Molecular basis of endothelial cell morphogenesis in three-dimensional extracellular matrices

Bioengineering methods for vascularizing organoids

Organoids, self-organizing three-dimensional (3D) structures derived from stem cells, offer unique advantages for studying organ development, modeling diseases, and screening potential therapeutics. However, their translational potential and ability to mimic complex in vivo functions are often hindered by the lack of an integrated vascular network. To address this critical limitation, bioengineering strategies are rapidly advancing to enable efficient vascularization of organoids. These methods encompass co-culturing organoids with various vascular cell types, co-culturing lineage-specific organoids with vascular organoids, co-differentiating stem cells into organ-specific and vascular lineages, using organoid-on-a-chip technology to integrate perfusable vasculature within organoids, and using 3D bioprinting to also create perfusable organoids. This review explores the field of organoid vascularization, examining the biological principles that inform bioengineering approaches. Additionally, this review envisions how the converging disciplines of stem cell biology, biomaterials, and advanced fabrication technologies will propel the creation of increasingly sophisticated organoid models, ultimately accelerating biomedical discoveries and innovations.

Tuning apicobasal polarity and junctional recycling in the hemogenic endothelium orchestrates the morphodynamic complexity of emerging pre-hematopoietic stem cells

Hematopoietic stem cells emerge in the embryo from an aortic-derived tissue called the hemogenic endothelium (HE). The HE appears to give birth to cells of different nature and fate but the molecular principles underlying this complexity are largely unknown. Here we show, in the zebrafish embryo, that two cell types emerge from the aortic floor with radically different morphodynamics. With the support of live imaging, we bring evidence suggesting that the mechanics underlying the two emergence types rely, or not, on apicobasal polarity establishment. While the first type is characterized by reinforcement of apicobasal polarity and maintenance of the apical/luminal membrane until release, the second type emerges via a dynamic process reminiscent of trans-endothelial migration. Interfering with Runx1 function suggests that the balance between the two emergence types depends on tuning apicobasal polarity at the level of the HE. In support of this and unexpectedly, we show that Pard3ba - one of the four Pard3 proteins expressed in the zebrafish - is sensitive to interference with Runx1 activity, in aortic endothelial cells. This supports the idea of a signaling cross talk controlling cell polarity and its associated features, between aortic and hemogenic cells. In addition, using new transgenic fish lines that express Junctional Adhesion Molecules and functional interference, we bring evidence for the essential role of ArhGEF11/PDZ-RhoGEF in controlling the HE-endothelial cell dynamic interface, including cell-cell intercalation, which is ultimately required for emergence completion. Overall, we highlight critical cellular and dynamic events of the endothelial-to-hematopoietic transition that support emergence complexity, with a potential impact on cell fate.

Arf6 is required for endocytosis and filamentous actin assembly during angiogenesis in vitro

OBJECTIVE

Endocytosis is a process vital to angiogenesis and vascular homeostasis. In pathologies where supraphysiological growth factor signaling underlies disease etiology, such as in diabetic retinopathy and solid tumors, strategies to limit chronic growth factor signaling by way of blunting endocytic processes have been shown to have tremendous clinical value. ADP ribosylation factor 6 (Arf6) is a small GTPase that promotes the assembly of actin necessary for clathrin-mediated and clathrin-independent endocytosis. In its absence, growth factor signaling is greatly diminished, which has been shown to ameliorate pathological signaling input in diseased vasculature. However, it is less clear if there are bystander effects related to loss of Arf6 on angiogenic behaviors. Our goal was to provide an analysis of Arf6's function in angiogenic endothelium, focusing on its role in actin and endocytosis as well as sprouting morphogenesis.

METHODS

Primary endothelial cells were cultured in both 2D and 3D environments. Here, endothelial cells were fixed and stained for various proteins or transfected with fluorescently-tagged constructs for live-cell imaging.

RESULTS

We found that Arf6 localized to both filamentous actin and sites of endocytosis in two-dimensional culture. Loss of Arf6 distorted both apicobasal polarity and reduced the total cellular filamentous actin content, which may be the primary driver underlying gross sprouting dysmorphogenesis in its absence.

CONCLUSIONS

Our findings highlight that endothelial Arf6 is a potent mediator of both actin regulation and endocytosis and is required for proper sprout formation.

Arf6 Regulates Endocytosis and Angiogenesis by Promoting Filamentous Actin Assembly

Clathrin-mediated endocytosis (CME) is a process vital to angiogenesis as well as general vascular homeostasis. In pathologies where supraphysiological growth factor signaling underlies disease etiology, such as in diabetic retinopathy and solid tumors, strategies to limit chronic growth factor signaling by way of CME have been shown to have tremendous clinical value. ADP ribosylation factor 6 (Arf6) is a small GTPase that promotes the assembly of actin necessary for CME. In its absence, growth factor signaling is greatly diminished, which has been shown to ameliorate pathological signaling input in diseased vasculature. However, it is less clear if there are bystander effects related to loss of Arf6 on angiogenic behaviors. Our goal was to provide a analysis of Arf6’s function in angiogenic endothelium, focusing on its role in lumenogenesis as well as its relation to actin and CME. We found that Arf6 localized to both filamentous actin and sites of CME in 2-dimensional culture. Loss of Arf6 distorted both apicobasal polarity and reduced the total cellular filamentous actin content, and this may be the primary driver underlying gross dysmorphogenesis during angiogenic sprouting in its absence. Our findings highlight that endothelial Arf6 is a potent mediator of both actin regulation and CME.

Programming the Self-Organization of Endothelial Cells into Perfusable Microvasculature

The construction of three-dimensional (3D) microvascular networks with defined structures remains challenging. Emerging bioprinting strategies provide a means of patterning endothelial cells (ECs) into the geometry of 3D microvascular networks, but the microenvironmental cues necessary to promote their self-organization into cohesive and perfusable microvessels are not well known. To this end, we reconstituted microvessel formation in vitro by patterning thin lines of closely packed ECs fully embedded within a 3D extracellular matrix (ECM) and observed how different microenvironmental parameters influenced EC behaviors and their self-organization into microvessels. We found that the inclusion of fibrillar matrices, such as collagen I, into the ECM positively influenced cell condensation into extended geometries such as cords. We also identified the presence of a high-molecular-weight protein(s) in fetal bovine serum that negatively influenced EC condensation. This component destabilized cord structure by promoting cell protrusions and destabilizing cell-cell adhesions. Endothelial cords cultured in the presence of fibrillar collagen and in the absence of this protein activity were able to polarize, lumenize, incorporate mural cells, and support fluid flow. These optimized conditions allowed for the construction of branched and perfusable microvascular networks directly from patterned cells in as little as 3 days. These findings reveal important design principles for future microvascular engineering efforts based on bioprinting and micropatterning techniques. Impact statement Bioprinting is a potential strategy to achieve microvascularization in engineered tissues. However, the controlled self-organization of patterned endothelial cells into perfusable microvasculature remains challenging. We used DNA Programmed Assembly of Cells to create cell-dense, capillary-sized cords of endothelial cells with complete control over their structure. We optimized the matrix and media conditions to promote self-organization and maturation of these endothelial cords into stable and perfusable microvascular networks.

