The requirement for fibroblasts in angiogenesis: fibroblast-derived matrix proteins are essential for endothelial cell lumen formation

Preparation of Cell-Paved and -Incorporated Polysaccharide Hollow Fibers Using a Microfluidic Device

Cellular constructs having hollow tubular structures are expected to be used as artificial blood vessels. We have recently demonstrated that water-insoluble polyion complexes (PICs) were formed from water-soluble polysaccharides with opposite charges at the interface of coaxial flows, which resulted in the formation of hollow fibers. In this study, both inside- and outside-cell-laden chondroitin sulfate C (CS)/chitosan (CHI) hollow fibers were prepared by utilizing a microfluidic device and modification with cell adhesive molecules. Loading of type I collagen (COL) and surface modification with fibronectin and gelatin using layer-by-layer assembly techniques improved the adhesion and spreading of fibroblast cells to/on the surface of CS/CHI hollow fibers. On the other hand, by suspending mesenchymal stem cells (MSCs) in the core flow solution, cells were successfully loaded in the walls of the hollow fibers. As the culture time extended, cells trapped in the PIC structures constituting the wall of the hollow fibers migrated to the interface between the hollow fibers and the medium: cells adhered to and stretched "on" the lumen surfaces in the COL-loaded fibers. In contrast, for the case of unmodified hollow fibers, it was difficult for cells to adhere to the lumen surfaces. Therefore, cell aggregates were formed "in" the lumen. Results of the live/dead assay and MTT assay clearly demonstrated that MSCs possessed certain levels of cell viability and proliferated for up to 10 days, especially for the cases of COL-loaded hollow fibers. On the basis of these results, the utility of the present hollow fibers in the formation of cellular constructs corresponding to blood vessels is also discussed.

Acellular polycaprolactone scaffolds laden with fibroblast/endothelial cell-derived extracellular matrix for bone regeneration

Inconsistencies in graft osteoconduction and osteoinduction present a clinical challenge in regeneration of large bone defects. Deposition of decellularized extracellular matrix (dECM) on tissue engineered scaffolds offers an alternative approach that can enhance these properties by mimicking bone's molecular complexity and direct infiltrating cells to repair damaged bone. However, dECMs derived from homogenous cell populations do not adequately simulate the heterogeneous and vascularized microenvironment of the bone. In this study, successive culture and decellularization of fibroblasts and endothelial cells (ECs) grown on polycaprolactone microfibers was used to develop a bioactive scaffold with heterogeneous dECM mimicking endothelial basement membrane. These scaffolds had greater amount of protein and minimally increased nucleic acid content than scaffolds with homogenous culture dECM. Coomassie Blue and antibody staining revealed extensive tube formation by ECs on fibroblast dECM. Fibroblast/endothelial dECM significantly enhanced osteoblast attachment, alkaline phosphatase activity, and osteocalcin- and osteopontin-positive extracellular mineral deposits. We demonstrated that the osteoconduction of dECMs can be tailored with the appropriate combination of cells to accelerate osteoblast mineral secretion. The overall concept can be expanded to generate increasingly more complex tissue constructs for regeneration of bone defects and other vascularized tissues.

Understanding Multicellularity: The Functional Organization of the Intercellular Space

The aim of this paper is to provide a theoretical framework to understand how multicellular systems realize functionally integrated physiological entities by organizing their intercellular space. From a perspective centered on physiology and integration, biological systems are often characterized as organized in such a way that they realize metabolic self-production and self-maintenance. The existence and activity of their components rely on the network they realize and on the continuous management of the exchange of matter and energy with their environment. One of the virtues of the organismic approach focused on organization is that it can provide an understanding of how biological systems are functionally integrated into coherent wholes. Organismic frameworks have been primarily developed by focusing on unicellular life. Multicellularity, however, presents additional challenges to our understanding of biological systems, related to how cells are capable to live together in higher-order entities, in such a way that some of their features and behaviors are constrained and controlled by the system they realize. Whereas most accounts of multicellularity focus on cell differentiation and increase in size as the main elements to understand biological systems at this level of organization, we argue that these factors are insufficient to provide an understanding of how cells are physically and functionally integrated in a coherent system. In this paper, we provide a new theoretical framework to understand multicellularity, capable to overcome these issues. Our thesis is that one of the fundamental theoretical principles to understand multicellularity, which is missing or underdeveloped in current accounts, is the functional organization of the intercellular space. In our view, the capability to be organized in space plays a central role in this context, as it enables (and allows to exploit all the implications of) cell differentiation and increase in size, and even specialized functions such as immunity. We argue that the extracellular matrix plays a crucial active role in this respect, as an evolutionary ancient and specific (non-cellular) control subsystem that contributes as a key actor to the functional specification of the multicellular space and to modulate cell fate and behavior. We also analyze how multicellular systems exert control upon internal movement and communication. Finally, we show how the organization of space is involved in some of the failures of multicellular organization, such as aging and cancer.

Fibronectin in malignancy: Cancer-specific alterations, protumoral effects, and therapeutic implications

Initial studies on cancer primarily focused on malignant cells themselves. The overarching narrative of cancer revolved around unchecked and rapidly proliferating cells. Special attention was given to the molecular, genetic, and metabolic profiles of isolated cancer cells in hopes of elucidating a critical factor in malignancy. However, the scope of cancer research has broadened over the past few decades to include the local environment around cancer. It has become increasingly apparent that the immune cells, vascular networks, and the extracellular matrix all have a part in cancer progression. The impact of the extracellular matrix is particularly fascinating and key stromal changes have been identified in various cancers. Pioneering work studying laminin and hyaluronate has shown that these molecules have vital roles in cancer progression. More recently, fibronectin has been included as an extracellular driver of malignancy. Fibronectin is thought to play a considerable, albeit poorly understood, role in cancer pathogenesis. In this review, we present fundamental studies that have investigated the impact of fibronectin in cancer. As an abundant component of the extracellular matrix, understanding the effect of this molecule has the potential to elucidate cancer biology.

Tumorigenic Interplay Between Macrophages and Collagenous Matrix in the Tumor Microenvironment

The tumor microenvironment is a heterogeneous tissue that in addition to tumor cells, contain tumor-associated cell types such as immune cells, fibroblasts, and endothelial cells. Considerably important in the tumor microenvironment is its noncellular component, namely, the extracellular matrix (ECM). In particular, the collagenous matrix is subjected to significant alterations in its composition and structure that create a permissive environment for tumor growth, invasion, and dissemination. Among tumor-infiltrating immune cells, tumor-associated macrophages (TAMs) are numerous in the tumor stroma and are locally educated to mediate important biological functions that profoundly affect tumor initiation, growth, and dissemination. While the influence of TAMs and mechanical properties of the collagenous matrix on tumor invasion and progression have been comprehensively investigated individually, their interaction within the complex tumor microenvironment was overlooked. This review summarizes accumulating evidence that indicate the existence of an intricate tumorigenic crosstalk between TAMs and collagenous matrix. A better mechanistic comprehension of this reciprocal interplay may open a novel arena for cancer therapeutics.

