Fine-tuning of a three-dimensional microcarrier-based angiogenesis assay for the analysis of endothelial-mesenchymal cell co-cultures in fibrin and collagen gels

Towards a More Objective and High-throughput Spheroid Invasion Assay Quantification Method

Multicellular spheroids embedded in 3D hydrogels are prominent in vitro models for 3D cell invasion. Yet, quantification methods for spheroid cell invasion that are high-throughput, objective and accessible are still lacking. Variations in spheroid sizes and the shapes of the cells within render it difficult to objectively assess invasion extent. The goal of this work is to develop a high-throughput quantification method of cell invasion into 3D matrices that minimizes sensitivity to initial spheroid size and cell spreading and provides precise integrative directionally-dependent metrics of invasion. By analyzing images of fluorescent cell nuclei, invasion metrics are automatically calculated at the pixel level. The initial spheroid boundary is segmented and automated calculations of the nuclear pixel distances from the initial boundary are used to compute common invasion metrics (i.e., the change in invasion area, mean distance) for the same spheroid at a later timepoint. We also introduce the area moment of inertia as an integrative metric of cell invasion that considers the invasion area as well as the pixel distances from the initial spheroid boundary. Further, we show that principal component analysis can be used to quantify the directional influence of a stimuli to invasion (e.g., due to a chemotactic gradient or contact guidance). To demonstrate the power of the analysis for cell types with different invasive potentials and the utility of this method for a variety of biological applications, the method is used to analyze the invasiveness of five different cell types. In all, implementation of this high-throughput quantification method results in consistent and objective analysis of 3D multicellular spheroid invasion. We provide the analysis code in both MATLAB and Python languages as well as a GUI for ease of use for researchers with a range of computer programming skills and for applications in a variety of biological research areas such as wound healing and cancer metastasis.

Precision Culture Scaling to Establish High-Throughput Vasculogenesis Models

Hydrogel-based 3D cell cultures can recapitulate (patho)physiological phenomena ex vivo. However, due to their complex multifactorial regulation, adapting these tissue and disease models for high-throughput screening workflows remains challenging. In this study, a new precision culture scaling (PCS-X) methodology combines statistical techniques (design of experiment and multiple linear regression) with automated, parallelized experiments and analyses to customize hydrogel-based vasculogenesis cultures using human umbilical vein endothelial cells and retinal microvascular endothelial cells. Variations of cell density, growth factor supplementation, and media composition are systematically explored to induce vasculogenesis in endothelial mono- and cocultures with mesenchymal stromal cells or retinal microvascular pericytes in 384-well plate formats. The developed cultures are shown to respond to vasculogenesis inhibitors in a compound- and dose-dependent manner, demonstrating the scope and power of PCS-X in creating parallelized tissue and disease models for drug discovery and individualized therapies.

Biomolecules based hydrogels and their potential biomedical applications: A comprehensive review

The need for biocompatible drug carriers has been significantly increased from the past few years. Researchers show great interest in the development of more versatile and sophisticated biomaterials based drug carriers. Hydrogels are beneficial drug carriers and easily release the controlled amount of drug at target site due to its tunable structure. The hydrogels made-up of potent biological macromolecules including collagen, gelatin, fibrin, elastin, fibroin, chitosan, starch, alginate, agarose and carrageenan have been proven as versatile biomaterials. These are three-dimensional polymeric networks, synthesized by crosslinking of hydrophilic polymers. The biological macromolecules based hydrogels containing therapeutic substances are used in a wide range of biomedical applications including wound healing, tissue engineering, cosmetics and contact lenses. However, many aspects related to hydrogels such as the mechanism of cross-linking and molecular entanglement are not clear. So, there is a need to do more research and exploration toward the extensive and cost-effective use of hydrogels. The present review article elaborately discusses the biomolecules based hydrogels and their possible biomedical applications in different fields.

New Solutions for Old Problems: How Reproductive Tissue Engineering Has Been Revolutionizing Reproductive Medicine

Acquired disorders and congenital defects of the male and female reproductive systems can have profound impacts on patients, causing sexual and endocrine dysfunction and infertility, as well as psychosocial consequences that affect their self-esteem, identity, sexuality, and relationships. Reproductive tissue engineering (REPROTEN) is a promising approach to restore fertility and improve the quality of life of patients with reproductive disorders by developing, replacing, or regenerating cells, tissues, and organs from the reproductive and urinary systems. In this review, we explore the latest advancements in REPROTEN techniques and their applications for addressing degenerative conditions in male and female reproductive organs. We discuss current research and clinical outcomes and highlight the potential of 3D constructs utilizing biomaterials such as scaffolds, cells, and biologically active molecules. Our review offers a comprehensive guide for researchers and clinicians, providing insights into how to reestablish reproductive tissue structure and function using innovative surgical approaches and biomaterials. We highlight the benefits of REPROTEN for patients, including preservation of fertility and hormonal production, reconstruction of uterine and cervical structures, and restoration of sexual and urinary functions. Despite significant progress, REPROTEN still faces ethical and technical challenges that need to be addressed. Our review underscores the importance of continued research in this field to advance the development of effective and safe REPROTEN approaches for patients with reproductive disorders.

Recent Developments in Biopolymer-Based Hydrogels for Tissue Engineering Applications

Hydrogels are being investigated for their application in inducing the regeneration of various tissues, and suitable conditions for each tissue are becoming more apparent. Conditions such as the mechanical properties, degradation period, degradation mechanism, and cell affinity can be tailored by changing the molecular structure, especially in the case of polymers. Furthermore, many high-functional hydrogels with drug delivery systems (DDSs), in which drugs or bioactive substances are contained in controlled hydrogels, have been reported. This review focuses on the molecular design and function of biopolymer-based hydrogels and introduces recent developments in functional hydrogels for clinical applications.

Angiogenesis Invasion Assay to Study Endothelial Cell Invasion and Sprouting Behavior

Angiogenesis is the formation of new blood vessels from the existing vasculature. It is a fundamental process in developmental biology but also a pathological event that initiates or aggravates many diseases. In this complex multistep process, endothelial cells are activated by angiogenic stimuli; undergo specialization in response to VEGF/Notch signaling; degrade the basement membrane of the parent vessel; sprout, migrate, and proliferate to form capillary tubes that branch; and ultimately anastomose with adjacent vessels. Here we describe an assay that mimics the invasion step in vitro. Human microvascular endothelial cells are confronted by a VEGF-enriched basement membrane material in a three-dimensional environment that promotes endothelial cell sprouting, tube formation, and anastomosis. After a few hours, endothelial cells have become tip cells, and vascular sprouts can be observed by phase contrast, fluorescence, or time-lapse microscopy. Sprouting endothelial cells express tip cell markers, display podosomes and filopodia, and exhibit cell dynamics similar to those of angiogenic endothelial cells in vivo. This model provides a system that can be manipulated genetically to study physiological or pathological angiogenesis and that can be used to screen compounds for pro-/anti-angiogenic properties. In this chapter, we describe the key steps in setting up this assay.