Capturing membrane trafficking events during 3D angiogenic development in vitro

OBJECTIVES

Vesicular trafficking dictates protein localization, functional activity, and half-life, providing a critically important regulatory step in tissue development; however, there is little information detailing endothelial-specific trafficking signatures. This is due, in part, to limitations in visualizing trafficking events in endothelial tissues. Our aim in this investigation was to explore the use of a 3-dimensional (3D) in vitro sprouting model to image endothelial membrane trafficking events.

METHODS

Endothelial cells were challenged to grow sprouts in a fibrin bead assay. Thereafter, spouts were transfected with fluorescent proteins and stained for various cell markers. Sprouts were then imaged for trafficking events using live and fixed-cell microscopy.

RESULTS

Our results demonstrate that fibrin bead sprouts have a strong apicobasal polarity marked by apical localization of proteins moesin and podocalyxin. Comparison of trafficking mediators Rab27a and Rab35 between 3D sprouts and 2D culture showed that vesicular carriers can be imaged at high resolution, exhibiting proper membrane polarity solely in 3D sprouts. Lastly, we imaged exocytic events of von Willebrand Factor and demonstrated a distinct imaging advantage for monitoring secretion events in 3D sprouts as compared with 2D culture.

CONCLUSIONS

Our results establish that the fibrin bead sprouting assay is well-suited for imaging of trafficking events during angiogenic growth.

Tissue Engineering Approaches to Uncover Therapeutic Targets for Endothelial Dysfunction in Pathological Microenvironments

Endothelial cell dysfunction plays a central role in many pathologies, rendering it crucial to understand the underlying mechanism for potential therapeutics. Tissue engineering offers opportunities for in vitro studies of endothelial dysfunction in pathological mimicry environments. Here, we begin by analyzing hydrogel biomaterials as a platform for understanding the roles of the extracellular matrix and hypoxia in vascular formation. We next examine how three-dimensional bioprinting has been applied to recapitulate healthy and diseased tissue constructs in a highly controllable and patient-specific manner. Similarly, studies have utilized organs-on-a-chip technology to understand endothelial dysfunction's contribution to pathologies in tissue-specific cellular components under well-controlled physicochemical cues. Finally, we consider studies using the in vitro construction of multicellular blood vessels, termed tissue-engineered blood vessels, and the spontaneous assembly of microvascular networks in organoids to delineate pathological endothelial dysfunction.

Endothelial k-RasV12 Expression Induces Capillary Deficiency Attributable to Marked Tube Network Expansion Coupled to Reduced Pericytes and Basement Membranes

OBJECTIVE

We sought to determine how endothelial cell (EC) expression of the activating k-Ras (kirsten rat sarcoma 2 viral oncogene homolog) mutation, k-RasV12, affects their ability to form lumens and tubes and interact with pericytes during capillary assembly Approach and Results: Using defined bioassays where human ECs undergo observable tubulogenesis, sprouting behavior, pericyte recruitment to EC-lined tubes, and pericyte-induced EC basement membrane deposition, we assessed the impact of EC k-RasV12 expression on these critical processes that are necessary for proper capillary network formation. This mutation, which is frequently seen in human ECs within brain arteriovenous malformations, was found to markedly accentuate EC lumen formation mechanisms, with strongly accelerated intracellular vacuole formation, vacuole fusion, and lumen expansion and with reduced sprouting behavior, leading to excessively widened tube networks compared with control ECs. These abnormal tubes demonstrate strong reductions in pericyte recruitment and pericyte-induced EC basement membranes compared with controls, with deficiencies in fibronectin, collagen type IV, and perlecan deposition. Analyses of signaling during tube formation from these k-RasV12 ECs reveals strong enhancement of Src (Src proto-oncogene, non-receptor tyrosine kinase), Pak2 (P21 [RAC1 (Rac family small GTPase 1)] activated kinase 2), b-Raf (v-raf murine sarcoma viral oncogene homolog B1), Erk (extracellular signal-related kinase), and Akt (AK strain transforming) activation and increased expression of PKCε (protein kinase C epsilon), MT1-MMP (membrane-type 1 matrix metalloproteinase), acetylated tubulin and CDCP1 (CUB domain-containing protein 1; most are known EC lumen regulators). Pharmacological blockade of MT1-MMP, Src, Pak, Raf, Mek (mitogen-activated protein kinase) kinases, Cdc42 (cell division cycle 42)/Rac1, and Notch markedly interferes with lumen and tube formation from these ECs.

CONCLUSIONS

Overall, this novel work demonstrates that EC expression of k-RasV12 disrupts capillary assembly due to markedly excessive lumen formation coupled with strongly reduced pericyte recruitment and basement membrane deposition, which are critical pathogenic features predisposing the vasculature to develop arteriovenous malformations.

Heterotypic Multicellular Spheroids as Experimental and Preclinical Models of Sprouting Angiogenesis

Sprouting angiogenesis is the common response of live tissues to physiological and pathological angiogenic stimuli. Its accurate evaluation is of utmost importance for basic research and practical medicine and pharmacology and requires adequate experimental models. A variety of assays for angiogenesis were developed, none of them perfect. In vitro approaches are generally less physiologically relevant due to the omission of essential components regulating the process. However, only in vitro models can be entirely non-xenogeneic. The limitations of the in vitro angiogenesis assays can be partially overcome using 3D models mimicking tissue O2 and nutrient gradients, the influence of the extracellular matrix (ECM), and enabling cell-cell interactions. Here we present a review of the existing models of sprouting angiogenesis that are based on the use of endothelial cells (ECs) co-cultured with perivascular or other stromal cells. This approach provides an excellent in vitro platform for further decoding of the cellular and molecular mechanisms of sprouting angiogenesis under conditions close to the in vivo conditions, as well as for preclinical drug testing and preclinical research in tissue engineering and regenerative medicine.