Human Ocular Angiogenesis-Inspired Vascular Models on an Injection-Molded Microfluidic Chip

Angiogenic sprouting, which is the growth of new blood vessels from pre-existing vessels, is orchestrated by cues from the cellular microenvironment, such as spatially controlled gradients of angiogenic factors. However, current in vitro models are less scalable for in-depth studies of angiogenesis. In this study, a plastic-based microfluidic chip is developed to reconstruct in vitro 3D vascular networks. The main disadvantages of the preexisting system are identified, namely, the low productivity and difficulty of experiments, and a breakthrough is suggested while minimizing disadvantages. The selection of plastic materials contributes to the productivity and usability of in vitro devices. By adopting this material, this chip offers simple fluid patterning, facilitating the construction of a cell-culture microenvironment. Compared with previous systems, the chip, which can form both inward and outwardly radial vascular sprouting, demonstrates the growth of functional, morphologically integral microvessels. The developed angiogenic model yields dose-dependent results for antiangiogenic drug screening. This model may contribute significantly not only to vascular studies under normal and pathological conditions, but also to fundamental research on the ocular neovascularization. Furthermore, it can be applied as a tool for more practical, extended preclinical research, providing an alternative to animal experiments.

Identification of a pro-angiogenic functional role for FSP1-positive fibroblast subtype in wound healing

Fibrosis accompanying wound healing can drive the failure of many different organs. Activated fibroblasts are the principal determinants of post-injury pathological fibrosis along with physiological repair, making them a difficult therapeutic target. Although activated fibroblasts are phenotypically heterogeneous, they are not recognized as distinct functional entities. Using mice that express GFP under the FSP1 or αSMA promoter, we characterized two non-overlapping fibroblast subtypes from mouse hearts after myocardial infarction. Here, we report the identification of FSP1-GFP⁺ cells as a non-pericyte, non-hematopoietic fibroblast subpopulation with a predominant pro-angiogenic role, characterized by in vitro phenotypic/cellular/ultrastructural studies and in vivo granulation tissue formation assays combined with transcriptomics and proteomics. This work identifies a fibroblast subtype that is functionally distinct from the pro-fibrotic αSMA-expressing myofibroblast subtype. Our study has the potential to shift our focus towards viewing fibroblasts as molecularly and functionally heterogeneous and provides a paradigm to approach treatment for organ fibrosis.

Activated fibroblasts are key contributors to tissue repair after cardiac injury. Here, Saraswati et al. identify and characterize a subpopulation of FSP1-positive cardiac fibroblasts with proangiogenic properties in infarcted hearts.

Engineering tumor vasculature on an injection-molded plastic array 3D culture (IMPACT) platform

Recent advances in microfluidic organ-on-a-chip technology have enabled the growth of 3D microphysiological systems for diverse biological studies. Fabrication and usage limitations inherent to conventional soft lithographic polydimethylsiloxane (PDMS) based microfluidic platforms drive demands for more accessible, standardized, and mass producible platforms for wider applications. Here, we introduce a novel injection-molded plastic array 3D culture (IMPACT) platform, a microfluidic system designed for easy and diverse patterning of 3D cellular hydrogel. The flexibility of the IMPACT platform enabled simultaneous high-content morphological profiling of the effect of nine different types of tumor cells on vascular formation. Moreover, screening of three different known anti-tumor drugs (5-FU, axitinib and cetuximab) was done at various delivered dosages. We observed distinct and expected molecular mechanism dependent response on both tumor and vasculature in response to treatment, confirming the applicability of the IMPACT as high-content drug testing tool. Therefore, we propose IMPACT as the next generation of 3D microfluidic co-culture platform compatible with any biological, clinical, and pharmaceutical investigations requiring robust high-throughput and high-content assays.

Mechanically Defined Microenvironment Promotes Stabilization of Microvasculature, Which Correlates with the Enrichment of a Novel Piezo-1+ Population of Circulating CD11b+ /CD115+ Monocytes

Vascularization is a critical step in the restoration of cellular homeostasis. Several strategies including localized growth factor delivery, endothelial progenitor cells, genetically engineered cells, gene therapy, and prevascularized implants have been explored to promote revascularization. But, long-term stabilization of newly induced vessels remains a challenge. It has been shown that fibroblasts and mesenchymal stem cells can stabilize newly induced vessels. However, whether an injected biomaterial alone can serve as an instructive environment for angiogenesis remains to be elucidated. It is reported here that appropriate vascular branching, and long-term stabilization can be promoted simply by implanting a hydrogel with stiffness matching that of fibrin clot. A unique subpopulation of circulating CD11b⁺ myeloid and CD11b⁺ /CD115⁺ monocytes that express the stretch activated cation channel Piezo-1, which is enriched prominently in the clot-like hydrogel, is identified. These findings offer evidence for a mechanobiology paradigm in angiogenesis involving an interplay between mechanosensitive circulating cells and mechanics of tissue microenvironment.

EphrinB2/EphB4 Signaling Regulates DPSCs to Induce Sprouting Angiogenesis of Endothelial Cells

Dental pulp stem cells (DPSCs) are capable of facilitating angiogenesis resembling pericytes when located adjacent to endothelial cells (ECs). Nevertheless, the precise mechanisms orchestrating their proangiogenic functions remain unclear. Using a 3-dimensional (3-D) fibrin gel model, we aimed to investigate whether EphrinB2/EphB4 signaling in DPSCs plays a role in supporting vascular morphogenesis mediated by ECs, together with the underlying mechanism involved. The EphrinB2/EphB4 signaling was inhibited either by a pharmacological inhibitor of EphB4 receptor or by knocking down the expressions of EphrinB2 and EphB4 using lentiviral small hairpin RNA (shRNA). DPSCs were either encapsulated in fibrin gel together with human umbilical vein endothelial cells (HUVECs) or cultured as a monolayer on top of HUVECs to investigate both paracrine and juxtacrine interactions simultaneously. Following 10 d of direct coculture, we found that pharmacological inhibition of EphrinB2/EphB4 signaling severely impaired vessel formation and laminin deposition. When directly cocultured with HUVECs, knockdown of EphrinB2 or EphB4 in DPSCs significantly inhibited endothelial sprouting, resulting in less capillary sprouts with reduced vessel length (P < 0.05). By contrast, when DPSCs were not in direct contact with HUVECs, attenuation of EphrinB2 or EphB4 expression levels in DPSCs did not exert any significant effects on capillary morphogenesis. Noticeably, exogenous stimulation with soluble EphrinB2-Fc or EphB4-Fc (1 µg/mL) enhanced vascular endothelial growth factor (VEGF) secretion from DPSCs, thereby moderately promoting angiogenic cascades in the fibrin matrix. This study, for the first time, reveals a crucial role of EphrinB2/EphB4 signaling in regulating the capacity of DPSCs to induce sprouting angiogenesis. These findings advance our understanding of postnatal angiogenesis and may have future regenerative medicine applications.

