biological probes. Immobilization of proteins and nucleotides, as well as use of magnetic field are all methods that will be further discussed. Finally, concluding remarks and future prospects will be presented.

Tailoring biomaterial surface properties to modulate host-implant interactions: implication in cardiovascular and bone therapy

Host body response to a foreign medical device plays a critical role in defining its fate post implantation. It is thus important to control host-material interactions by designing innovative implant surfaces. In the recent years, biochemical and topographical features have been explored as main target to produce this new type of bioinert or bioresponsive implants. The review discusses specific biofunctional materials and strategies to achieve a precise control over implant surface properties and presents possible solutions to develop next generation of implants, particularly in the fields of bone and cardiovascular therapy.

Guided tissue regeneration in heart valve replacement: from preclinical research to first-in-human trials

Heart valve tissue-guided regeneration aims to offer a functional and viable alternative to current prosthetic replacements. Not requiring previous cell seeding and conditioning in bioreactors, such exceptional tissue engineering approach is a very fascinating translational regenerative strategy. After in vivo implantation, decellularized heart valve scaffolds drive their same repopulation by recipient's cells for a prospective autologous-like tissue reconstruction, remodeling, and adaptation to the somatic growth of the patient. With such a viability, tissue-guided regenerated conduits can be delivered as off-the-shelf biodevices and possess all the potentialities for a long-lasting resolution of the dramatic inconvenience of heart valve diseases, both in children and in the elderly. A review on preclinical and clinical investigations of this therapeutic concept is provided with evaluation of the issues still to be well deliberated for an effective and safe in-human application.

Adlayer-mediated antibody immobilization to stainless steel for potential application to endothelial progenitor cell capture

This work describes the straightforward surface modification of 316L stainless steel with BTS, S-(11-trichlorosilylundecanyl)-benzenethiosulfonate, a thiol-reactive trichlorosilane cross-linker molecule designed to form intermediary coatings with subsequent biofunctionalization capability. The strategy is more specifically exemplified with the immobilization of intact antibodies and their Fab' fragments. Both surface derivatization steps are thoroughly characterized by means of X-ray photoelectron spectroscopy. The antigen binding capability of both types of biofunctionalized surfaces is subsequently assessed by fluorescence microscopy. It was determined that BTS adlayers achieve robust immobilization of both intact and fragmented antibodies, while preserving antigen binding activity. Another key finding was the observation that the Fab' fragment immobilization strategy would constitute a preferential option over that involving intact antibodies in the context of in vivo capture of endothelial progenitor cells in stent applications.

Microfluidic chip system for the selection and enrichment of cell binding aptamers

Aptamers are promising cell targeting ligands for several applications such as for the diagnosis, therapy, and drug delivery. Especially, in the field of regenerative medicine, stem cell specific aptamers have an enormous potential. Using the combinatorial chemistry process SELEX (Systematic Evolution of Ligands by Exponential enrichment), aptamers are selected from a huge oligonucleotide library consisting of approximately 10(15) different oligonucleotides. Here, we developed a microfluidic chip system that can be used for the selection of cell specific aptamers. The major drawbacks of common cell-SELEX methods are the inefficient elimination of the unspecifically bound oligonucleotides from the cell surface and the unspecific binding/uptake of oligonucleotides by dead cells. To overcome these obstacles, a microfluidic device, which enables the simultaneous performance of dielectrophoresis and electrophoresis in the same device, was designed. Using this system, viable cells can be selectively assembled by dielectrophoresis between the electrodes and then incubated with the oligonucleotides. To reduce the rate of unspecifically bound sequences, electrophoretic fields can be applied in order to draw loosely bound oligonucleotides away from the cells. Furthermore, by increasing the flow rate in the chip during the iterative rounds of SELEX, the selection pressure can be improved and aptamers with higher affinities and specificities can be obtained. This new microfluidic device has a tremendous capability to improve the cell-SELEX procedure and to select highly specific aptamers.