Heterotypic Tumor Spheroids in Agitation-Based Cultures: A Scaffold-Free Cell Model That Sustains Long-Term Survival of Endothelial Cells

Endothelial cells (ECs) are an important component of the tumor microenvironment, playing key roles in tumor development and progression that span from angiogenesis to immune regulation and drug resistance. Heterotypic tumor spheroids are one of the most widely used in vitro tumor microenvironment models, presenting improved recapitulation of tumor microenvironments compared to 2D cultures, in a simple and low-cost setup. Heterotypic tumor spheroid models incorporating endothelial cells have been proposed but present multiple limitations, such as the short culture duration typically obtained, the use of animal-derived matrices, and poor reproducibility; the diversity of culture conditions employed hinders comparison between studies and standardization of relevant culture parameters. Herein, we developed long-term cultures of triple heterotypic spheroids composed of the HCC1954 tumor cell line, human fibroblasts, and ECs. We explored culture parameters potentially relevant for EC maintenance, such as tumor cell line, seeding cell number, cell ratio, and agitation vs. static culture. In HCC1954-based spheroids, we observed maintenance of viable EC for up to 1 month of culture in agitation, with retention of the identity markers CD31 and von Willebrand factor. At the optimized tumor cell:fibroblast:EC ratio of 1:3:10, HCC1954-based spheroids had a higher EC area/total spheroid area at 1 month of culture than the other cell ratios tested. EC maintenance was tumor cell line-dependent, and in HCC1954-based spheroids it was also dependent on the presence of fibroblasts and agitation. Moreover, vascular endothelial growth factor (VEGF) supplementation was not required for maintenance of EC, as the factor was endogenously produced. ECs co-localized with fibroblasts, which accumulated preferentially in the core of the spheroids and secreted EC-relevant extracellular matrix proteins, such as collagen I and IV. This simple model setup does not rely on artificial or animal-derived scaffolds and can serve as a useful tool to explore the culture parameters influencing heterotypic spheroids, contributing to model standardization, as well as to explore molecular cross talk of ECs within the tumor microenvironment, and potentially its effects on drug response.

Stem cell-based vascularization of microphysiological systems

Microphysiological systems (MPSs) (i.e., tissue or organ chips) exploit microfluidics and 3D cell culture to mimic tissue and organ-level physiology. The advent of human induced pluripotent stem cell (hiPSC) technology has accelerated the use of MPSs to study human disease in a range of organ systems. However, in the reduction of system complexity, the intricacies of vasculature are an often-overlooked aspect of MPS design. The growing library of pluripotent stem cell-derived endothelial cell and perivascular cell protocols have great potential to improve the physiological relevance of vasculature within MPS, specifically for in vitro disease modeling. Three strategic categories of vascular MPS are outlined: self-assembled, interface focused, and 3D biofabricated. This review discusses key features and development of the native vasculature, linking that to how hiPSC-derived vascular cells have been generated, the state of the art in vascular MPSs, and opportunities arising from interdisciplinary thinking.

Cell-Matrix Interactions in the Eye: From Cornea to Choroid

The extracellular matrix (ECM) plays a crucial role in all parts of the eye, from maintaining clarity and hydration of the cornea and vitreous to regulating angiogenesis, intraocular pressure maintenance, and vascular signaling. This review focuses on the interactions of the ECM for homeostasis of normal physiologic functions of the cornea, vitreous, retina, retinal pigment epithelium, Bruch's membrane, and choroid as well as trabecular meshwork, optic nerve, conjunctiva and tenon's layer as it relates to glaucoma. A variety of pathways and key factors related to ECM in the eye are discussed, including but not limited to those related to transforming growth factor-β, vascular endothelial growth factor, basic-fibroblastic growth factor, connective tissue growth factor, matrix metalloproteinases (including MMP-2 and MMP-9, and MMP-14), collagen IV, fibronectin, elastin, canonical signaling, integrins, and endothelial morphogenesis consistent of cellular activation-tubulogenesis and cellular differentiation-stabilization. Alterations contributing to disease states such as wound healing, diabetes-related complications, Fuchs endothelial corneal dystrophy, angiogenesis, fibrosis, age-related macular degeneration, retinal detachment, and posteriorly inserted vitreous base are also reviewed.

Control of endothelial tubulogenesis by Rab and Ral GTPases, and apical targeting of caveolin-1-labeled vacuoles

Here, we examine known GTPase regulators of vesicle trafficking events to assess whether they affect endothelial cell (EC) lumen and tube formation. We identify novel roles for the small GTPases Rab3A, Rab3B, Rab8A, Rab11A, Rab27A, RalA, RalB and caveolin-1 in co-regulating membrane trafficking events that control EC lumen and tube formation. siRNA suppression of individual GTPases such as Rab3A, Rab8A, and RalB markedly inhibit tubulogenesis, while greater blockade is observed with combinations of siRNAs such as Rab3A and Rab3B, Rab8A and Rab11A, and RalA and RalB. These combinations of siRNAs also disrupt very early events in lumen formation including the formation of intracellular vacuoles. In contrast, knockdown of the endocytosis regulator, Rab5A, fails to inhibit EC tube formation. Confocal microscopy and real-time videos reveal that caveolin-1 strongly labels intracellular vacuoles and localizes to the EC apical surface as they fuse to form the luminal membrane. In contrast, Cdc42 and Rab11A localize to a perinuclear, subapical region where intracellular vacuoles accumulate and fuse during lumen formation. Our new data demonstrates that EC tubulogenesis is coordinated by a series of small GTPases to control polarized membrane trafficking events to generate, deliver, and fuse caveolin-1-labeled vacuoles to create the apical membrane surface.

Tumor cell-organized fibronectin maintenance of a dormant breast cancer population

Tumors can undergo long periods of dormancy, with cancer cells entering a largely quiescent, nonproliferative state before reactivation and outgrowth. To understand the role of the extracellular matrix (ECM) in regulating tumor dormancy, we created an in vitro cell culture system with carefully controlled ECM substrates to observe entrance into and exit from dormancy with live imaging. We saw that cell populations capable of surviving entrance into long-term dormancy were heterogeneous, containing quiescent, cell cycle-arrested, and actively proliferating cells. Cell populations capable of entering dormancy formed an organized, fibrillar fibronectin matrix via αvβ3 and α5β1 integrin adhesion, ROCK-generated tension, and TGFβ2 stimulation, and cancer cell outgrowth after dormancy required MMP-2-mediated fibronectin degradation. We propose this approach as a useful, in vitro method to study factors important in regulating dormancy, and we used it here to elucidate a role for fibronectin deposition and MMP activation.

Self-assembly, buckling and density-invariant growth of three-dimensional vascular networks

The experimental actualization of organoids modelling organs from brains to pancreases has revealed that much of the diverse morphologies of organs are emergent properties of simple intercellular 'rules' and not the result of top-down orchestration. In contrast to other organs, the initial plexus of the vascular system is formed by aggregation of cells in the process known as vasculogenesis. Here we study this self-assembling process of blood vessels in three dimensions through a set of simple rules that align intercellular apical-basal and planar cell polarity. We demonstrate that a fully connected network of tubes emerges above a critical initial density of cells. Through planar cell polarity, our model demonstrates convergent extension, and this polarity furthermore allows for both morphology-maintaining growth and growth-induced buckling. We compare this buckling with the special vasculature of the islets of Langerhans in the pancreas and suggest that the mechanism behind the vascular density-maintaining growth of these islets could be the result of growth-induced buckling.