MK5 haplodeficiency decreases collagen deposition and scar size during post-myocardial infarction wound repair

MK5 is a protein serine/threonine kinase activated by p38, ERK3, and ERK4 MAPKs. MK5 mRNA and immunoreactivity are detected in mouse cardiac fibroblasts, and MK5 haplodeficiency attenuates the increase in collagen 1-α1 mRNA evoked by pressure overload. The present study examined the effect of MK5 haplodeficiency on reparative fibrosis following myocardial infarction (MI). Twelve-week-old MK5^+/- and wild-type littermate (MK5^+/+) mice underwent ligation of the left anterior descending coronary artery (LADL). Surviving mice were euthanized 8 or 21 days post-MI. Survival rates did not differ significantly between MK5^+/+ and MK5^+/- mice, with rupture of the LV wall being the primary cause of death. Echocardiographic imaging revealed similar increases in LV end-diastolic diameter, myocardial performance index, and wall motion score index in LADL-MK5^+/+ and LADL-MK5^+/- mice. Area at risk did not differ between LADL-MK5^+/+ and LADL-MK5^+/- hearts. In contrast, infarct size, scar area, and scar collagen content were reduced in LADL-MK5^+/- hearts. Immunohistochemical analysis of mice experiencing heart rupture revealed increased MMP-9 immunoreactivity in the infarct border zone of LADL-MK5^+/- hearts compared with LADL-MK5^+/+. Although inflammatory cell infiltration was similar in LADL-MK5^+/+ and LADL-MK5^+/- hearts, angiogenesis was more pronounced in the infarct border zone of LADL-MK5^+/- mice. Characterization of ventricular fibroblasts revealed reduced motility and proliferation in fibroblasts isolated from MK5^-/- mice compared with those from both wild-type and haplodeficient mice. siRNA-mediated knockdown of MK5 in fibroblasts from wild-type mice also impaired motility. Hence, reduced MK5 expression alters fibroblast function and scar morphology but not mortality post-MI. NEW & NOTEWORTHY MK5/PRAK is a protein serine/threonine kinase activated by p38 MAPK and/or atypical MAPKs ERK3/4. MK5 haplodeficiency reduced infarct size, scar area, and scar collagen content post-myocardial infarction. Motility and proliferation were reduced in cultured MK5-null cardiac myofibroblasts.

Epidermal Growth Factor Like-domain 7 and miR-126 are abnormally expressed in diffuse Systemic Sclerosis fibroblasts

Systemic sclerosis (SSc) is characterized by microangiopathy with impaired reparative angiogenesis and fibrosis. Epidermal Growth Factor Like-domain 7 (EGFL7), firstly described in endothelial cells plays a pivotal role in angiogenesis. Fibroblasts (FBs) are involved in vascular remodeling, under physiological and pathological conditions. In this study, we investigated: (i) the expression of EGFL7 and its miR-126 in patients affected by diffuse cutaneous SSc (dcSSc); (ii) the ability of Transforming Growth Factor-beta (TGF-β) to modulate EGFL7 expression; (iii) the ability of EGFL7 to modulate COL1A1 expression and proliferation/migration, and (iv) the functional role of EGFL7 on angiogenesis. Patients were divided in 2 subsets: patients fulfilling the classification criteria in less than one year from Raynaud’s Phenomenon onset (Early Onset Subset–EOS), and all the others (Long Standing Subset–LSS). We show that EGFL7 expression is increased in EOS dcSSc skin and cultured FBs. EGFL7 is inducible by TGF-β on Healthy Controls (HC) FBs but not in SSc-FBs. EGFL7 decreases COL1A1 expression in EOS SSc-FBs while EGFL7 silencing up-regulates COL1A1 expression. EGFL7 promotes migration/invasion of EOS SSc-FBs but not proliferation. Finally, SSc-FBs, partially inhibit angiogenesis in organotypic coculture assays, and this is reversed by treatment with human recombinant (rh)EGFL7. We conclude that EGFL7 and its specific microRNA miR-126 may be involved in the pathogenesis of SSc vasculopathy and fibrosis.

PGE2 -induced migration of human brain endothelial cell is mediated though protein kinase A in cooperation of EP receptors

PGE₂ plays a critical role in angiogenesis, ischemic, and neuro-inflammatory disorders of the brain, which breakdown the blood-brain barrier (BBB). However, the effects of PGE₂ on human brain endothelial cell (HBECs) migration, a key process in the angiogenic response and BBB stability, are not well defined. In this study, we investigated the mechanism of PGE₂ in HBECs migration in vitro. Here we showed that PGE₂ stimulated migration of HBECs in a dose-time and matrix-dependent manner, evaluated by the Boyden chamber assay, but other prostanoids failed to do so. PGE₂ receptor (EP2; butaprost), EP3 (sulprostone), and EP4 (PGE₁ -OH) receptor agonists stimulated HBECs migration, but the silencing of EP significantly attenuated this effect. EP1 agonist (11-trinor PGE₁ ) had no effect on HBECs migration on silencing of the EP1 receptor. We further showed that PGE₂ stimulated cAMP production and activated protein kinase A (PKA), whereas pretreatment with the adenyl cyclase inhibitor (dideoxyadenosine; 1 μM) or PKA inhibitors, H89 (0.5 μM)/PKAI (1 μM), completely abrogated PGE₂ -induced migration. Furthermore, silencing of the EP2/EP4 receptors significantly inhibited PGE₂ -induced cAMP and PKA activation, whereas EP3 receptor silencing failed to do so. These results suggest that PGE₂ regulates HBEC migration via cooperation of EP2, EP3, and EP4 receptors. Coupling of PGE₂ to these receptors resulted in increased production of cAMP, which regulates HBEC migration via PKA pathway. The elucidation of molecular events involved is critical for the development of targeted strategies to treat cerebrovascular diseases associated with dysregulated angiogenesis.

Battling adhesions: from understanding to prevention

Adhesions represent a major burden in clinical practice, particularly following abdominal, intrauterine, pericardial and tendon surgical procedures. Adhesions are initiated by a disruption in the epithelial or mesothelial layer of tissue, which leads to fibrin adhesion sites due to the downregulation of fibrinolytic activity and an increase in fibrin deposition. Hence, the metabolic events involved in tissue healing, coagulation, inflammation, fibrinolysis and angiogenesis play a pivotal role in adhesion formation. Understanding these events, their interactions and their influence on the development of post-surgical adhesion is crucial for the development of effective therapies to prevent them. Mechanical barriers, antiadhesive agents and combination thereof are customarily used in the battle against adhesions. Although these systems seem to be effective at reducing adhesions in clinical procedures, their prevention remains still elusive, imposing the need for new antiadhesive strategies.

Characterization of Endothelial and Smooth Muscle Cells From Different Canine Vessels

Vasculature performs a critical function in tissue homeostasis, supply of oxygen and nutrients, and the removal of metabolic waste products. Vascular problems are implicated in a large variety of pathologies and accurate in vitro models resembling native vasculature are of great importance. Unfortunately, existing in vitro models do not sufficiently reflect their in vivo counterpart. The complexity of vasculature requires the examination of multiple cell types including endothelial cells (ECs) and vascular smooth muscle cells (VSMCs), as well as vessel location in the body from which they originate. The use of canine blood vessels provides a way to study vasculature with similar vessel size and physiology compared to human vasculature. We report an isolation procedure that provides the possibility to isolate both the endothelial and smooth muscle cells from the same vessels simultaneously, enabling new opportunities in investigating vasculature behavior. Canine primary ECs and VSMCs were isolated from the vena cava, vena porta and aorta. All tissue sources were derived from three donors for accurate comparison and to reduce inter-animal variation. The isolation and purification of the two distinct cell types was confirmed by morphology, gene- and protein-expression and function. As both cell types can be derived from the same vessel, this approach allows accurate modeling of vascular diseases and can also be used more widely, for example, in vascular bioreactors and tissue engineering designs. Additionally, we identified several new genes that were highly expressed in canine ECs, which may become candidate genes for novel EC markers. In addition, we observed transcriptional and functional differences between arterial- and venous-derived endothelium. Further exploration of the transcriptome and physiology of arteriovenous differentiation of primary cells may have important implications for a better understanding of the fundamental behavior of the vasculature and pathogenesis of vascular disease.