Unraveling Muscle Impairment Associated With COVID-19 and the Role of 3D Culture in Its Investigation

COVID-19, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has been considered a public health emergency, extensively investigated by researchers. Accordingly, the respiratory tract has been the main research focus, with some other studies outlining the effects on the neurological, cardiovascular, and renal systems. However, concerning SARS-CoV-2 outcomes on skeletal muscle, scientific evidence is still not sufficiently strong to trace, treat and prevent possible muscle impairment due to the COVID-19. Simultaneously, there has been a considerable amount of studies reporting skeletal muscle damage in the context of COVID-19. Among the detrimental musculoskeletal conditions associated with the viral infection, the most commonly described are sarcopenia, cachexia, myalgia, myositis, rhabdomyolysis, atrophy, peripheral neuropathy, and Guillain-Barré Syndrome. Of note, the risk of developing sarcopenia during or after COVID-19 is relatively high, which poses special importance to the condition amid the SARS-CoV-2 infection. The yet uncovered mechanisms by which musculoskeletal injury takes place in COVID-19 and the lack of published methods tailored to study the correlation between COVID-19 and skeletal muscle hinder the ability of healthcare professionals to provide SARS-CoV-2 infected patients with an adequate treatment plan. The present review aims to minimize this burden by both thoroughly exploring the interaction between COVID-19 and the musculoskeletal system and examining the cutting-edge 3D cell culture techniques capable of revolutionizing the study of muscle dynamics.

Available In Vitro Models for Human Satellite Cells from Skeletal Muscle

Skeletal muscle accounts for almost 40% of the total adult human body mass. This tissue is essential for structural and mechanical functions such as posture, locomotion, and breathing, and it is endowed with an extraordinary ability to adapt to physiological changes associated with growth and physical exercise, as well as tissue damage. Moreover, skeletal muscle is the most age-sensitive tissue in mammals. Due to aging, but also to several diseases, muscle wasting occurs with a loss of muscle mass and functionality, resulting from disuse atrophy and defective muscle regeneration, associated with dysfunction of satellite cells, which are the cells responsible for maintaining and repairing adult muscle. The most established cell lines commonly used to study muscle homeostasis come from rodents, but there is a need to study skeletal muscle using human models, which, due to ethical implications, consist primarily of in vitro culture, which is the only alternative way to vertebrate model organisms. This review will survey in vitro 2D/3D models of human satellite cells to assess skeletal muscle biology for pre-clinical investigations and future directions.

Establishment of a 3D model of tumor-driven angiogenesis to study the effects of anti-angiogenic drugs on pericyte recruitment

Hepatocellular carcinoma (HCC), as a well-vascularized tumor, has attracted increasing attention in antiangiogenic therapies. Notably, emerging studies reveal that the long-term administration of antiangiogenic drugs induces hypoxia in tumors. Pericytes, which play a vital role in vascular stabilization and maturation, have been documented to be associated with antiangiogenic drug-induced tumor hypoxia. However, the role of antiangiogenic agents in regulating pericyte behavior still remains elusive. In this study, by using immunostaining analysis, we first demonstrated that tumors obtained from HCC patients were highly angiogenic, in which vessels were irregularly covered by pericytes. Therefore, we established a new 3D model of tumor-driven angiogenesis by culturing endothelial cells, pericytes, cancer stem cells (CSCs) and mesenchymal stem cells (MSCs) with microcarriers in order to investigate the effects and mechanisms exerted by antiangiogenic agents on pericyte recruitment during tumor angiogenesis. Interestingly, microcarriers, as supporting matrices, enhanced the interactions between tumor cells and the extracellular matrix (ECM), promoted malignancy of tumor cells and increased tumor angiogenesis within the 3D model, as determined by qRT-PCR and immunostaining. More importantly, we showed that zoledronic acid (ZA) reversed the inhibited pericyte recruitment, which was induced by sorafenib (Sora) treatment, through fostering the expression and activation of ErbB1/ErbB2 and PDGFR-β in pericytes, in both an in vitro 3D model and an in vivo xenograft HCC mouse model. Hence, our model provides a more pathophysiologically relevant platform for the assessment of therapeutic effects of antiangiogenic compounds and identification of novel pharmacological targets, which might efficiently improve the benefits of antiangiogenic treatment for HCC patients.

3D in vitro models of skeletal muscle: myopshere, myobundle and bioprinted muscle construct

Typical two-dimensional (2D) culture models of skeletal muscle-derived cells cannot fully recapitulate the organization and function of living muscle tissues, restricting their usefulness in in-depth physiological studies. The development of functional 3D culture models offers a major opportunity to mimic the living tissues and to model muscle diseases. In this respect, this new type of in vitro model significantly increases our understanding of the involvement of the different cell types present in the formation of skeletal muscle and their interactions, as well as the modalities of response of a pathological muscle to new therapies. This second point could lead to the identification of effective treatments. Here, we report the significant progresses that have been made the last years to engineer muscle tissue-like structures, providing useful tools to investigate the behavior of resident cells. Specifically, we interest in the development of myopshere- and myobundle-based systems as well as the bioprinting constructs. The electrical/mechanical stimulation protocols and the co-culture systems developed to improve tissue maturation process and functionalities are presented. The formation of these biomimetic engineered muscle tissues represents a new platform to study skeletal muscle function and spatial organization in large number of physiological and pathological contexts.

Cancer Stem Cell Microenvironment Models with Biomaterial Scaffolds In Vitro

Defined by its potential for self-renewal, differentiation and tumorigenicity, cancer stem cells (CSCs) are considered responsible for drug resistance and relapse. To understand the behavior of CSC, the effects of the microenvironment in each tissue are a matter of great concerns for scientists in cancer biology. However, there are many complicated obstacles in the mimicking the microenvironment of CSCs even with current advanced technology. In this context, novel biomaterials have widely been assessed as in vitro platforms for their ability to mimic cancer microenvironment. These efforts should be successful to identify and characterize various CSCs specific in each type of cancer. Therefore, extracellular matrix scaffolds made of biomaterial will modulate the interactions and facilitate the investigation of CSC associated with biological phenomena simplifying the complexity of the microenvironment. In this review, we summarize latest advances in biomaterial scaffolds, which are exploited to mimic CSC microenvironment, and their chemical and biological requirements with discussion. The discussion includes the possible effects on both cells in tumors and microenvironment to propose what the critical factors are in controlling the CSC microenvironment focusing the future investigation. Our insights on their availability in drug screening will also follow the discussion.