Predicting the Uncertain Future of Aptamer-Based Diagnostics and Therapeutics

Despite the great promise of nucleic acid aptamers in the areas of diagnostics and therapeutics for their facile in vitro development, lack of immunogenicity and other desirable properties, few truly successful aptamer-based products exist in the clinical or other markets. Core reasons for these commercial deficiencies probably stem from industrial commitment to antibodies including a huge financial investment in humanized monoclonal antibodies and a general ignorance about aptamers and their performance among the research and development community. Given the early failures of some strong commercial efforts to gain government approval and bring aptamer-based products to market, it may seem that aptamers are doomed to take a backseat to antibodies forever. However, the key advantages of aptamers over antibodies coupled with niche market needs that only aptamers can fill and more recent published data still point to a bright commercial future for aptamers in areas such as infectious disease and cancer diagnostics and therapeutics. As more researchers and entrepreneurs become familiar with aptamers, it seems inevitable that aptamers will at least be considered for expanded roles in diagnostics and therapeutics. This review also examines new aptamer modifications and attempts to predict new aptamer applications that could revolutionize biomedical technology in the future and lead to marketed products.

Progress in interventional cardiology: challenges for the future

Cardiovascular disease is the leading cause of death in the western and developing countries. Percutaneous transluminal coronary interventions have become the most prevalent treatment option for coronary artery disease; however, due to serious complications, such as stent thrombosis and in-stent restenosis (ISR), the efficacy and safety of the procedure remain important issues to address. Strategies to overcome these aspects are under extensive investigation. In this review, we summarise relevant milestones during the time to overcome these limitations of coronary stents, such as the development of polymer-free drug-eluting stents (DES) to avoid pro-inflammatory response due to the polymer coating or the developement of stents with cell-directing drugs to, simultaneously, improve re-endothelialisation and inhibit ISR amongst other techniques most recently developed, which have not fully entered the clinical stage. Also the novel concept of fully biodegradable DES featured by the lack of a permanent foreign body promises to be a beneficial and applicable tool to restore a natural vessel with maintained vasomotion and to enable optional subsequent surgical revascularisation.

Accelerating in situ endothelialisation of cardiovascular bypass grafts

The patency of synthetic cardiovascular grafts in the long run is synonymous with their ability to inhibit the processes of intimal hyperplasia, thrombosis and calcification. In the human body, the endothelium of blood vessels exhibits characteristics that inhibit such processes. As such it is not surprising that research in tissue engineering is directed towards replicating the functionality of the natural endothelium in cardiovascular grafts. This can be done either by seeding the endothelium within the lumen of the grafts prior to implantation or by designing the graft such that in situ endothelialisation takes place after implantation. Due to certain difficulties identified with in vitro endothelialisation, in situ endothelialisation, which will be the focus of this article, has garnered interest in the last years. To promote in situ endothelialisation, the following aspects can be taken into account: (1) Endothelial progenital cell mobilization, adhesion and proliferation; (2) Regulating differentiation of progenitor cells to mature endothelium; (3) Preventing thrombogenesis and inflammation during endothelialisation. This article aims to review and compile recent developments to promote the in situ endothelialisation of cardiovascular grafts and subsequently improve their patency, which can also have widespread implications in the field of tissue engineering.

In vivo tissue engineering: mimicry of homing factors for self-endothelialization of blood-contacting materials

Thrombogenicity of foreign surfaces is the major obstacle in cardiovascular interventions. Despite enormous advances in biomaterials research, the hemocompatibility of blood-contacting materials is still not satisfactory and the native endothelium still represents the ideal surface for blood contact. Circulating adult endothelial progenitor cells (EPCs) in the human blood provide an excellent source of autologous stem cells for the in vivo self-endothelialization of blood-contacting materials. For this purpose, material surfaces can be coated with capture molecules mimicking natural homing factors to attract circulating EPCs. Hitherto, several ligands, such as aptamers, monoclonal antibodies, peptides, selectins and their ligands, or magnetic molecules, are used to biofunctionalize surfaces for the capturing of EPCs directly from patient's bloodstream onto blood-contacting materials. Subsequently, attracted EPCs can differentiate into endothelial cells and generate an autologous endothelium. The in vivo self-endothelialization of blood-contacting materials prevents the recognition of them as a foreign body; this opens up revolutionary new prospects for future clinical stem-cell and tissue engineering strategies.