Macrophage and Fibroblast Interactions in Biomaterial-Mediated Fibrosis

Biomaterial-mediated inflammation and fibrosis remain a prominent challenge in designing materials to support tissue repair and regeneration. Despite the many biomaterial technologies that have been designed to evade or suppress inflammation (i.e., delivery of anti-inflammatory drugs, hydrophobic coatings, etc.), many materials are still subject to a foreign body response, resulting in encapsulation of dense, scar-like extracellular matrix. The primary cells involved in biomaterial-mediated fibrosis are macrophages, which modulate inflammation, and fibroblasts, which primarily lay down new extracellular matrix. While macrophages and fibroblasts are implicated in driving biomaterial-mediated fibrosis, the signaling pathways and spatiotemporal crosstalk between these cell types remain loosely defined. In this review, the role of M1 and M2 macrophages (and soluble cues) involved in the fibrous encapsulation of biomaterials in vivo is investigated, with additional focus on fibroblast and macrophage crosstalk in vitro along with in vitro models to study the foreign body response. Lastly, several strategies that have been used to specifically modulate macrophage and fibroblast behavior in vitro and in vivo to control biomaterial-mediated fibrosis are highlighted.

Preparation of iPS cell-derived CD31+ endothelial cells using three-dimensional suspension culture

A well-organised vascular network is essential for metabolic exchange to maintain homoeostasis in the body. Therefore, for progress in regenerative medicine, it is particularly important to establish methods of vascularization in bioengineered three-dimensional (3D) functional tissues. In addition, it is necessary to develop methods to supply a large number of iPS cell-derived endothelial cells for fabricating the vascular network structure. There are already many reports on the method of inducing the differentiation of endothelial cells from iPS cells using 2D culture. However, there are few reports on methods for preparing a large number of iPS cell-derived endothelial cells. Therefore, we developed methods for inducing vascular endothelial cells from human inducible pluripotent stem (hiPS) cells using 3D suspension culture. hiPS cell-derived CD31+ cells expressed several endothelial marker genes and formed endothelial cell network structures, similar to human umbilical vein endothelial cells. These results indicate that hiPS cell-derived CD31+ cells may be a useful cell source for pre-vascularised network structures in 3D functional tissues, and it is important to develop 3D mass culture system for preparing a large number of cells to fabricate bioengineered tissues.

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.

Microfluidic-Based 3D Engineered Microvascular Networks and Their Applications in Vascularized Microtumor Models

The microvasculature plays a critical role in human physiology and is closely associated to various human diseases. By combining advanced microfluidic-based techniques, the engineered 3D microvascular network model provides a precise and reproducible platform to study the microvasculature in vitro, which is an essential and primary component to engineer organ-on-chips and achieve greater biological relevance. In this review, we discuss current strategies to engineer microvessels in vitro, which can be broadly classified into endothelial cell lining-based methods, vasculogenesis and angiogenesis-based methods, and hybrid methods. By closely simulating relevant factors found in vivo such as biomechanical, biochemical, and biological microenvironment, it is possible to create more accurate organ-specific models, including both healthy and pathological vascularized microtissue with their respective vascular barrier properties. We further discuss the integration of tumor cells/spheroids into the engineered microvascular to model the vascularized microtumor tissue, and their potential application in the study of cancer metastasis and anti-cancer drug screening. Finally, we conclude with our commentaries on current progress and future perspective of on-chip vascularization techniques for fundamental and clinical/translational research.

Microfluidic-based vascularized microphysiological systems

Microphysiological systems have emerged in the last decade to provide an alternative to in vivo models in basic science and pharmaceutical research. In the field of vascular biology, in particular, there has been a lack of a suitable in vitro model exhibiting a three-dimensional structure and the physiological function of vasculature integrated with organ-on-a-chip models. The rapid development of organ-on-a-chip technology is well positioned to fulfill unmet needs. Recently, functional integration of vasculature with diverse microphysiological systems has been increasing. This recent trend corresponds to emerging research interest in how the vascular system contributes to various physiological and pathological conditions. This innovative platform has undergone significant development, but adoption of this technology by end-users and researchers in biology is still a work in progress. Therefore, it is critical to focus on simplification and standardization to promote the distribution and acceptance of this technology by the end-users. In this review, we will introduce the latest developments in vascularized microphysiological systems and summarize their outlook in basic research and drug screening applications.

Consensus guidelines for the use and interpretation of angiogenesis assays

The formation of new blood vessels, or angiogenesis, is a complex process that plays important roles in growth and development, tissue and organ regeneration, as well as numerous pathological conditions. Angiogenesis undergoes multiple discrete steps that can be individually evaluated and quantified by a large number of bioassays. These independent assessments hold advantages but also have limitations. This article describes in vivo, ex vivo, and in vitro bioassays that are available for the evaluation of angiogenesis and highlights critical aspects that are relevant for their execution and proper interpretation. As such, this collaborative work is the first edition of consensus guidelines on angiogenesis bioassays to serve for current and future reference.

Assessing the Minimum Time-Period of Normoxic Preincubation for Stable Adipose Stromal Cell-Derived Vascular Networks

Introduction

Pre-vascularization of tissue engineered grafts is a promising strategy to facilitate their improved viability following in vivo implantation. In this process, endothelial cells (ECs) form capillary-like networks that can anastomose with host vasculature. Adipose-derived stromal cells (ASCs) are a commonly used cell population for tissue engineering and contain a subpopulation of ECs capable of assembling into robust vascular networks and anastomosing with the host. However, their initial vascular assembly is significantly impaired in hypoxic conditions (2% O₂). In this study, we explored the minimum period of normoxic (20% O₂) pre-treatment required to enable the formation of stable vascular networks.

Methods

ASC-derived vascular structures were allowed to preassemble in fibrin hydrogels in normoxia for 0, 2, 4, or 6 days and then transplanted into hypoxic environments for 6 days. Total vascular length, pericyte coverage, cell proliferation, apoptosis rates, and ECM production was assessed.

Results

Vascular assembly increased with time over the 6 days of culture. We found that 4 days was the minimum period of time required for stable vascular assembly. We compared the major differences in cell behavior and network structure at Days 2 and 4. Neither proliferation nor apoptosis differed, however, the Day 4 time-point was associated with a significant increase in pericyte coverage (46.1 ± 2.6%) compared to Day 2 (24.3 ± 5.3%).

Conclusions

These data suggest oxygen tension may be a mediator of EC-pericyte interactions during vascular assembly. Pre-vascularization strategies should incorporate a normoxic period of to enable successful vascular formation and development.

HAMSCs/HBMSCs coculture system ameliorates osteogenesis and angiogenesis against glucolipotoxicity

Osteoporosis and vascular lesions induced by glucolipotoxicity are common complications of diabetes mellitus (DM). In order to deal with these complications, we designed a new therapeutic strategy, i.e. coculture system containing human amnion-derived mesenchymal stem cells (HAMSCs) and human bone marrow mesenchymal stem cells (HBMSCs). Two in vitro coculture models, transwell and mixed cocultures, were proposed for 7 days with variable HAMSCs: HBMSCs ratios. Then, supernatant from each coculture was used to reverse the deficiency of HBMSCs and human umbilical vein endothelial cells (HUVECs) impaired by high glucose and palmitic acid (GP). We found that glucolipotoxicity caused by GP remarkably inhibited cell proliferation, osteogenic differentiation and superoxide dismutase (SOD) activity, as well as induced the reactive oxygen species (ROS) level in HBMSCs. Meanwhile, glucolipotoxicity suppressed cell proliferation, tube formation capacity and angiogenic potential of HUVECs. Though, HAMSCs/HBMSCs coculture system reduced HBMSCs dysfunction by antioxidant properties and promoted angiogenesis in HUVECs. The mixed HAMSCs/HBMSCs coculture at the optimal ratio of 3/1 showed significantly greater cell proliferation, antioxidant properties, osteogenic and angiogenic differentiation than HBMSCs or HUVECs alone. In conclusion, the current coculture system of HAMSCs/HBMSCs can be a potential therapeutic material for advancing bone and vascular regeneration against DM-induced glucolipotoxicity.