A microcarrier-based spheroid 3D invasion assay to monitor dynamic cell movement in extracellular matrix

Background

Cell invasion through extracellular matrix (ECM) is a critical step in tumor metastasis. To study cell invasion in vitro, the internal microenvironment can be simulated via the application of 3D models.

Results

This study presents a method for 3D invasion examination using microcarrier-based spheroids. Cell invasiveness can be evaluated by quantifying cell dispersion in matrices or tracking cell movement through time-lapse imaging. It allows measuring of cell invasion and monitoring of dynamic cell behavior in three dimensions. Here we show different invasive capacities of several cell types using this method. The content and concentration of matrices can influence cell invasion, which should be optimized before large scale experiments. We also introduce further analysis methods of this 3D invasion assay, including manual measurements and homemade semi-automatic quantification. Finally, our results indicate that the position of spheroids in a matrix has a strong impact on cell moving paths, which may be easily overlooked by researchers and may generate false invasion results.

Conclusions

In all, the microcarrier-based spheroids 3D model allows exploration of adherent cell invasion in a fast and highly reproducible way, and provides informative results on dynamic cell behavior in vitro.

Biofabrication of thick vascularized neo-pedicle flaps for reconstructive surgery

Wound chronicity due to intrinsic and extrinsic factors perturbs adequate lesion closure and reestablishment of the protective skin barrier. Immediate and proper care of chronic wounds is necessary for a swift recovery and a reduction of patient vulnerability to infection. Advanced therapies supplemented with standard wound care procedures have been clinically implemented to restore aberrant tissue; however, these treatments are ineffective if local vasculature is too compromised to support minimally-invasive strategies. Autologous "flaps", which are tissues equipped with their own hierarchical vascular supply, can be harvested from one region of the patient and transplanted to the wound where it is reperfused upon microsurgical anastomosis to appropriate recipient vessels. Despite the success of autologous flap transfer, these procedures are extremely invasive, incur obligatory donor-site morbidity, and require sufficient donor-tissue availability, microsurgical expertise, and specialized equipment. 3D-bioprinting modalities, such as extrusion-based bioprinting, can be used to address the clinical constraints of autologous flap transfer, primarily addressing donor-site morbidity and tissue availability. This advancement in regenerative medicine allows the biofabrication of heterogeneous tissue structures with high shape fidelity and spatial resolution to generate biomimetic constructs with the anatomically-precise geometries of native tissue to ensure tissue-specific function. Yet, meaningful progress toward this clinical application has been limited by the lack of vascularization required to meet the nutrient and oxygen demands of clinically relevant tissue volumes. Thus, various criteria for the fabrication of functional tissues with hierarchical, patent vasculature must be considered when implementing 3D-bioprinting technologies for deep, chronic wounds.

Current Challenges of Bioprinted Tissues Toward Clinical Translation

IMPACT STATEMENT

This review has a broad overview of the current challenges of bioprinted tissues towards clinical translations and future directions to overcome those challenges. The development of this field has a huge impact on the situation of an insufficient number of organ donors for life-saving organ transplantations.

In Vitro Tissue-Engineered Skeletal Muscle Models for Studying Muscle Physiology and Disease

Healthy skeletal muscle possesses the extraordinary ability to regenerate in response to small-scale injuries; however, this self-repair capacity becomes overwhelmed with aging, genetic myopathies, and large muscle loss. The failure of small animal models to accurately replicate human muscle disease, injury and to predict clinically-relevant drug responses has driven the development of high fidelity in vitro skeletal muscle models. Herein, the progress made and challenges ahead in engineering biomimetic human skeletal muscle tissues that can recapitulate muscle development, genetic diseases, regeneration, and drug response is discussed. Bioengineering approaches used to improve engineered muscle structure and function as well as the functionality of satellite cells to allow modeling muscle regeneration in vitro are also highlighted. Next, a historical overview on the generation of skeletal muscle cells and tissues from human pluripotent stem cells, and a discussion on the potential of these approaches to model and treat genetic diseases such as Duchenne muscular dystrophy, is provided. Finally, the need to integrate multiorgan microphysiological systems to generate improved drug discovery technologies with the potential to complement or supersede current preclinical animal models of muscle disease is described.

Mesenchymal stem cells promote lymphangiogenic properties of lymphatic endothelial cells

Lymphatic metastasis is one of the main prognostic factors concerning long-term survival of cancer patients. In this regard, the molecular mechanisms of lymphangiogenesis are still rarely explored. Also, the interactions between stem cells and lymphatic endothelial cells (LEC) in humans have not been well examined. Therefore, the main objective of this study was to assess the interactions between mesenchymal stem cells (MSC) and LEC using in vitro angiogenesis assays. Juvenile LEC were stimulated with VEGF-C, bFGF, MSC-conditioned medium (MSC-CM) or by co-culture with MSC. LEC proliferation was assessed using a MTT assay. Migration of the cells was determined with a wound healing assay and a transmigration assay. To measure the formation of lymphatic sprouts, LEC spheroids were embedded in collagen or fibrin gels. The LEC's capacity to form capillary-like structures was assessed by a tube formation assay on Matrigel^® . The proliferation, migration and tube formation of LEC could be significantly enhanced by MSC-CM and by co-culture with MSC. The effect of stimulation with MSC-CM was stronger compared to stimulation with the growth factors VEGF-C and bFGF in proliferation and transmigration assays. Sprouting was stimulated by VEGF-C, bFGF and by MSC-CM. With this study, we demonstrate the potent stimulating effect of the MSC secretome on proliferation, migration and tube formation of LEC. This indicates an important role of MSC in lymphangiogenesis in pathological as well as physiological processes.

A planar model of the vessel wall from cellularized-collagen scaffolds: focus on cell–matrix interactions in mono-, bi- and tri-culture models

An easy to prepare and manipulate model of the vascular wall in a planar shape to investigate physiological and pathological processes of vascular tissues.