Real Microgravity Influences the Cytoskeleton and Focal Adhesions in Human Breast Cancer Cells

With the increasing number of spaceflights, it is crucial to understand the changes occurring in human cells exposed to real microgravity (r-µg) conditions. We tested the effect of r-µg on MCF-7 breast cancer cells with the objective to investigate cytoskeletal alterations and early changes in the gene expression of factors belonging to the cytoskeleton, extracellular matrix, focal adhesion, and cytokines. In the Technische Experimente unter Schwerelosigkeit (TEXUS) 54 rocket mission, we had the opportunity to conduct our experiment during 6 min of r-µg and focused on cytoskeletal alterations of MCF-7 breast cancer cells expressing the Lifeact-GFP marker protein for the visualization of F-actin as well as the mCherry-tubulin fusion protein using the Fluorescence Microscopy Analysis System (FLUMIAS) for fast live-cell imaging under r-µg. Moreover, in a second mission we investigated changes in RNA transcription and morphology in breast cancer cells exposed to parabolic flight (PF) maneuvers (31st Deutsches Zentrum für Luft- und Raumfahrt (DLR) PF campaign). The MCF-7 cells showed a rearrangement of the F-actin and tubulin with holes, accumulations in the tubulin network, and the appearance of filopodia- and lamellipodia-like structures in the F-actin cytoskeleton shortly after the beginning of the r-µg period. PF maneuvers induced an early up-regulation of KRT8, RDX, TIMP1, CXCL8 mRNAs, and a down-regulation of VCL after the first parabola. E-cadherin protein was significantly reduced and is involved in cell adhesion processes, and plays a significant role in tumorigenesis. Changes in the E-cadherin protein synthesis can lead to tumor progression. Pathway analyses indicate that VCL protein has an activating effect on CDH1. In conclusion, live-cell imaging visualized similar changes as those occurring in thyroid cancer cells in r-µg. This result indicates the presence of a common mechanism of gravity perception and sensation.

Construction of Continuous Capillary Networks Stabilized by Pericyte-like Perivascular Cells

IMPACT STATEMENT

Construction of capillary networks is a fundamental challenge for the development of three-dimensional (3D) tissue engineering. However, it is not well understood how to construct stable capillary networks that maintain a luminal size similar to that of capillary structures in vivo (i.e., <10 μm diameter). In this study, we demonstrated the construction of stable capillary networks covered by pericyte-like perivascular cells using an in vitro 3D angiogenesis model by optimizing interactions between endothelial cells and perivascular cells. Our 3D angiogenesis model can be combined with 3D culture of epithelial cells in the context of vascularization of 3D tissue-engineered constructs.

Fabrication of centimeter-scale and geometrically arbitrary vascular networks using in vitro self-assembly

One of the largest challenges facing the field of tissue engineering is the incorporation of a functional vasculature, allowing effective nourishment of graft tissue beyond diffusion length scales. Here, we demonstrate a methodology for inducing the robust self-assembly of endothelial cells into stable three-dimensional perfusable networks on millimeter and centimeter length scales. Utilizing broadly accessible cell strains and reagents, we have rigorously tested a state space of cell densities (0.5-2.0 × 10⁶ cell/mL) and collagen gel densities (2-6 mg/mL) that result in robust vascular network formation. Further, over the range of culture conditions with which we observed robust network formation, we advanced image processing algorithms and quantitative metrics to assess network connectivity, coverage, tortuosity, lumenization, and vessel diameter. These data demonstrate that decreasing collagen density produced more connected networks with higher coverage. Finally, we demonstrated that this methodology results in the formation of perfusable networks, is extensible to arbitrary geometries and centimeter scales, and results in networks that remain stable for 21 days without the need for the co-culture of supporting cells. Given the robustness and accessibility, this system is ideal for studies of tissue-scale biology, as well as future studies on the formation and remodeling of larger engineered graft tissues.

Human Induced Pluripotent Stem Cell-Derived Endothelial Cells for Three-Dimensional Microphysiological Systems

Microphysiological systems (MPS), or "organ-on-a-chip" platforms, aim to recapitulate in vivo physiology using small-scale in vitro tissue models of human physiology. While significant efforts have been made to create vascularized tissues, most reports utilize primary endothelial cells that hinder reproducibility. In this study, we report the use of human induced pluripotent stem cell-derived endothelial cells (iPS-ECs) in developing three-dimensional (3D) microvascular networks. We established a CDH5-mCherry reporter iPS cell line, which expresses the vascular endothelial (VE)-cadherin fused to mCherry. The iPS-ECs demonstrate physiological functions characteristic of primary endothelial cells in a series of in vitro assays, including permeability, response to shear stress, and the expression of endothelial markers (CD31, von Willibrand factor, and endothelial nitric oxide synthase). The iPS-ECs form stable, perfusable microvessels over the course of 14 days when cultured within 3D microfluidic devices. We also demonstrate that inhibition of TGF-β signaling improves vascular network formation by the iPS-ECs. We conclude that iPS-ECs can be a source of endothelial cells in MPS providing opportunities for human disease modeling and improving the reproducibility of 3D vascular networks.

Flow dynamics control endothelial permeability in a microfluidic vessel bifurcation model

Endothelial barrier function is known to be regulated by a number of molecular mechanisms; however, the role of biomechanical signals associated with blood flow is comparatively less explored. Biomimetic microfluidic models comprised of vessel analogues that are lined with endothelial cells (ECs) have been developed to help answer several fundamental questions in endothelial mechanobiology. However, previously described microfluidic models have been primarily restricted to single straight or two parallel vessel analogues, which do not model the bifurcating vessel networks typically present in physiology. Therefore, the effects of hemodynamic stresses that arise due to bifurcating vessel geometries on ECs are not well understood. Here, we introduce and characterize a microfluidic model that mimics both the flow conditions and the endothelial/extracellular matrix (ECM) architecture of bifurcating blood vessels to systematically monitor changes in endothelial permeability mediated by the local flow dynamics at specific locations along the bifurcating vessel structure. We show that bifurcated fluid flow (BFF) that arises only at the base of a vessel bifurcation and is characterized by stagnation pressure of ∼38 dyn cm-2 and approximately zero shear stress induces significant decrease in EC permeability compared to the static control condition in a nitric oxide (NO)-dependent manner. Similarly, intravascular laminar shear stress (LSS) (3 dyn cm-2) oriented tangential to ECs located downstream of the vessel bifurcation also causes a significant decrease in permeability compared to the static control condition via the NO pathway. In contrast, co-application of transvascular flow (TVF) (∼1 μm s-1) with BFF and LSS rescues vessel permeability to the level of the static control condition, which suggests that TVF has a competing role against the stabilization effects of BFF and LSS. These findings introduce BFF at the base of vessel bifurcations as an important regulator of vessel permeability and suggest a mechanism by which local flow dynamics control vascular function in vivo.

Plocabulin, a novel tubulin-binding agent, inhibits angiogenesis by modulation of microtubule dynamics in endothelial cells

BACKGROUND

Vascular supply of tumors is one of the main targets for cancer therapy. Here, we investigated if plocabulin (PM060184), a novel marine-derived microtubule-binding agent, presents antiangiogenic and vascular-disrupting activities.