Engineering of vascularized 3D cell constructs to model cellular interactions through a vascular network

Current in vitro 3D culture models lack a vascular system to transport oxygen and nutrients, as well as cells, which is essential to maintain cellular viability and functions. Here, we describe a microfluidic method to generate a perfusable vascular network that can form inside 3D multicellular spheroids and functionally connect to microchannels. Multicellular spheroids containing endothelial cells and lung fibroblasts were embedded within a hydrogel inside a microchannel, and then, endothelial cells were seeded into both sides of the hydrogel so that angiogenic sprouts from the cell spheroids and the microchannels were anastomosed to form a 3D vascular network. Solution containing cells and reagents can be perfused inside the cell spheroids through the vascular network by injecting it into a microchannel. This method can be used to study cancer cell migration towards 3D co-culture spheroids through a vascular network. We recapitulated a bone-like microenvironment by culturing multicellular spheroids containing osteo-differentiated mesenchymal stem cells (MSCs), as well as endothelial cells, and fibroblasts in the device. After the formation of vascularized spheroids, breast cancer cells were injected into a microchannel connected to a vascular network and cultured for 7 days on-chip to monitor cellular migration. We demonstrated that migration rates of the breast cancer cells towards multicellular spheroids via blood vessels were significantly higher in the bone-like microenvironment compared with the microenvironment formed by undifferentiated MSCs. These findings demonstrate the potential value of the 3D vascularized spheroids-on-a-chip for modeling in vivo-like cellular microenvironments, drug delivery through blood vessels, and cellular interactions through a vascular network.

Establishment and characterization of an embryonic pericyte cell line

OBJECTIVE

Pericytes are specialized perivascular cells embedded within the basement membrane. These cells envelope the abluminal surface of endothelial cells and promote microvessel homeostasis. Recent discoveries of unique pericyte functions, particularly in neural tissues, underscore the need for overcoming existing challenges in establishing a functionally validated pericyte cell line. Here, we present methodologies for addressing these challenges as well as an embryonic pericyte cell line for use with in vitro and ex vivo experimental models.

METHODS

We isolated an enriched population of NG2:DsRed+ pericytes from E12.5 mice. This pericyte cell line was compared to MEFs with respect to gene expression, cell morphology and migration, and engagement with endothelial cells during junction stabilization and angiogenesis.

RESULTS

NG2+ pericytes displayed gene expression patterns, cell morphology, and 2D migration behaviors distinct from MEFs. In three different vessel formation models, pericytes from this line migrated to and incorporated into developing vessels. When co-cultured with HUVECs, these pericytes stimulated more robust VE-Cadherin junctions between HUVECs as compared to MEFs, as well as contributed to HUVEC organization into primitive vascular structures.

CONCLUSIONS

Our data support use of this pericyte cell line in a broad range of models to further understand pericyte functionality during normal and pathological conditions.

Vasculature on the clock: Circadian rhythm and vascular dysfunction

The master mammalian circadian clock (i.e. central clock), located in the suprachiasmatic nucleus of the hypothalamus, orchestrates the synchronization of the daily behavioural and physiological rhythms to better adapt the organism to the external environment in an anticipatory manner. This central clock is entrained by a variety of signals, the best established being light and food. However, circadian cycles are not simply the consequences of these two cues but are generated by endogenous circadian clocks. Indeed, clock machinery is found in mainly all tissues and cell types, including cells of the vascular system such as endothelial cells, fibroblasts, smooth muscle cells and stem cells. This machinery physiologically contributes to modulate the daily vascular function, and its disturbance therefore plays a major role in the pathophysiology of vascular dysfunction. Therapies targeting the circadian rhythm may therefore be of benefit against vascular disease.

Heparin Functionalized Injectable Cryogel with Rapid Shape-Recovery Property for Neovascularization

Cryogel based scaffolds have high porosity with interconnected macropores that may provide cell compatible microenvironment. In addition, cryogel based scaffolds can be utilized in minimally invasive surgery due to its sponge-like properties, including rapid shape recovery and injectability. Herein, we developed an injectable cryogel by conjugating heparin to gelatin as a carrier for vascular endothelial growth factor (VEGF) and fibroblasts in hindlimb ischemic disease. Our gelatin/heparin cryogel showed gelatin concentration-dependent mechanical properties, swelling ratios, interconnected porosities, and elasticities. In addition, controlled release of VEGF led to effective angiogenic responses both in vitro and in vivo. Furthermore, its sponge-like properties enabled cryogels to be applied as an injectable carrier system for in vivo cells and growth factor delivery. Our heparin functionalized injectable cryogel facilitated the angiogenic potential by facilitating neovascularization in a hindlimb ischemia model.

Proteomic analysis of cerebrospinal fluid from children with central nervous system tumors identifies candidate proteins relating to tumor metastatic spread

Central nervous system (CNS) tumors are the most common solid tumors in childhood. Since the sensitivity of combined cerebrospinal fluid (CSF) cytology and radiological neuroimaging in detecting meningeal metastases remains relatively low, we sought to characterize the CSF proteome of patients with CSF tumors to identify biomarkers predictive of metastatic spread. CSF samples from 27 children with brain tumors and 13 controls (extra-CNS non-Hodgkin lymphoma) were processed using core-shell hydrogel nanoparticles, and analyzed with reverse-phase liquid chromatography/electrospray tandem mass spectrometry (LC-MS/MS). Candidate proteins were identified with Fisher's exact test and/or a univariate logistic regression model. Reverse phase protein array (RPPA), Western blot (WB), and ELISA were used in the training set and in an independent set of CFS samples (60 cases, 14 controls) to validate our discovery findings. Among the 558 non-redundant proteins identified by LC-MS/MS, 147 were missing from the CSF database at http://www.biosino.org. Fourteen of the 26 final top-candidate proteins were chosen for validation with WB, RPPA and ELISA methods. Six proteins (type 1 collagen, insulin-like growth factor binding protein 4, procollagen C-endopeptidase enhancer 1, glial cell-line derived neurotrophic factor receptor α2, inter-alpha-trypsin inhibitor heavy chain 4, neural proliferation and differentiation control protein-1) revealed the ability to discriminate metastatic cases from controls. Combining a unique dataset of CSFs from pediatric CNS tumors with a novel enabling nanotechnology led us to identify CSF proteins potentially related to metastatic status.