Cardiovascular diseases related to ionizing radiation: The risk of low-dose exposure (Review)

Traditionally, non-cancer diseases are not considered as health risks following exposure to low doses of ionizing radiation. Indeed, non-cancer diseases are classified as deterministic tissue reactions, which are characterized by a threshold dose. It is judged that below an absorbed dose of 100 mGy, no clinically relevant tissue damage occurs, forming the basis for the current radiation protection system concerning non-cancer effects. Recent epidemiological findings point, however, to an excess risk of non-cancer diseases following exposure to lower doses of ionizing radiation than was previously thought. The evidence is the most sound for cardiovascular disease (CVD) and cataract. Due to limited statistical power, the dose-risk relationship is undetermined below 0.5 Gy; however, if this relationship proves to be without a threshold, it may have considerable impact on current low-dose health risk estimates. In this review, we describe the CVD risk related to low doses of ionizing radiation, the clinical manifestation and the pathology of radiation-induced CVD, as well as the importance of the endothelium models in CVD research as a way forward to complement the epidemiological data with the underlying biological and molecular mechanisms.

The artificial ovary: current status and future perspectives

Cryopreservation and transplantation of ovarian tissue has proved to be a promising technique to safeguard fertility in cancer patients. However, with some types of cancer, there is a risk of transmitting malignant cells present in the cryopreserved tissue, so transplantation after disease remission is not advisable. To restore fertility in these patients, some research teams have been developing a transplantable artificial ovary, whose main goal is to mimic the natural organ. It should be composed of a matrix that encapsulates and protects follicles, as well as ovarian cells, which are necessary for follicle survival and development. This article reviews progress made in the creation of a transplantable artificial ovary and discusses future trends for its development.

Fabrication of Biodegradable Synthetic Vascular Networks and Their Use as a Model of Angiogenesis

One of the greatest challenges currently faced in tissue engineering is the incorporation of vascular networks within tissue-engineered constructs. The aim of this study was to develop a technique for producing a perfusable, 3-dimensional, cell-friendly model of vascular structures that could be used to study the factors affecting angiogenesis and vascular biology in engineered systems in more detail. Initially, biodegradable synthetic pseudovascular networks were produced via the combination of robocasting and electrospinning techniques. The internal surfaces of the vascular channels were then recellularized with human dermal microvascular endothelial cells (HDMECs) with and without the presence of human dermal fibroblasts (HDFs) on the outer surface of the scaffold. After 7 days in culture, channels that had been reseeded with HDMECs alone demonstrated irregular cell coverage. However, when using a co-culture of HDMECs inside and HDFs outside the vascular channels, coverage was found to be continuous throughout the internal channel. Using this cell combination, collagen gels loaded with vascular endothelial growth factor were deposited onto the outer surface of the scaffold and cultured for a further 7 days. After this, endothelial cell outgrowth from within the channels into the collagen gel was observed, showing that the engineered vasculature maintains its capacity for angiogenesis. Furthermore, the HDMECs appeared to have formed perfusable tubules within the gel. These results show promising steps towards the development of an in vitro platform for studying angiogenesis and vascular biology in a tissue engineering context.

Endothelial sprouting and network formation in collagen- and fibrin-based modular microbeads

A modular tissue engineering approach may have advantages over current therapies in providing rapid and sustained revascularization of ischemic tissue. In this study, modular protein microbeads were prepared from pure fibrin (FIB) and collagen-fibrin composites (COL-FIB) using a simple water-in-oil emulsification technique. Human endothelial cells and fibroblasts were embedded directly in the microbead matrix. The resulting microbeads were generally spheroidal with a diameter of 100-200μm. Cell viability was high (75-80% viable) in microbeads, but was marginally lower than in bulk hydrogels of corresponding composition (85-90% viable). Cell proliferation was significantly greater in COL-FIB microbeads after two weeks in culture, compared to pure FIB microbeads. Upon embedding of microbeads in a surrounding fibrin hydrogel, endothelial cell networks formed inside the microbead matrix and extended into the surrounding matrix. The number of vessel segments, average segment length, and number of branch points was higher in FIB samples, compared to COL-FIB samples, resulting in significantly longer total vessel networks. Anastomosis of vessel networks from adjacent microbeads was also observed. These studies demonstrate that primitive vessel networks can be formed by modular protein microbeads containing embedded endothelial cells and fibroblasts. Such microbeads may find utility as prevascularized tissue modules that can be delivered minimally invasively as a therapy to restore blood flow to ischemic tissues.

STATEMENT OF SIGNIFICANCE

Vascularization is critically important for tissue engineering and regenerative medicine, and materials that support and/or promote neovascularization are of value both for translational applications and for mechanistic studies and discovery-based research. Therefore, we fabricated small modular microbeads formulated from pure fibrin (FIB) and collagen-fibrin (COL-FIB) containing endothelial cells and supportive fibroblasts. We explored how cells encapsulated within these materials form microvessel-like networks both within and outside of the microbeads when embedded in larger 3D matrices. FIB microbeads were found to initiate more extensive sprouting into the surrounding ECM in vitro. These results represent an important step towards our goal of developing injectable biomaterial modules containing preformed vascular units that can rapidly restore vascularization to an ischemic tissue in vivo.

Engineering of a disulfide loop instead of a Zn binding loop restores the anti-proliferative, anti-angiogenic and anti-tumor activities of the N-terminal fragment of endostatin: Mechanistic and therapeutic insights

Although considerable effort has been devoted to understanding the molecular mechanism of endostatin's anti-cancer activity, the role of its Zn bound N-terminal loop has not been completely clarified. To investigate whether Zn binding or the N-terminal loop is involved in the anti-cancer properties of endostatin, we compared the structure and biological activity of a native Zn binding endostatin peptide (ES-Zn) with three variants: a Zn free variant (ES), a variant containing both a Zn binding site and a disulfide bond (ES-SSZn), and a variant including a disulfide loop but incapable of Zn binding (ES-SS). Spectroscopic studies indicated that ES-Zn and ES-SS consist of random coil and β structures, whereas ES-SSZn and ES fold into random coils. Theoretical analysis proposed that ES-Zn and ES-SS have a similar binding site to αVβ3 integrin. The anti-proliferative activity of endostatin was retained by all peptides except ES, and the in vitro anti-angiogenic property was preserved in ES-Zn and ES-SS. Remarkably, breast tumor growth and CD31 activity were inhibited more effectively by ES-SS than by ES-Zn. Therefore, a correlation exists between the N-terminal loop and anti-cancer properties of endostatin fragment and a disulfide loop may be more promising than a Zn binding loop for inhibiting tumor growth.