METHODS

The effects of plocabulin on microtubule network and dynamics were studied on HUVEC endothelial cells. We have also studied its effects on capillary tube structures formation or destabilization in three-dimensional collagen matrices. In vivo experiments were performed on different tumor cell lines.

RESULTS

In vitro studies show that, at picomolar concentrations, plocabulin inhibits microtubule dynamics in endothelial cells. This subsequently disturbs the microtubule network inducing changes in endothelial cell morphology and causing the collapse of angiogenic vessels, or the suppression of the angiogenic process by inhibiting the migration and invasion abilities of endothelial cells. This rapid collapse of the endothelial tubular network in vitro occurs in a concentration-dependent manner and is observed at concentrations lower than that affecting cell survival. The in vitro findings were confirmed in tumor xenografts where plocabulin treatment induced a large reduction in vascular volume and induction of extensive necrosis in tumors, consistent with antivascular effects.

CONCLUSIONS

Altogether, these data suggest that an antivascular mechanism is contributing to the antitumor activities of plocabulin.

In Vitro Mimetic Models for the Bone-Cartilage Interface Regeneration

In embryonic development, pure cartilage structures are in the basis of bone-cartilage interfaces. Despite this fact, the mature bone and cartilage structures can vary greatly in composition and function. Nevertheless, they collaborate in the osteochondral region to create a smooth transition zone that supports the movements and forces resulting from the daily activities. In this sense, all the hierarchical organization is involved in the maintenance and reestablishment of the equilibrium in case of damage. Therefore, this interface has attracted a great deal of interest in order to understand the mechanisms of regeneration or disease progression in osteoarthritis. With that purpose, in vitro tissue models (either static or dynamic) have been studied. Static in vitro tissue models include monocultures, co-cultures, 3D cultures, and ex vivo cultures, mostly cultivated in flat surfaces, while dynamic models involve the use of bioreactors and microfluidic systems. The latter have emerged as alternatives to study the cellular interactions in a more authentic manner over some disadvantages of the static models. The current alternatives of in vitro mimetic models for bone-cartilage interface regeneration are overviewed and discussed herein.

CLASP1 regulates endothelial cell branching morphology and directed migration

Endothelial cell (EC) branching is critically dependent upon the dynamic nature of the microtubule (MT) cytoskeleton. Extracellular matrix (ECM) mechanosensing is a prominent mechanism by which cytoskeletal reorganization is achieved; yet how ECM-induced signaling is able to target cytoskeletal reorganization intracellularly to facilitate productive EC branching morphogenesis is not known. Here, we tested the hypothesis that the composition and density of the ECM drive the regulation of MT growth dynamics in ECs by targeting the MT stabilizing protein, cytoplasmic linker associated protein 1 (CLASP1). High-resolution fluorescent microscopy coupled with computational image analysis reveal that CLASP1 promotes slow MT growth on glass ECMs and promotes short-lived MT growth on high-density collagen-I and fibronectin ECMs. Within EC branches, engagement of either high-density collagen-I or high-density fibronectin ECMs results in reduced MT growth speeds, while CLASP1-dependent effects on MT dynamics promotes elevated numbers of short, branched protrusions that guide persistent and directed EC migration.

Summary: CLASP1 modulates microtubule dynamics with sub-cellular specificity in response to extracellular matrix density and composition. CLASP1 effects on microtubules promote short, branched protrusions that guide persistent and directional EC migration. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.

Biological Interaction Between Human Gingival Fibroblasts and Vascular Endothelial Cells for Angiogenesis: A Co-culture Perspective

Advancement in cell culture protocols, multidisciplinary research approach, and the need of clinical implication to reconstruct damaged or diseased tissues has led to the establishment of three-dimensional (3D) test systems for regeneration and repair. Regenerative therapies, including dental tissue engineering, have been pursued as a new prospect to repair and rebuild the diseased/lost oral tissues. Interactions between the different cell types, growth factors, and extracellular matrix components involved in angiogenesis are vital in the mechanisms of new vessel formation for tissue regeneration. In vitro pre-vascularization is one of the leading scopes in the tissue-engineering field. Vascularization strategies that are associated with co-culture systems have proved that there is communication between different cell types with mutual beneficial effects in vascularization and tissue regeneration in two-dimensional or 3D cultures. Endothelial cells with different cell populations, including osteoblasts, smooth muscle cells, and fibroblasts in a co-culture have shown their ability to advocate pre-vascularization. In this review, a co-culture perspective of human gingival fibroblasts and vascular endothelial cells is discussed with the main focus on vascularization and future perspective of this model in regeneration and repair.

Improvement of endothelial progenitor outgrowth cell (EPOC)-mediated vascularization in gelatin-based hydrogels through pore size manipulation

In addition to chemical compositions, physical properties of scaffolds, such as pore size, can also influence vascularization within the scaffolds. A larger pore has been shown to improve host vascular tissue invasion into scaffolds. However, the influence of pore sizes on vascularization by endothelial cells directly encapsulated in hydrogels remains unknown. In this study, micro-cavitary hydrogels with different pore sizes were created in gelatin-methacrylate hydrogels with dissolvable gelatin microspheres (MS) varying in sizes. The effect of pore sizes on vascular network formation by endothelial progenitor outgrowth cells (EPOCs) encapsulated in hydrogels was then investigated both in vitro and in vivo. When cultured in vitro, vascular networks were formed around pore structures in micro-cavitary hydrogels. The middle pore size supported best differentiation of EPOCs and thus best hydrogel vascularization in vitro. When implantation in vivo, functional connections between encapsulated EPOCs and host vasculature micro-cavitary hydrogels were established. Vascularization in vivo was promoted best in hydrogels with the large pore size due to the increased vascular tissue invasion. These results highlight the difference between in vitro and in vivo culture conditions and indicate that pore sizes shall be designed for in vitro and in vivo hydrogel vascularization respectively. Pore sizes for hydrogel vascularization in vitro shall be middle ones and pore sizes for hydrogel vascularization in vivo shall be large ones.

STATEMENT OF SIGNIFICANCE

This study reveals that the optimal pore size for hydrogel vascularization in vitro and in vivo is different. The middle pore size supported best differentiation of EPOCs and thus best hydrogel vascularization in vitro, while vascularization in vivo was promoted best in hydrogels with the large pore size due to the increased vascular tissue invasion. These results highlight the difference between in vitro and in vivo culture conditions and indicate that pore sizes shall be designed for in vitro and in vivo hydrogel vascularization respectively. Pore sizes for hydrogel vascularization in vitro shall be middle ones and pore sizes for hydrogel vascularization in vivo shall be large ones.