Perfusable Vascular Network with a Tissue Model in a Microfluidic Device

A spheroid (a multicellular aggregate) is regarded as a good model of living tissues in the human body. Despite the significant advancement in the spheroid cultures, a perfusable vascular network in the spheroids remains a critical challenge for long-term culture required to maintain and develop their functions, such as protein expressions and morphogenesis. The protocol presents a novel method to integrate a perfusable vascular network within the spheroid in a microfluidic device. To induce a perfusable vascular network in the spheroid, angiogenic sprouts connected to microchannels were guided to the spheroid by utilizing angiogenic factors from human lung fibroblasts cultured in the spheroid. The angiogenic sprouts reached the spheroid, merged with the endothelial cells co-cultured in the spheroid, and formed a continuous vascular network. The vascular network could perfuse the interior of the spheroid without any leakage. The constructed vascular network may be further used as a route for supply of nutrients and removal of waste products, mimicking blood circulation in vivo. The method provides a new platform in spheroid culture toward better recapitulation of living tissues.

Vascularization in Craniofacial Bone Tissue Engineering

Craniofacial bones, separate from the appendicular skeleton, bear a significant amount of strain and stress generated from mastication-related muscles. Current research on the regeneration of craniofacial bone focuses on the reestablishment of an elaborate vascular network. In this review, current challenges and efforts particularly in advances of scaffold properties and techniques for vascularization remodeling in craniofacial bone tissue engineering will be discussed. A microenvironment of ischemia and hypoxia in the biomaterial core drives propagation and reorganization of endothelial progenitor cells (EPCs) to assemble into a primitive microvascular framework. Co-culture strategies and delivery of vasculogenic molecules enhance EPCs' differentiation and stimulate the host regenerative response to promote vessel sprouting and strength. To optimize structural and vascular integration, well-designed microstructures of scaffolds are biologically considered. Proper porous structures, matrix stiffness, and surface morphology of scaffolds have a profound influence on cell behaviors and thus affect revascularization. In addition, advanced techniques facilitating angiogenesis and vaculogenesis have also been discussed. Oxygen delivery biomaterials, scaffold-free cell sheet techniques, and arteriovenous loop-induced axial vascularization strategies bring us new understanding and powerful strategies to manage revascularization of large craniofacial bone defects. Although promising histological results have been achieved, the efficient perfusion and functionalization of newly formed vessels are still challenging.

The ablation of the matricellular protein EMILIN2 causes defective vascularization due to impaired EGFR-dependent IL-8 production affecting tumor growth

EMILIN2 is an extracellular matrix constituent playing an important role in angiogenesis; however, the underlying mechanism is unknown. Here we show that EMILIN2 promotes angiogenesis by directly binding epidermal growth factor receptor (EGFR), which enhances interleukin-8 (IL-8) production. In turn, IL-8 stimulates the proliferation and migration of vascular endothelial cells. Emilin2 null mice were generated and exhibited delayed retinal vascular development, which was rescued by the administration of the IL-8 murine ortholog MIP-2. Next, we assessed tumor growth and tumor-associated angiogenesis in these mice. Tumor cell growth in Emilin2 null mice was impaired as well as the expression of MIP-2. The vascular density of the tumors developed in Emilin2 null mice was prejudiced and vessels perfusion, as well as response to chemotherapy, decreased. Accordingly, human tumors expressing high levels of EMILIN2 were more responsive to chemotherapy. These results point at EMILIN2 as a key microenvironmental cue affecting vessel formation and unveil the possibility to develop new prognostic tools to predict chemotherapy efficacy.

Incorporating Pericytes into an Endothelial Cell Bead Sprouting Assay

Angiogenesis is the growth of new vessels from pre-existing vasculature and is an important component of many biological processes, including embryogenesis and development, wound healing, tumor growth and metastasis, and ocular and cardiovascular diseases. Effective in vitro models that recapitulate the biology of angiogenesis are needed to appropriately study this process and identify mechanisms of regulation that can be ultimately targeted for novel therapeutic strategies. The bead angiogenesis assay has been previously demonstrated to recapitulate the multiple stages of endothelial sprouting in vitro. However, a limitation of this assay is a lack of endothelial - mural cell interactions, which are key to the molecular and phenotypic regulation of endothelial cell function in vivo. The protocol given here presents a methodology for the incorporation of mural cells into the bead angiogenesis assay and demonstrates a tight association of endothelial cells and pericytes during sprouting in vitro. The protocol also details a methodology for effective silencing of target genes using siRNA in endothelial cells for mechanistic studies. Altogether, this protocol provides an in vitro assay that more appropriately models the diverse cell types involved in sprouting angiogenesis, and provides a more physiologically-relevant platform for therapeutic assessment and novel discovery of mechanisms of angiogenesis regulation.

Assessing angiogenic responses induced by primary human prostate stromal cells in a three-dimensional fibrin matrix assay

Accurate modeling of angiogenesis in vitro is essential for guiding the preclinical development of novel anti-angiogenic agents and treatment strategies. The formation of new blood vessels is a multifactorial and multi-stage process dependent upon paracrine factors produced by stromal cells in the local microenvironment. Mesenchymal stem cells (MSCs) are multipotent cells in adults that can be recruited to sites of inflammation and tissue damage where they aid in wound healing through regenerative, trophic, and immunomodulatory properties. Primary stromal cultures derived from human bone marrow, normal prostate, or prostate cancer tissue are highly enriched in MSCs and stromal progenitors. Using conditioned media from these primary cultures, a robust pro-angiogenic response was observed in a physiologically-relevant three-dimensional fibrin matrix assay. To evaluate the utility of this assay, the allosteric HDAC4 inhibitor tasquinimod and the anti-VEGF monoclonal antibody bevacizumab were used as model compounds with distinct mechanisms of action. While both agents had a profound inhibitory effect on endothelial sprouting, only bevacizumab induced significant regression of established vessels. Additionally, the pro-angiogenic properties of MSCs derived from prostate cancer patients provides further evidence that selective targeting of this population may be of therapeutic benefit.

The role of fibrinolysis inhibition in engineered vascular networks derived from endothelial cells and adipose-derived stem cells

Background

Co-cultures of endothelial cells with mesenchymal stem cells currently represent one of the most promising approaches in providing oxygen and nutrient supply for microvascular tissue engineering. Still, to translate this model into clinics several in vitro parameters including growth medium and scaffold degradation need to be fine-tuned.

Methods

We recently described the co-culture of adipose-derived stem cells with endothelial cells in fibrin, resulting in capillary formation in vitro as well as their perfusion in vivo. Here, we aimed to further characterise microvascular tube formation in fibrin by determining the role of scaffold degradation, thrombin concentration and culture conditions on vascularisation.

Results

We observed that inhibition of cell-mediated fibrin degradation by the commonly used inhibitor aprotinin resulted in impaired vascular network formation. Aprotinin had no effect on laminin and collagen type IV deposition or formation of tube-like structures in scaffold-free co-culture, indicating that poor vascularisation of fibrin clots is primarily caused by inhibition of plasminogen-driven fibrinolysis. Co-culture in plasminogen- and factor XIII-depleted fibrin did not result in different vascular network density compared to controls. Furthermore, we demonstrate that thrombin negatively affects vascular network density at high concentrations. However, only transient activation of incorporated endothelial cells by thrombin could be observed, thus excluding a long-term inflammatory response in tissue-engineered micro-capillaries. Finally, we show that vascularisation of fibrin scaffolds in basal medium is undermined because of increased fibrinolytic activity leading to scaffold destabilisation without aprotinin.

Conclusions

Taken together, our data reveal a critical role of fibrinolysis inhibition in in vitro cell-mediated vascularisation of fibrin scaffolds.

Electronic supplementary material

The online version of this article (10.1186/s13287-017-0764-2) contains supplementary material, which is available to authorized users.