Effects of hydroxyapatite on endothelial network formation in collagen/fibrin composite hydrogels in vitro and in vivo

Co-culture of endothelial cells (EC) and mesenchymal stem cells (MSC) results in robust vascular network formation in constrained 3-D collagen/fibrin (COL/FIB) composite hydrogels. However, the ability to form endothelial networks is lost when such gels are allowed to compact via cell-mediated remodeling. In this study, we created co-cultures of human EC and human MSC in both constrained and unconstrained COL/FIB matrices and systematically added nanoparticulate hydroxyapatite (HA, 0-20 mg ml(-1)), a bone-like mineral that has been shown to have pro-vasculogenic effects. Constructs cultured for 7 days were assayed for gel compaction, vascular network formation, and mechanical properties. In vitro, robust endothelial network formation was observed in constrained COL/FIB constructs without HA, but this response was significantly inhibited by addition of 5, 10, or 20 mg ml(-1) HA. In unconstrained matrices, network formation was abolished in pure COL/FIB constructs but was rescued by 1.25 or 2.5 mg ml(-1) HA, while higher levels again inhibited vasculogenesis. HA inhibited gel compaction in a dose-dependent manner, which was not correlated to endothelial network formation. HA affected initial stiffness of the gels, but gel remodeling abrogated this effect. Subcutaneous implantation of COL/FIB with 0, 2.5 or 2 0mg ml(-1) HA in the mouse resulted in increased perfusion at the implant site, with no significant differences between materials. Histology at day 7 showed both host and human CD31-stained vasculature infiltrating the implants. These findings are relevant to the design of materials and scaffolds for orthopedic tissue engineering, where both vasculogenesis and formation of a mineral phase are required for regeneration.

Using Del-1 to tip the angiogenic balance in endothelial cells in modular constructs

Modular tissue engineering is a method of building vascularized tissue-engineered constructs. Submillimeter-sized collagen pieces (modules) coated with a layer of endothelial cells (EC; vascular component), and with embedded functional cells, are self-assembled into a larger, three-dimensional tissue. In this study, we examined the use of developmental endothelial locus-1 (Del-1), an extracellular matrix protein with proangiogenic properties, as a means of tipping the angiogenic balance in human umbilical vein endothelial cells incorporated in modular tissue-engineered constructs. The motivation was to enhance the vascularization of these constructs upon transplantation in vivo, in this case, without the use of exogenous mesenchymal stromal cells. EC were transduced using a lentiviral construct to overexpress Del-1. The Del-1 EC formed more sprouts in a fibrin gel sprouting assay in vitro compared with eGFP (control) transduced EC, as expected. Del-1 EC had a distinct profile of gene expression (upregulation of matrix metalloproteinase-9 [MMP-9], urokinase-type plasminogen activator [uPA/PLAU], vascular endothelial growth factor [VEGF-A], and intercellular adhesion molecule-1 [ICAM-1]; downregulation of angiopoietin-2 [Ang2]), also supporting the notion of "tipping the angiogenic balance". On the other hand, contrary to our expectations, when Del-1 EC-coated modules were implanted subcutaneously in a severe combined immunodeficient/beige animal model, the proangiogenic effect of Del-1 was less remarkable. There was only a small increase in the number of blood vessels formed in Del-1 implants compared with the eGFP implants, and only few blood vessels formed at the implant site in both cases. This was presumed due to limited EC survival after transplantation. We speculate that if we could improve EC survival in our study (for example, by adding other prosurvival factors or supporting cells), we would see a greater Del-1-induced angiogenic benefit in vivo as a consequence of increased Del-1 secretion by a higher number of surviving cells.

Angiogenic endothelial cell invasion into fibrin is stimulated by proliferating smooth muscle cells

These studies aimed to determine the effect of smooth muscle cells (SMCs) on angiogenic behavior of endothelial cells (ECs) within fibrin hydrogels, an extracellular matrix (ECM) commonly used in tissue engineering. We developed a 3-D, fibrin-based co-culture assay of angiogenesis consisting of aggregates of SMCs with ECs seeded onto the aggregates' surface. Using digital fluorescence micrography, EC matrix invasion was quantified by average length of sprouts (ALS) and density of sprout formation (DSF). We demonstrated that ECs and SMCs co-invade into the ECM in close proximity to one another. ECs that were co-cultured with SMCs demonstrated increased invasion compared to ECs that were cultured alone at all time points. At Day 19, the ALS of ECs in co-culture was 327+/-58μm versus 70+/-11μm of ECs cultured alone (p=.01). The DSF of co-cultured ECs was also significantly greater than that of ECs cultured alone (p=.007 on Day 19). This appeared to be a function of both increased EC invasion as well as improved persistence of EC sprout networks. At 7days, ECs in co-culture with proliferation-inhibited SMCs previously treated with Mitomycin-C (MMC) demonstrated significantly attenuated sprouting compared to ECs co-cultured with SMCs that were untreated with MMC (82+/-14μm versus 205+/-32μm; p<.05). In assays in which multiple co-culture aggregates were cultured within a single hydrogel, we observed directional invasion of sprouts preferentially towards the other aggregates within the hydrogel. In co-culture assays without early EC/SMC contact, the ALS of ECs cultured in the presence of SMCs was significantly greater than those cultured in the absence of SMCs by Day 3 (320+/-21μm versus 187+/-16μm; p<.005). We conclude that SMCs augment EC matrix invasion into 3-D fibrin hydrogels, at least in part resulting from SMC proliferative and invasive activities. Directed invasion between co-culture aggregates and augmented angiogenesis in the absence of early contact suggests a paracrine mechanism for the observed results.

Towards constructing extracellular matrix-mimetic hydrogels: an elastic hydrogel constructed from tandem modular proteins containing tenascin FnIII domains

Protein-based hydrogels have been developed for various biomedical applications where they provide artificial extracellular microenvironments that mimic the physical and biochemical characteristics of natural extracellular matrices (ECMs). In natural ECMs, a large number of proteins are tandem modular proteins consisting of many individually folded functional domains that confer structural and biological functionalities. Such tandem modular proteins are promising building blocks for constructing ECM-mimetic biomaterials. However, their use for such purposes has not been explored extensively. Tenascin-C (TNC) is an ECM tandem modular protein and plays an important role in mechanotransduction by regulating important cell-matrix interactions. The third FnIII domain of TNC (TNfn3) contains an RGD sequence and is known to bind integrins. Here we use the TNfn3 domain and resilin sequence-based tandem modular protein FRF4RF4R (F represents the TNfn3 domain and R represents the resilin sequence, respectively) as a building block to construct protein-based ECM-mimetic hydrogels. The tandem modular protein-based building block FRF4RF4R closely mimics the architecture of the naturally occurring tandem modular ECM protein TNC and incorporates intact RGD-containing FnIII domains. Our results demonstrate that tandem modular proteins containing TNfn3 can be readily photochemically crosslinked into elastic hydrogels, whose Young's modulus can be tuned by the concentration of the tandem modular protein solution. In vitro studies demonstrate that none of the photochemical crosslinking reaction components are cytotoxic at the level tested, and the hydrogel supports the spread of human lung fibroblast cells. Our results demonstrate that FRF4RF4R-based hydrogel is a novel ECM-mimetic hydrogel.