One-staged aptamer-based isolation and application of endothelial progenitor cells in a porcine myocardial infarction model

A multitude of stem cell types has been extensively studied and used for myocardial regenerative therapy. Amongst these endothelial progenitor cells form a promising source. In our present study, we investigated a one-staged approach for isolation and application of autologous endothelial progenitor cells in a pig model of myocardial infarction. Endothelial progenitor cell isolation was performed by immediately preprocedural bone marrow aspiration and consecutive positive selection by aptamer-based magnetic cell sorting. Animals were divided in three groups receiving endothelial progenitor cells, saline, or no intramyocardial injection respectively. Postprocedural follow-up consisted of weekly echocardiographic evaluations. Postmortem histological analysis after four weeks focused on detection of transplanted PKH26-positive endothelial progenitor cells and neovascularization within the infarcted myocardium. A significant difference in left ventricular ejection fraction could not be shown between the three groups. PKH26-stained endothelial progenitor cells could be found in the endothelial progenitor cells transplanted group, although detection was scarce. Large-sized capillaries were found to be significantly more in endothelial progenitor cells treated myocardium. The one-stage approach of endothelial progenitor cells isolation and application presented herein offers a new therapeutic concept. Even though a beneficial impact on myocardial function could not be assessed, increased neovascularization may indicate positive effects on remodeling processes. Being able to harvest endothelial progenitor cells right before application provides a wider scope of action in urgent cases.

Immobilization of DNA aptamers via plasma polymerized allylamine film to construct an endothelial progenitor cell-capture surface

The endothelial progenitor cells (EPCs) capture stent has drawn increasing attentions and become one of the most promising concepts for the next generation vascular stent. In this regard, it is of great significance to immobilize a molecule with the ability to bind EPC for rapid in vivo endothelialization with high specificity. In this work, a facile two-step method aimed at constructing a coating with specific EPC capturing aptamers is reported. The processes involves as the first-step deposition of plasma polymerized allylamine (PPAam) on a substrate to introduce amine groups, followed by the electrostatic adsorption of a 34 bases single strand DNA sequence to the PPAam surface as a second step (PPAam-DNA). Grazing incidence attenuated total reflection Fourier transform infrared spectroscopy (GATR-FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the successful immobilization of the aptamers. Quartz crystal microbalance with dissipation (QCM-D) real time monitoring result shows that about 175 ng/cm(2) aptamers were conjugated onto the PPAam surface. The interactions between the modified surfaces and human umbilical vein endothelial cells (ECs), smooth muscle cells (SMCs), and murine induced EPCs derived from mesenchymal stem cells (MSCs) were also investigated. It was demonstrated that PPAam-DNA samples could capture more EPCs, and present a cellular friendly surface for the proliferation of both EPCs and ECs but no effect on the hyperplasia of SMCs. Also, the co-culture results of 3 types of cells confirmed that the aptamer could specifically bond EPCs rather than ECs and SMCs, suggesting the competitive adhesion advantage of EPCs to ECs and SMCs. These data demonstrate that the EPC aptamer has large potential for designing an EPC captured stent and other vascular grafts with targeted in situ endothelialization.

Importance of rigorous in vitro evaluation of prospective cell binding aptamers

Hitherto, several aptamers have been selected against cell surface molecules. The use of these aptamers for in vivo applications requires the prior in-depth in vitro evaluation of cell specific binding. Here, we demonstrate the in vitro tests, which are imperatively necessary to evaluate aptamers prior to in vivo applications. Exemplarily, the target binding of a chemically synthesized model aptamer containing phosphorothioate linkages was tested after the induction of the target protein expression on the cell surface by using flow cytometry. Furthermore, different cell types were used to compare the binding of the aptamer. Different single stranded DNA oligonucleotides were selected as negative controls to evaluate sequence specific binding of the aptamer to the cells. In further experiments, the aptamer binding to the target cells was determined in a mixture containing human plasma and peripheral blood cells to simulate the binding of the aptamer to target cells in human whole blood. In this study, we demonstrated the compelling necessity of the in vitro binding tests with the selected aptamers using target and non-target cells, the use of appropriate nonsense aptamers to validate the sequence specific binding of aptamers, and the evaluation of target binding in human plasma containing blood proteins and cells. Thus, we recommend the use of described methods to validate the target specific binding of newly selected aptamers prior to in vivo applications.