Evaluating the potential of endothelial cells derived from human induced pluripotent stem cells to form microvascular networks in 3D cultures

A major translational challenge in the fields of therapeutic angiogenesis and regenerative medicine is the need to create functional microvasculature. The purpose of this study was to assess whether a potentially autologous endothelial cell (EC) source derived from human induced pluripotent stem cells (iPSC-ECs) can form the same robust, stable microvasculature as previously documented for other sources of ECs. We utilized a well-established in vitro assay, in which endothelial cell-coated (iPSC-EC or HUVEC) beads were co-embedded with fibroblasts in a 3D fibrin matrix to assess their ability to form stable microvessels. iPSC-ECs exhibited a five-fold reduction in capillary network formation compared to HUVECs. Increasing matrix density reduced sprouting, although this effect was attenuated by distributing the NHLFs throughout the matrix. Inhibition of both MMP- and plasmin-mediated fibrinolysis was required to completely block sprouting of both HUVECs and iPSC-ECs. Further analysis revealed MMP-9 expression and activity were significantly lower in iPSC-EC/NHLF co-cultures than in HUVEC/NHLF co-cultures at later time points, which may account for the observed deficiencies in angiogenic sprouting of the iPSC-ECs. Collectively, these findings suggest fundamental differences in EC phenotypes must be better understood to enable the promise and potential of iPSC-ECs for clinical translation to be realized.

Sprouting angiogenesis induces significant mechanical heterogeneities and ECM stiffening across length scales in fibrin hydrogels

Matrix stiffness is a well-established instructive cue in two-dimensional cell cultures. Its roles in morphogenesis in 3-dimensional (3D) cultures, and the converse effects of cells on the mechanics of their surrounding microenvironment, have been more elusive given the absence of suitable methods to quantify stiffness on a length-scale relevant for individual cell-extracellular matrix (ECM) interactions. In this study, we applied traditional bulk rheology and laser tweezers-based active microrheology to probe mechanics across length scales during the complex multicellular process of capillary morphogenesis in 3D, and further characterized the relative contributions of neovessels and supportive stromal cells to dynamic changes in stiffness over time. Our data show local ECM stiffness was highly heterogeneous around sprouting capillaries, and the variation progressively increased with time. Both endothelial cells and stromal support cells progressively stiffened the ECM, with the changes in bulk properties dominated by the latter. Interestingly, regions with high micro-stiffness did not necessarily correlate with remodeled regions of high ECM density as shown by confocal reflectance microscopy. Collectively, these findings, especially the large spatiotemporal variations in local stiffness around cells during morphogenesis in soft 3D fibrin gels, underscore that characterizing ECM mechanics across length scales. provides an opportunity to attain a deeper mechanobiological understanding of the microenvironment's roles in cell fate and tissue patterning.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Self-assembly of vascularized tissue to support tumor explants in vitro

Testing the efficacy of cancer drugs requires functional assays that recapitulate the cell populations, anatomy and biological responses of human tumors. Although current animal models and in vitro cell culture platforms are informative, they have significant shortcomings. Mouse models can reproduce tissue-level and systemic responses to tumor growth and treatments observed in humans, but xenografts from patients often do not grow, or require months to develop. On the other hand, current in vitro assays are useful for studying the molecular bases of tumorigenesis or drug activity, but often lack the appropriate in vivo cell heterogeneity and natural microenvironment. Therefore, there is a need for novel tools that allow rapid analysis of patient-derived tumors in a robust and representative microenvironment. We have developed methodology for maintaining harvested tumor tissue in vitro by placing them in a support bed with self-assembled stroma and vasculature. The harvested biopsy or tumor explant integrates with the stromal bed and vasculature, providing the correct extracellular matrix (collagen I, IV, fibronectin), associated stromal cells, and a lumenized vessel network. Our system provides a new tool that will allow ex vivo drug-screening and can be adapted for the guidance of patient-specific therapeutic strategies.

Subcellular Interactions during Vascular Morphogenesis in 3D Cocultures between Endothelial Cells and Fibroblasts

Background: Increasing the complexity of in vitro systems to mimic three-dimensional tissues and the cellular interactions within them will increase the reliability of data that were previously collected with in vitro systems. In vivo vascularization is based on complex and clearly defined cell–matrix and cell–cell interactions, where the extracellular matrix (ECM) seems to play a very important role. The aim of this study was to monitor and visualize the subcellular and molecular interactions between endothelial cells (ECs), fibroblasts, and their surrounding microenvironment during vascular morphogenesis in a three-dimensional coculture model. Methods: Quantitative and qualitative analyses during the generation of a coculture tissue construct consisting of endothelial cells and fibroblasts were done using transmission electron microscopy and immunohistochemistry. Results: Dynamic interactions were found in cocultures between ECs, between fibroblasts (FBs), between ECs and FBs, and between the cells and the ECM. Microvesicles were involved in intercellular information transfer. FBs took an active and physical part in the angiogenesis process. The ECM deposited by the cells triggered endothelial angiogenic activity. Capillary-like tubular structures developed and matured. Moreover, some ECM assembled into a basement membrane (BM) having three different layers equivalent to those seen in vivo. Finally, the three-dimensional in vitro construct mirrored the topography of histological tissue sections. Conclusion: Our results visualize the importance of the physical contact between all cellular and acellular components of the cocultures.

Loss of tenascin X gene function impairs injury-induced stromal angiogenesis in mouse corneas

To determine the contribution by tenascin X (Tnx) gene expression to corneal stromal angiogenesis, the effects were determined of its loss on this response in TNX knockout (KO) mice. In parallel, the effects of such a loss were evaluated on vascular endothelial growth factor (VEGF) and transforming growth factor β1 (TGFβ1) gene and protein expression in fibroblasts and macrophages in cell culture. Histological, immunohistochemical and quantitative RT‐PCR changes determined if Tnx gene ablation on angiogenic gene expression, inflammatory cell infiltration and neovascularization induced by central corneal stromal cauterization. The role was determined of Tnx function in controlling VEGF‐A or TGFβ1 gene expression by comparing their expression levels in ocular fibroblasts and macrophages obtained from wild‐type (WT) and body‐wide Tnx KO mice. Tnx was up‐regulated in cauterized cornea. In Tnx KO, macrophage invasion was attenuated, VEGF‐A and its cognate receptor mRNA expression along with neovascularization were lessened in Tnx KOs relative to the changes occurring in their WT counterpart. Loss of Tnx instead up‐regulated in vivo mRNA expression of anti‐angiogenic VEGF‐B but not VEGF‐A. On the other hand, TGFβ1 mRNA expression declined in Tnx KO cultured ocular fibroblasts. Loss of Tnx gene expression caused VEGF‐A expression to decline in macrophages. Tnx gene expression contributes to promoting TGFβ1 mRNA expression in ocular fibroblasts and VEGF‐A in macrophages, macrophage invasion, up‐regulation of VEGF‐A expression and neovascularization in an injured corneal stroma. On the other hand, it suppresses anti‐angiogenic VEGF‐B mRNA expression in vivo.