Enhanced survival and neurite network formation of human umbilical cord blood neuronal progenitors in three-dimensional collagen constructs

Umbilical cord blood (CB) stem cells have been proposed for cell-based therapeutic applications for diverse diseases of the CNS. We hypothesized that tissue-engineering strategies may extend the efficacy of these approaches by improving the long-term viability and function of stem cell-derived neuronal progenitors. To test our hypothesis, we explored the survival and differentiation of human CB-derived neuronal progenitors (HUCBNP) in a three-dimensional (3D) collagen construct. In contrast to two-dimensional culture conditions, the cells survived in 3D for an extended period of time of more than 2 months. Under 3D conditions, HUCBNP underwent spontaneous neuronal differentiation, which was further enhanced by treatment with neuronal conditioned medium (CM) and nerve growth factor (NGF). Neurite outgrowth, quantified by assessing the fractal dimension (D f) of the complex neuronal networks, was significantly enhanced under 3D conditions in the presence of CM/NGF, concomitant with a reduced expression of the early neuronal marker nestin (1.9-fold), and increased levels of mature neuronal markers such as MAP-2 (3.6-fold), β-tubulin (1.5-fold), and neuronal specific enolase (6.6-fold) and the appearance of the synaptic marker synaptophysin. To assess the feasibility for clinical usage, HUCBNP were also isolated from frozen CB samples and cultured under 3D conditions. The data indicate the essential complete preservation of neurotrophic (survival) and neurotropic (neurite outgrowth) properties. In conclusion, 3D culture conditions are proposed as an essential step for both maintenance of CB neuronal progenitors in vitro and for investigating specific features of neuronal differentiation towards future use in regenerative therapy.

Fluid pressure is a magnitude-dependent modulator of early endothelial tubulogenic activity: implications related to a potential tissue-engineering control parameter

A significant barrier to the success of engineered tissues is the inadequate transport of nutrients and gases to, and waste away from, cells within the constructs, after implantation. Generation of microtubular networks by endothelial cells in engineered constructs to mimic the in vivo transport scheme is essential for facilitating tissue survival by promoting the in vitro formation of microvessels that integrate with host microvasculature, after implantation. Previously, we reported that select pressures stimulate endothelial proliferation involving protubulogenic molecules such as fibroblast growth factor-2 (FGF-2) and vascular endothelial growth factor-C (VEGF-C). Based on this, we investigated fluid pressure as a selective modulator of early tubulogenic activity with the intent of assessing the potential utility of this mechanical stimulus as a tissue-engineering control parameter. For this purpose, we used a custom pressure system to expose two-dimensional (2D) and three-dimensional (3D) cultures of endothelial cells to static pressures of 0 (controls), 20, or 40 mmHg for 3 days. Compared to controls, 2D endothelial cultures exposed to 20, but not 40 mmHg, exhibited significantly (p<0.05) enhanced cell growth that depended on VEGF receptor-3 (VEGFR-3), a receptor for VEGF-C. Moreover, endothelial cells grown on microbeads and suspended in 3D collagen gels under 20 mmHg, but not 40 mmHg, displayed significantly (p<0.05) increased sprout formation. Interestingly, pressure-dependent proliferation and sprout formation occurred in parallel with pressure-sensitive upregulation of VEGF-C and VEGFR-3 expression and were sensitive to local FGF-2 levels. Collectively, the results of the present study provided evidence that early endothelial-related tubulogenic activity depends on local hydrostatic pressure levels in the context of local growth factor conditions. In addition to relevance to microvascular diseases associated with interstitial hypertension (e.g., cancer and glaucoma), these findings provided first insight into the potential utility of hydrostatic pressure as a fine-tune control parameter to optimize microvascularization of tissue-engineering constructs in the in vitro setting before their implantation.

Matrix composition regulates three-dimensional network formation by endothelial cells and mesenchymal stem cells in collagen/fibrin materials

Co-cultures of endothelial cells (EC) and mesenchymal stem cells (MSC) in three-dimensional (3D) protein hydrogels can be used to recapitulate aspects of vasculogenesis in vitro. MSC provide paracrine signals that stimulate EC to form vessel-like structures, which mature as the MSC transition to the role of mural cells. In this study, vessel-like network formation was studied using 3D collagen/fibrin (COL/FIB) matrices seeded with embedded EC and MSC and cultured for 7 days. The EC:MSC ratio was varied from 5:1, 3:2, 1:1, 2:3 and 1:5. The matrix composition was varied at COL/FIB compositions of 100/0 (pure COL), 60/40, 50/50, 40/60 and 0/100 (pure FIB). Vasculogenesis was markedly decreased in the highest EC:MSC ratio, relative to the other cell ratios. Network formation increased with increasing fibrin content in composite materials, although the 40/60 COL/FIB and pure fibrin materials exhibited the same degree of vasculogenesis. EC and MSC were co-localized in vessel-like structures after 7 days and total cell number increased by approximately 70%. Mechanical property measurements showed an inverse correlation between matrix stiffness and network formation. The effect of matrix stiffness was further investigated using gels made with varying total protein content and by crosslinking the matrix using the dialdehyde glyoxal. This systematic series of studies demonstrates that matrix composition regulates vasculogenesis in 3D protein hydrogels, and further suggests that this effect may be caused by matrix mechanical properties. These findings have relevance to the study of neovessel formation and the development of strategies to promote vascularization in transplanted tissues.

How blood vessel networks are made and measured

Tissue and organ viability depends on the proper systemic distribution of cells, nutrients, and oxygen through blood vessel networks. These networks arise in part via angiogenic sprouting. Vessel sprouting involves the precise coordination of several endothelial cell processes including cell-cell communication, cell migration, and proliferation. In this review, we discuss zebrafish and mammalian models of blood vessel sprouting and the quantification methods used to assess vessel sprouting and network formation in these models. We also review the mechanisms involved in angiogenic sprouting, and we propose that the process consists of distinct stages. Sprout initiation involves endothelial cell interactions with neighboring cells and the environment to establish a specialized tip cell responsible for leading the emerging sprout. Furthermore, local sprout guidance cues that spatially regulate this outward migration are discussed. We also examine subsequent events, such as sprout fusion and lumenization, that lead to maturation of a nascent sprout into a patent blood vessel.