O2-controllable hydrogels for studying cellular responses to hypoxic gradients in three dimensions in vitro and in vivo

Oxygen (O₂) acts as a potent upstream regulator of cell function. In both physiological and pathophysiological microenvironments, the O₂ concentration is not uniformly distributed but instead follows a gradient that depends on distance from oxygen-carrying blood vessels. Such gradients have a particularly important role in development, tissue regeneration, and tumor growth. In this protocol, we describe how to use our previously reported gelatin-based O₂-controllable hydrogels that can provide hypoxic microenvironments in vitro. The hydrogel polymeric network is formed via a laccase-mediated cross-linking reaction. In this reaction, laccase catalyzes diferulic acid (diFA) formation to form hydrogels with an O₂-consuming reaction. Cells, such as cancer or endothelial cells, as well as tumor/tissue grafts, can be encapsulated in the hydrogels during hydrogel formation and then analyzed for cellular responses to 3D hypoxic gradients and to elucidate the underlying mechanisms governing these responses. Importantly, oxygen gradients can be precisely controlled in standard cell/tissue culture conditions and in vivo. This platform has been applied to study vascular morphogenesis in response to hypoxia and to understand how oxygen gradients mediate cancer cell behavior. Herein, we describe the means to validate the assay from polymer synthesis and characterization-which take 1-2 weeks and include verification of ferulic acid (FA) conjugation, rheological measurements, and O₂ monitoring-to the study of cellular responses and use in rodent models. Time courses for biological experiments using this hydrogel are variable, and thus they may range from hours to weeks, depending on the application and user end goal.

Human lung fibroblast-derived matrix facilitates vascular morphogenesis in 3D environment and enhances skin wound healing

Extracellular matrix (ECM) is crucial to many aspects of vascular morphogenesis and maintenance of vasculature function. Currently the recapitulation of angiogenic ECM microenvironment is still challenging, due mainly to its diverse components and complex organization. Here we investigate the angiogenic potential of human lung fibroblast-derived matrix (hFDM) in creating a three-dimensional (3D) vascular construct. hFDM was obtained via decellularization of in vitro cultured human lung fibroblasts and analyzed via immunofluorescence staining and ELISA, which detect multiple ECM macromolecules and angiogenic growth factors (GFs). Human umbilical vein endothelial cells (HUVECs) morphology was more elongated and better proliferative on hFDM than on gelatin-coated substrate. To prepare 3D construct, hFDM is collected, quantitatively analyzed, and incorporated in collagen hydrogel (Col) with HUVECs. Capillary-like structure (CLS) formation at 7day was significantly better with the groups containing higher doses of hFDM compared to the Col group (control). Moreover, the group (Col/hFDM/GFs) with both hFDM and angiogenic GFs (VEGF, bFGF, SDF-1) showed the synergistic activity on CLS formation and found much larger capillary lumen diameters with time. Further analysis of hFDM via angiogenesis antibody array kit reveals abundant biochemical cues, such as angiogenesis-related cytokines, GFs, and proteolytic enzymes. Significantly up-regulated expression of VE-cadherin and ECM-specific integrin subunits was also noticed in Col/hFDM/GFs. In addition, transplantation of Col/hFMD/GFs with HUVECs in skin wound model presents more effective re-epithelialization, many regenerated hair follicles, better transplanted cells viability, and advanced neovascularization. We believe that current system is a very promising platform for 3D vasculature construction in vitro and for cell delivery toward therapeutic applications in vivo.

STATEMENT OF SIGNIFICANCE

Functional 3D vasculature construction in vitro is still challenging due to the difficulty of recapitulating the complex angiogenic extracellular matrix (ECM) environment. Herein, we present a simple and practical method to create an angiogenic 3D environment via incorporation of human lung fibroblast-derived matrix (hFDM) into collagen hydrogel. We found that hFDM offers a significantly improved angiogenic microenvironment for HUVECs on 2D substrates and in 3D construct. A synergistic effect of hFDM and angiogenic growth factors has been well confirmed in 3D condition. The prevascularized 3D collagen constructs also facilitate skin wound healing. We believe that current system should be a convenient and powerful platform in engineering 3D vasculature in vitro, and in delivering cells for therapeutic purposes in vivo.

In Vitro Multitissue Interface Model Supports Rapid Vasculogenesis and Mechanistic Study of Vascularization across Tissue Compartments

A significant challenge facing tissue engineers is the design and development of complex multitissue systems, including vascularized tissue-tissue interfaces. While conventional in vitro models focus on either vasculogenesis (de novo formation of blood vessels) or angiogenesis (vessels sprouting from existing vessels or endothelial monolayers), successful therapeutic vascularization strategies will likely rely on coordinated integration of both processes. To address this challenge, we developed a novel in vitro multitissue interface model in which human endothelial colony forming cell (ECFC)-encapsulated tissue spheres are embedded within a surrounding tissue microenvironment. This highly reproducible approach exploits biphilic surfaces (nanostructured surfaces with distinct superhydrophobic and hydrophilic regions) to (i) support tissue compartments with user-specified matrix composition and physical properties as well as cell type and density and (ii) introduce boundary conditions that prevent the cell-mediated tissue contraction routinely observed with conventional three-dimensional monodispersion cultures. This multitissue interface model was applied to test the hypothesis that independent control of cell-extracellular matrix (ECM) and cell-cell interactions would affect vascularization within the tissue sphere as well as across the tissue-tissue interface. We found that high-cell-density tissue spheres containing 5 × 10(6) ECFCs/mL exhibit rapid and robust vasculogenesis, forming highly interconnected, stable (as indicated by type IV collagen deposition) vessel networks within only 3 days. Addition of adipose-derived stromal cells (ASCs) in the surrounding tissue further enhanced vasculogenesis within the sphere as well as angiogenic vessel elongation across the tissue-tissue boundary, with both effects being dependent on the ASC density. Overall, results show that the ECFC density and ECFC-ASC crosstalk, in terms of paracrine and mechanophysical signaling, are critical determinants of vascularization within a given tissue compartment and across tissue interfaces. This new in vitro multitissue interface model and the associated mechanistic insights it yields provide guiding principles for the design and optimization of multitissue vascularization strategies for research and clinical applications.

Human iPSC-derived endothelial cell sprouting assay in synthetic hydrogel arrays

Activation of vascular endothelial cells (ECs) by growth factors initiates a cascade of events during angiogenesis in vivo consisting of EC tip cell selection, sprout formation, EC stalk cell proliferation, and ultimately vascular stabilization by support cells. Although EC functional assays can recapitulate one or more aspects of angiogenesis in vitro, they are often limited by undefined substrates and lack of dependence on key angiogenic signaling axes. Here, we designed and characterized a chemically-defined model of endothelial sprouting behavior in vitro using human induced pluripotent stem cell-derived endothelial cells (iPSC-ECs). We rapidly encapsulated iPSC-ECs at high density in poly(ethylene glycol) (PEG) hydrogel spheres using thiol-ene chemistry and subsequently encapsulated cell-dense hydrogel spheres in a cell-free hydrogel layer. The hydrogel sprouting array supported pro-angiogenic phenotype of iPSC-ECs and supported growth factor-dependent proliferation and sprouting behavior. iPSC-ECs in the sprouting model responded appropriately to several reference pharmacological angiogenesis inhibitors of vascular endothelial growth factor, NF-κB, matrix metalloproteinase-2/9, protein kinase activity, and β-tubulin, which confirms their functional role in endothelial sprouting. A blinded screen of 38 putative vascular disrupting compounds from the US Environmental Protection Agency's ToxCast library identified six compounds that inhibited iPSC-EC sprouting and five compounds that were overtly cytotoxic to iPSC-ECs at a single concentration. The chemically-defined iPSC-EC sprouting model (iSM) is thus amenable to enhanced-throughput screening of small molecular libraries for effects on angiogenic sprouting and iPSC-EC toxicity assessment.