Vascularisation for cardiac tissue engineering: the extracellular matrix

Cardiovascular diseases present a major socio-economic burden. One major problem underlying most cardiovascular and congenital heart diseases is the irreversible loss of contractile heart muscle cells, the cardiomyocytes. To reverse damage incurred by myocardial infarction or by surgical correction of cardiac malformations, the loss of cardiac tissue with a thickness of a few millimetres needs to be compensated. A promising approach to this issue is cardiac tissue engineering. In this review we focus on the problem of in vitro vascularisation as implantation of cardiac patches consisting of more than three layers of cardiomyocytes (> 100 μm thick) already results in necrosis. We explain the need for vascularisation and elaborate on the importance to include non-myocytes in order to generate functional vascularised cardiac tissue. We discuss the potential of extracellular matrix molecules in promoting vascularisation and introduce nephronectin as an example of a new promising candidate. Finally, we discuss current biomaterial- based approaches including micropatterning, electrospinning, 3D micro-manufacturing technology and porogens. Collectively, the current literature supports the notion that cardiac tissue engineering is a realistic option for future treatment of paediatric and adult patients with cardiac disease.

The Influence of Hyaluronic Acid and Glioblastoma Cell Coculture on the Formation of Endothelial Cell Networks in Gelatin Hydrogels

Glioblastoma (GBM) is the most common and deadly form of brain cancer. Interactions between GBM cells and vasculature in vivo contribute to poor clinical outcomes, with GBM-induced vessel co-option, regression, and subsequent angiogenesis strongly influencing GBM invasion. Here, elements of the GBM perivascular niche are incorporated into a methacrylamide-functionalized gelatin hydrogel as a means to examine GBM-vessel interactions. The complexity of 3D endothelial cell networks formed from human umbilical vein endothelial cells and normal human lung fibroblasts as a function of hydrogel properties and vascular endothelial growth factor (VEGF) presentation is presented. While overall length and branching of the endothelial cell networks decrease with increasing hydrogel stiffness and incorporation of brain-mimetic hyaluronic acid, it can be separately altered by changing the vascular cell seeding density. It is shown that covalent incorporation of VEGF supports network formation as robustly as continuously available soluble VEGF. The impact of U87-MG GBM cells on the endothelial cell networks is subsequently investigated. GBM cells localize in proximity to the endothelial cell networks and hasten network regression in vitro. Together, this in vitro platform recapitulates the close association between GBM cells and vessel structures as well as elements of vessel co-option and regression preceding angiogenesis in vivo.

pH and Glucose Dual-Responsive Injectable Hydrogels with Insulin and Fibroblasts as Bioactive Dressings for Diabetic Wound Healing

pH and glucose dual-responsive injectable hydrogels were prepared through the cross-linking of Schiff's base and phenylboronate ester using phenylboronic-modified chitosan, poly(vinyl alcohol) and benzaldehyde-capped poly(ethylene glycol). Protein drugs and live cells could be incorporated into the hydrogels during the in situ cross-linking, displaying sustained and pH/glucose-triggered drug release from the hydrogels and cell viability and proliferation in the three-dimensional hydrogel matrix as well. Hence, the hydrogels with insulin and fibroblasts were considered as bioactive dressings for diabetic wound healing. A streptozotocin-induced diabetic rat model was used to evaluate the efficacy of hydrogel dressings in wound repair. The results revealed that the incorporation of insulin and L929 in the hydrogels could promote neovascularization and collagen deposition and enhance the wound-healing process of diabetic wounds. Thus, the drug- and cell-loaded hydrogels have promising potential in wound healing as a medicated system for various therapeutic proteins and live cells.

Low levels of physiological interstitial flow eliminate morphogen gradients and guide angiogenesis

Convective transport can significantly distort spatial concentration gradients. Interstitial flow is ubiquitous throughout living tissue, but our understanding of how interstitial flow affects concentration gradients in biological processes is limited. Interstitial flow is of particular interest for angiogenesis because pathological and physiological angiogenesis is associated with altered interstitial flow, and both interstitial flow and morphogen gradients (e.g., vascular endothelial growth factor, VEGF) can potentially stimulate and guide new blood vessel growth. We designed an in vitro microfluidic platform to simulate 3D angiogenesis in a tissue microenvironment that precisely controls interstitial flow and spatial morphogen gradients. The microvascular tissue was developed from endothelial colony forming cell-derived endothelial cells extracted from cord blood and stromal fibroblasts in a fibrin extracellular matrix. Pressure in the microfluidic lines was manipulated to control the interstitial flow. A mathematical model of mass and momentum transport, and experimental studies with fluorescently labeled dextran were performed to validate the platform. Our data demonstrate that at physiological interstitial flow (0.1-10 μm/s), morphogen gradients were eliminated within hours, and angiogenesis demonstrated a striking bias in the opposite direction of interstitial flow. The interstitial flow-directed angiogenesis was dependent on the presence of VEGF, and the effect was mediated by αvβ3 integrin. We conclude that under physiological conditions, growth factors such as VEGF and fluid forces work together to initiate and spatially guide angiogenesis.

Procollagen C-proteinase enhancer 1 (PCPE-1) functions as an anti-angiogenic factor and enhances epithelial recovery in injured cornea

Procollagen C-proteinase enhancer 1 (PCPE-1) has been characterized as a protein capable of enhancing the activity of bone morphogenetic protein 1/tolloid-like proteinases in the biosynthetic processing of C-propeptides from procollagens I-III. This processing step is thought necessary to the formation of collagen I-III monomers capable of forming fibrils. Thus, PCPE-1 is predicted to play an important role in scarring, as scar tissue is predominantly composed of fibrillar collagen. Corneal scarring is of great clinical importance, as it leads to loss of visual acuity and, in severe cases, blindness. Here, we investigate a possible role for PCPE-1 in corneal scarring. Although differences in corneal opacity associated with scarring following injury of Pcolce ^-/- and wild-type (WT) mice using full-thickness excision or alkali burn models of corneal injury were not grossly apparent, differences in procollagen I processing levels between Pcolce ^-/- and WT primary corneal keratocytes were consistent with a role for PCPE-1 in corneal collagen deposition. An unexpected finding was that neoangiogenesis, which follows alkali burn cornea injury, was strikingly increased in Pcolce ^-/- cornea, compared to WT. A series of aortic ring assays confirmed the anti-angiogenic effects of PCPE-1. Another unexpected finding was of abnormalities of epithelial basement membrane and of re-epithelialization following Pcolce ^-/- corneal injury. Thus, PCPE-1 appears to be of importance as an anti-angiogenic factor and in re-epithelialization following injury in cornea and perhaps in other tissues as well.

Accelerated oral wound healing using a pre-vascularized mucosal cell sheet

Cell sheets with pre-vascularization have recently been developed but remain relatively untested in oral wound healing. Therefore, we examined the potential utility of our newly developed pre-vascularized mucosal cell sheets in oral wound healing. Mucosal keratinocytes, fibroblasts, and endothelial progenitor cells were primarily cultured for in vitro cell expansion from mucosa and blood of Sprague-Dawley rats. Mucosal cell sheets were generated using cultured keratinocytes and plasma fibrin (K sheet) or keratinocytes and a mixture of fibrin, fibroblasts, and endothelial cells (PV sheet). Autologous sheets were transplanted on deep wounds in the buccal region of rats. The gross and histological characteristics of wound healing were compared among control wound, K sheet, and PV sheet groups. We successfully cultured and expanded keratinocytes, fibroblasts, and endothelial progenitor cells in vitro for generating mucosal cell sheets with or without pre-vascularization. In the in vivo oral wound model, compared with the control wound, the PV sheet group exhibited rapid wound closure more prominently than the K sheet group. The histological healing in the PV sheet group was similar to that in rat normal buccal mucosa without fibrosis. The pre-vascularized mucosal cell sheet exhibited in vivo efficacy in oral wound healing by promoting accelerated healing.