Paracrine factors released by GATA-4 overexpressed mesenchymal stem cells increase angiogenesis and cell survival

Transplanted mesenchymal stem cells (MSC) release soluble factors that contribute to cardiac repair and vascular regeneration. We hypothesized that overexpression of GATA-4 enhances the MSC secretome, thereby increasing cell survival and promoting postinfarction cardiac angiogenesis. MSCs harvested from male rat bone marrow were transduced with GATA-4 (MSC(GATA-4)) using the murine stem cell virus retroviral expression system; control cells were either nontransduced (MSC(bas)) or transduced with empty vector (MSC(Null)). Compared with these control cells, MSC(GATA-4) were shown by immunofluorescence, real-time PCR, and Western blotting to have higher expression of GATA-4. An increased expression of angiogenic factors in MSC(GATA-4) and higher MSC resistance against hypoxia were observed. Human umbilical vein endothelial cells (HUVEC) treated with MSC(GATA-4) conditioned medium exhibited increased formation of capillary-like structures and promoted migration, compared with HUVECs treated with MSC(Null) conditioned medium. MSC(GATA-4) were injected into the peri-infarct region in an acute myocardial infarction model in Sprague-Dawley rats developed by ligation of the left anterior descending coronary artery. Survival of MSC(GATA-4), determined by Sry expression, was increased at 4 days postengraftment. MSC(GATA-4)-treated animals showed significantly improved cardiac function as assessed by echocardiography. Furthermore, fluorescent microsphere and histological studies revealed increased blood flow and blood vessel density and reduced infarction size in MSC(GATA-4)-treated animals. We conclude that GATA-4 overexpression in MSCs increased both MSC survival and angiogenic potential in ischemic myocardium and may therefore represent a novel and efficient therapeutic approach for postinfarct remodeling.

Engineering more than a cell: vascularization strategies in tissue engineering

Host integration and performance of engineered tissues have been severely limited by the lack of robust strategies to generate patent vascularization and tissue perfusion. This review highlights a selection of exciting developments in vascularization approaches for tissue engineering research. Current strategies for vascularization in tissue engineering are related to growth factor signaling and delivery, cell transplantation, bioactive smart matrix materials, and directed fabrication. Application of these techniques to in vivo models has resulted in a number of robust host vascular responses, especially with synergistic and engineered bioactive systems. The future outlook of the field includes refinement and development of new technologies for vascularization and combining these techniques with functional repair models for metabolically active tissues and relevant disease states.

Complex temporal regulation of capillary morphogenesis by fibroblasts

Interactions between endothelial and stromal cells are important for vascularization of regenerating tissue. Fibroblasts (FBs) are responsible for expression of angiogenic growth factors and matrix metalloproteinases, as well as collagen deposition and fibrotic myocardial remodeling. Recently, self-assembling peptide nanofibers were described as a promising environment for cardiac regeneration due to its synthetic nature and control over physiochemical properties. In this study, peptide nanofibers were used as a model system to quantify the dual role of fibroblasts in mediating angiogenesis chemically via expression of angiogenic factors and mechanically via cell-mediated scaffold disruption, extracellular matrix deposition, and remodeling. Human microvascular endothelial cells (ECs), FBs, or cocultures were cultured in three-dimensional nanofibers for up to 6 days. The peptide nanofiber microenvironment supported cell migration, capillary network formation, and cell survival in the absence of detectable scaffold contraction and proteolytic degradation. FBs enhanced early capillary network formation by "assisting" EC migration and increasing vascular endothelial growth factor and Angiopoietin-1 expression in a temporal manner. EC-FB interactions attenuated FB matrix metalloproteinase-2 expression while increasing collagen I deposition, resulting in greater construct stiffness and a more stable microenvironment in cocultures. Whereas FBs are critical for initial steps of angiogenesis in the absence of external angiogenic stimulation, coordinated efforts by ECs and FBs are required for a balance between cell-mediated scaffold disruption, extracellular matrix deposition, and remodeling at later time points. The findings of this study also emphasize the importance of developing a microenvironment that supports cell-cell interactions and cell migration, thus contributing toward an optimal environment for successful cardiac regeneration strategies.

Molecular mediators of angiogenesis

Angiogenesis, or the formation of new blood vessels from the preexisting vasculature, is a key component in numerous physiologic and pathologic responses and has broad impact in many medical and surgical specialties. In this review, we discuss the key cellular steps that lead to the neovascularization of tissues and highlight the main molecular mechanisms and mediators in this process. We include discussions on proteolytic enzymes, cell-matrix interactions, and pertinent cell signaling pathways and end with a survey of the mechanisms that lead to the stabilization and maturation of neovasculatures.

Transfer of two-dimensional patterns of human umbilical vein endothelial cells into fibrin gels to facilitate vessel formation

Two-dimensional cell patterns prepared on substrate surfaces by an electrochemical-based biolithography method have been transferred into fibrin gels prepared in situ. Line patterns of human umbilical vein endothelial cells (HUVECs) in the gel that was strained after the transfer formed a linear vessel-like structure within 8 days.

Analysis of MMP-dependent cell migration and invasion

Analysing cell migration and invasion is of interest to many investigators as they mimic a part of physiological or pathological events. In this chapter, methods to analyse MMP-dependent 2D and 3D cell migration are described in detail. To study 2D cell migration, the phagokinetic track motility assay can be used. To study 3D cell migration, Matrigel invasion assay or microcarrier beads invasion assay can be used to obtain quantitative results.

In vitro 3D model for human vascularized adipose tissue

The clinical need for both three-dimensional (3D) soft tissue replacements and in vitro adipose tissue models continues to grow. In this study, we evaluated structural and functional characteristics of an in vitro 3D coculture model of vascularized adipose tissue. Tomato red-infected human adipose tissue-derived mesenchymal stem cells (hASCs) and green fluorescence protein-infected human umbilical vein endothelial cells were cocultured on 3D aqueous-derived silk scaffolds for 2 weeks. Confocal microscopy images demonstrated viability of cocultures and organization of both cell types over time. Endothelial cells aligned with time, and further histological analyses revealed continuous endothelial lumen formation in both differentiated and undifferentiated cocultures. Differentiated adipose cocultures secreted significantly higher levels of leptin than undifferentiated cocultures at 1 and 2 weeks. Additionally, lipid accumulation was demonstrated with Oil Red O staining, where positive staining was higher in the differentiated cocultures. A promising in vitro approach for the vascularization of tissue-engineered adipose tissue, and the ability to vascularize a construct containing hASCs was demonstrated. The strategy outlined provides a basis for the formation of other in vitro vascularized tissues as well as a path forward for the sustainable formation of soft tissue due to the use of slowly degrading silk scaffolds.