STATEMENT OF SIGNIFICANCE

Angiogenesis assays that are commonly used for drug screening and toxicity assessment applications typically utilize natural substrates like Matrigel(TM) that are difficult to spatially pattern, costly, ill-defined, and may exhibit lot-to-lot variability. Herein, we describe a novel angiogenic sprouting assay using chemically-defined, bioinert poly(ethylene glycol) hydrogels functionalized with biomimetic peptides to promote cell attachment and degradation in a reproducible format that may mitigate the need for natural substrates. The quantitative assay of angiogenic sprouting here enables precise control over the initial conditions and can be formulated into arrays for screening. The sprouting assay here was dependent on key angiogenic signaling axes in a screen of angiogenesis inhibitors and a blinded screen of putative vascular disrupting compounds from the US-EPA.

Knockdown of ezrin suppresses the migration and angiogenesis of human umbilical vein endothelial cells in vitro

Progressive tumor growth is dependent on angiogenesis. The mechanisms by which endothelial cells (ECs) are incorporated to develop new blood vessels are not well understood. Recent studies reveal that the ezrin radixin moesin (ERM) family members are key regulators of cellular activities such as adhesion, morphogenetic change, and migration. We hypothesized that ezrin, one of the ERM family members, may play important roles in ECs organization during angiogenesis, and new vessels formation in preexisting tissues. To test this hypothesis, in this study, we investigated the effects of ezrin gene silencing on the migration and angiogenesis of human umbilical vein endothelial cells (HUVECs) in vitro. HUVECs were transfected with plasmids with ezrin-targeting short hairpin RNA by using the lipofectamine-2000 system. Wound assay in vitro and three-dimensional culture were used to detect the migration and angiogenesis capacity of HUVECs. The morphological changes of transfected cells were observed by confocal and phase contrast microscopy. Our results demonstrated that the decreased expression of ezrin in HUVECs significantly induced the morphogenetic changes and cytoskeletal reorganization of the transfected cells, and also reduced cell migration and angiogenesis capacity in vitro, suggesting that ezrin play an important role in the process of HUVECs migration and angiogenesis.

Phosphorylation of Threonine 794 on Tie1 by Rac1/PAK1 Reveals a Novel Angiogenesis Regulatory Pathway

The endothelial receptor tyrosine kinase (RTK) Tie1 was discovered over 20 years ago, yet its precise function and mode of action remain enigmatic. To shed light on Tie1’s role in endothelial cell biology, we investigated a potential threonine phosphorylation site within the juxtamembrane domain of Tie1. Expression of a non-phosphorylatable mutant of this site (T794A) in zebrafish (Danio rerio) significantly disrupted vascular development, resulting in fish with stunted and poorly branched intersomitic vessels. Similarly, T794A-expressing human umbilical vein endothelial cells formed significantly shorter tubes with fewer branches in three-dimensional Matrigel cultures. However, mutation of T794 did not alter Tie1 or Tie2 tyrosine phosphorylation or downstream signaling in any detectable way, suggesting that T794 phosphorylation may regulate a Tie1 function independent of its RTK properties. Although T794 is within a consensus Akt phosphorylation site, we were unable to identify a physiological activator of Akt that could induce T794 phosphorylation, suggesting that Akt is not the physiological Tie1-T794 kinase. However, the small GTPase Ras-related C3 botulinum toxin substrate 1 (Rac1), which is required for angiogenesis and capillary morphogenesis, was found to associate with phospho-T794 but not the non-phosphorylatable T794A mutant. Pharmacological activation of Rac1 induced downstream activation of p21-activated kinase (PAK1) and T794 phosphorylation in vitro, and inhibition of PAK1 abrogated T794 phosphorylation. Our results provide the first demonstration of a signaling pathway mediated by Tie1 in endothelial cells, and they suggest that a novel feedback loop involving Rac1/PAK1 mediated phosphorylation of Tie1 on T794 is required for proper angiogenesis.

Inexpensive production of near-native engineered stromas

Although the self-assembly approach is an efficient method for the production of engineered physiological and pathological tissues, avoiding the use of exogenous materials, it nevertheless remains expensive and requires dexterity, which are features incompatible with large-scale production. We propose a modification to this technique to make easier the production of mesenchymal compartment, to reduce the cost and to improve the histological quality of the self-assembled tissues. The stroma produced by this novel approach allowed epithelial cell differentiation, resulting in a pseudostratified epithelium that shared several features with native tissues. The incorporation of endothelial cells in the reconstructed mesenchyme formed a three-dimensional capillary-like network, positive for CD31 and von Willebrand factor and surrounded by NG2 positive cells. It could limit self-contraction of the resulting tissue by recruiting α-Smooth Muscle Actin positive cells. With this new technique, which is relatively inexpensive and easy to use in a research laboratory set-up, near-native stromas can now be produced with minimal handling time. Copyright © 2015 John Wiley & Sons, Ltd.

TGFβ Triggers miR-143/145 Transfer From Smooth Muscle Cells to Endothelial Cells, Thereby Modulating Vessel Stabilization

RATIONALE

The miR-143/145 cluster is highly expressed in smooth muscle cells (SMCs), where it regulates phenotypic switch and vascular homeostasis. Whether it plays a role in neighboring endothelial cells (ECs) is still unknown.

OBJECTIVE

To determine whether SMCs control EC functions through passage of miR-143 and miR-145.

METHODS AND RESULTS

We used cocultures of SMCs and ECs under different conditions, as well as intact vessels to assess the transfer of miR-143 and miR-145 from one cell type to another. Imaging of cocultured cells transduced with fluorescent miRNAs suggested that miRNA transfer involves membrane protrusions known as tunneling nanotubes. Furthermore, we show that miRNA passage is modulated by the transforming growth factor (TGF) β pathway because both a specific transforming growth factor-β (TGFβ) inhibitor (SB431542) and an shRNA against TGFβRII suppressed the passage of miR-143/145 from SMCs to ECs. Moreover, miR-143 and miR-145 modulated angiogenesis by reducing the proliferation index of ECs and their capacity to form vessel-like structures when cultured on matrigel. We also identified hexokinase II (HKII) and integrin β 8 (ITGβ8)-2 genes essential for the angiogenic potential of ECs-as targets of miR-143 and miR-145, respectively. The inhibition of these genes modulated EC phenotype, similarly to miR-143 and miR-145 overexpression in ECs. These findings were confirmed by ex vivo and in vivo approaches, in which it was shown that TGFβ and vessel stress, respectively, triggered miR-143/145 transfer from SMCs to ECs.