Fibrin: a versatile scaffold for tissue engineering applications

Tissue engineering combines cell and molecular biology with materials and mechanical engineering to replace damaged or diseased organs and tissues. Fibrin is a critical blood component responsible for hemostasis, which has been used extensively as a biopolymer scaffold in tissue engineering. In this review we summarize the latest developments in organ and tissue regeneration using fibrin as the scaffold material. Commercially available fibrinogen and thrombin are combined to form a fibrin hydrogel. The incorporation of bioactive peptides and growth factors via a heparin-binding delivery system improves the functionality of fibrin as a scaffold. New technologies such as inkjet printing and magnetically influenced self-assembly can alter the geometry of the fibrin structure into appropriate and predictable forms. Fibrin can be prepared from autologous plasma, and is available as glue or as engineered microbeads. Fibrin alone or in combination with other materials has been used as a biological scaffold for stem or primary cells to regenerate adipose tissue, bone, cardiac tissue, cartilage, liver, nervous tissue, ocular tissue, skin, tendons, and ligaments. Thus, fibrin is a versatile biopolymer, which shows a great potential in tissue regeneration and wound healing.

Update on therapeutic vascularization strategies

The ability to exploit angiogenesis and vascularization as a therapeutic strategy will be of enormous benefit to a wide range of medical and tissue-engineering applications. Angiogenic growth factor and cell-based therapies have thus far failed to produce a robust healing response in clinical trials for a variety of ischemic diseases, while engineered tissue substitutes are still size-limited by a lack of vascularization. The purpose of this review is to investigate current research advances in therapeutic vascularization strategies applied to ischemic disease states, tissue engineering and regenerative medicine. Recent advances are discussed that focus on better regulation of growth factor delivery and attempts to better mimic natural processes by delivering combinations of multiple growth factors, cells and bioactive materials in the right spatial and temporal setting. Some unconventional approaches and novel therapeutic targets that hold significant potential are also discussed.

Enhanced angiogenesis in porous collagen-chitosan scaffolds loaded with angiogenin

Artificial dermis lacks a vascular network, and angiogenesis is slow in vivo. Controlled delivery of angiogenin (ANG), a potent inducer of angiogenesis, should promote angiogenesis in artificial dermis. In this study, a porous collagen-chitosan scaffold was fabricated and heparinized using N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide (EDC) and N-hydroxysuccinimide (NHS) with a freeze-drying method. Using radioiodine labeling, the effect of heparin on the binding of ANG to the scaffold was studied. The release of ANG from the heparinized scaffold was investigated using a radioiodine labeling method or an enzyme-linked immunosorbent assay method. In vivo angiogenesis of the scaffold was studied for 28 days. All scaffolds possess three-dimensional porous structures, and their mean pore sizes increase upon EDC-NHS cross-linking. The binding of ANG to the scaffold showed a linear correlation with ANG concentration. With ANG concentrations of 160 ng/mL, the binding of ANG to the heparinized scaffold was 36.5%. In vitro, ANG was released from the heparinized scaffold in a controlled manner. The presence of ANG enhanced the angiogenesis of the heparinized scaffold after subcutaneous implantation into rabbits. The results of this study indicate that a porous collagen-chitosan scaffold loaded with ANG may be valuable in the development of artificial dermis requiring enhanced angiogenesis.

Formation of composite endothelial cell-mesenchymal stem cell islets: a novel approach to promote islet revascularization

OBJECTIVE

Mesenchymal stem cells (MSCs) contribute to endothelial cell (EC) migration by producing proteases, thereby paving the way into the tissues for ECs. MSCs were added to our previously described composite EC islets as a potential means to improve their capacity for islet angiogenesis.

RESEARCH DESIGN AND METHODS

Human islets were coated with primary human bone marrow-derived MSCs and dermal microvascular ECs. The capacity of ECs, with or without MSCs, to adhere to and grow into human islets was analyzed. The survival and functionality of these composite islets were evaluated in a dynamic perifusion assay, and their capacity for angiogenesis in vitro was assessed in a three-dimensional fibrin gel assay.

RESULTS

ECs proliferated after culture in MSC-conditioned medium, and MSCs improved the EC coverage threefold compared with EC islets alone. Islet survival in vitro and the functionality of the composite islets after culture were equal to those of control islets. The EC-MSC islets showed a twofold increase in total sprout formation compared with EC islets, and vascular sprouts emanating from the EC-MSC-islet surface showed migration of ECs into the islets and also into the surrounding matrix, either alone or in concert with MSCs.

CONCLUSIONS

EC proliferation, sprout formation, and ingrowth of ECs into the islets were enhanced by MSCs. The use of composite EC-MSC islets may have beneficial effects on revascularization and immune regulation. The technique presented allows for pretreatment of donor islets with recipient-derived ECs and MSCs as a means of improving islet engraftment.

Quantitative assessment of neuronal differentiation in three-dimensional collagen gels using enhanced green fluorescence protein expressing PC12 pheochromocytoma cells

There is a paucity of quantitative methods for evaluating the morphological differentiation of neuronal cells in a three-dimensional (3-D) system to assist in quality control of neural tissue engineering constructs for use in reparative medicine. Neuronal cells tend to aggregate in the 3-D scaffolds, hindering the application of two-dimensional (2-D) morphological methods to quantitate neuronal differentiation. To address this problem, we developed a stable transfectant green fluorescence protein (GFP)-PC12 neuronal cell model, in which the differentiation process in 3-D can be monitored with high sensitivity by fluorescence microscopy. Under 2-D conditions, the green cells showed collagen adherence, round morphology, proliferation properties, expression of the nerve growth factor (NGF) receptors TrkA and p75(NTR), stimulation of extracellular signal-regulated kinase phosphorylation by NGF and were able to differentiate in a dose-dependent manner upon NGF treatment, like wild-type (wt)-PC12 cells. When grown within 3-D collagen gels, upon NGF treatment, the GFP-PC12 cells differentiated, expressing long neurite outgrowths. We describe here a new validated method to measure NGF-induced differentiation in 3-D. Having properties similar to those of wt-PC12 and an ability to grow and differentiate in 3-D structures, these highly visualized GFP-expressing PC12 cells may serve as an ideal model for investigating various aspects of differentiation to serve in neural engineering.

Spatiotemporal control over growth factor signaling for therapeutic neovascularization

Many of the qualitative roles of growth factors involved in neovascularization have been delineated, but it is unclear yet from an engineering perspective how to use these factors as therapies. We propose that an approach that integrates quantitative spatiotemporal measurements of growth factor signaling using 3-D in vitro and in vivo models, mathematic modeling of factor tissue distribution, and new delivery technologies may provide an opportunity to engineer neovascularization on demand.