CONCLUSIONS

Our results demonstrate that miR-143 and miR-145 act as communication molecules between SMCs and ECs to modulate the angiogenic and vessel stabilization properties of ECs.

Tissue vascularization through 3D printing: Will technology bring us flow?

BACKGROUND

Though in vivo models provide the most physiologically relevant environment for studying tissue function, in vitro studies provide researchers with explicit control over experimental conditions and the potential to develop high throughput testing methods. In recent years, advancements in developmental biology research and imaging techniques have significantly improved our understanding of the processes involved in vascular development. However, the task of recreating the complex, multi-scale vasculature seen in in vivo systems remains elusive.

RESULTS

3D bioprinting offers a potential method to generate controlled vascular networks with hierarchical structure approaching that of in vivo networks. Bioprinting is an interdisciplinary field that relies on advances in 3D printing technology along with advances in imaging and computational modeling, which allow researchers to monitor cellular function and to better understand cellular environment within the printed tissue.

CONCLUSIONS

As bioprinting technologies improve with regards to resolution, printing speed, available materials, and automation, 3D printing could be used to generate highly controlled vascularized tissues in a high throughput manner for use in regenerative medicine and the development of in vitro tissue models for research in developmental biology and vascular diseases.

Combined effect of insulin-like growth factor-1 and CC chemokine ligand 2 on angiogenic events in endothelial cells

Therapeutic angiogenesis may be applied in medical conditions to promote stimulation of angiogenesis. Angiogenesis is a multistep process, which includes endothelial cell proliferation, migration, and tube formation, which is mediated by various angiogenic polypeptides. Thus, studies that elucidate the cellular mechanisms involved in these processes are necessary to develop novel therapeutic strategies. This study investigated the in vitro effects of the pro-angiogenic factors, insulin-like growth factor-1 (IGF-1) and/or chemokine (CC motif) ligand 2 (CCL2), on endothelial cells. Flow cytometry analysis showed that IGF-1 and CCL2 treatment did not interfere with IGF-1 receptor (IGF-1R) expression, but CCL2 treatment increased CCL2 receptor (CCR2) expression. Immunofluorescence analysis revealed that the IGF-1/CCL2 combination induced a greater increase in fibronectin deposition, but the treatments did not alter the expression of the fibronectin receptors, CD49e and CD44. The interaction of fibronectin with cytokines demonstrated that IGF-1/CCL2 promoted changes in intermediate F-actin remodeling that may result in increased endothelial cell adhesion and cell migration mediated by fibronectin. Furthermore, IGF-1/CCL2 stimulated endothelial cells, grown on fibronectin, to form capillary-like structures and intercellular lumina with greater luminal area. These data suggest that IGF-1/CCL2 combination and a fibronectin matrix may contribute to the angiogenesis process to stimulate adhesion, migration, and tube formation by endothelial cells as a result of F-actin remodeling.

Mechanisms of tubulogenesis and endothelial phenotype expression by MSCs

Stem cell-based therapies are a promising new avenue for treating ischemic disease and chronic wounds. Mesenchymal stem cells (MSCs) have a proven ability to augment the neovascularization processes necessary for wound healing and are widely popular as an autologous source of progenitor cells. Our lab has previously reported on PEGylated fibrin as a unique hydrogel that promotes spontaneous tubulogenesis of encapsulated MSCs without exogenous factors. However, the mechanisms underlying this process have remained unknown. To better understand the therapeutic value of PEGylated fibrin delivery of MSCs, we sought to clarify the relationship between biomaterial properties and cell behavior. Here we find that fibrin PEGylation does not dramatically alter the macroscopic mechanical properties of the fibrin-based matrix (less than 10% difference). It does, however, dramatically reduce the rate of diffusion through the gel matrix. PEGylated fibrin enhances the tubulogenic growth of encapsulated MSCs demonstrating fluid-filled lumens by interconnected MSCs. Image analysis gave a value of 4320 ± 1770 μm total network length versus 618 ± 443 μm for unmodified fibrin. PEGylation promotes the endothelial phenotype of encapsulated MSCs--compared to unmodified fibrin--as evidenced by higher levels of endothelial markers (von Willebrand factor, 2.2-fold; vascular endothelial cadherin, 1.8-fold) and vascular endothelial growth factor (VEGF, up to 1.8-fold). Prospective analysis of underlying molecular pathways demonstrated that this endothelial-like MSC behavior is sensitively modulated by hypoxic stress, but not VEGF supplementation as evidenced by a significant increase in VEGF and MMP-2 secretion per cell under hypoxia. Further gain-of-function studies under hypoxic stress demonstrated that hypoxia culture of MSCs in unmodified fibrin could increase both vWF and VE-cadherin levels to values that were not significantly different than cells cultured in PEGylated fibrin. This result corroborated our hypothesis that the diffusion-limited environment of PEGylated fibrin is augmenting endothelial differentiation cues provided by unmodified fibrin. However, MSC networks lack platelet endothelial cell adhesion molecule-1 (PECAM-1) expression, which indicates incomplete differentiation towards an endothelial cell type. Collectively, the data here supports a revised understanding of MSC-derived neovascularization that contextualizes their behavior and utility as a hybrid endothelial-stromal cell type, with mixed characteristics of both populations.

Electrospun anisotropic architectures and porous structures for tissue engineering

Recent advances in electrospun anisotropic architectures and porous structures, as well as their applications in tissue engineering, are presented.

Design of growth factor sequestering biomaterials

Growth factors (GFs) are major regulatory proteins that can govern cell fate, migration, and organization. Numerous aspects of the cell milieu can modulate cell responses to GFs, and GF regulation is often achieved by the native extracellular matrix (ECM). For example, the ECM can sequester GFs and thereby control GF bioavailability. In addition, GFs can exert distinct effects depending on whether they are sequestered in solution, at two-dimensional interfaces, or within three-dimensional matrices. Understanding how the context of GF sequestering impacts cell function in the native ECM can instruct the design of soluble or insoluble GF sequestering moieties, which can then be used in a variety of bioengineering applications. This Feature Article provides an overview of the natural mechanisms of GF sequestering in the cell milieu, and reviews the recent bioengineering approaches that have sequestered GFs to modulate cell function. Results to date demonstrate that the cell response to GF sequestering depends on the affinity of the sequestering interaction, the spatial proximity of sequestering in relation to cells, the source of the GF (supplemented or endogenous), and the phase of the sequestering moiety (soluble or insoluble). We highlight the importance of context for the future design of biomaterials that can leverage endogenous molecules in the cell milieu and mitigate the need for supplemented factors.

In vitro pre-vascularisation of tissue-engineered constructs A co-culture perspective

In vitro pre-vascularization is one of the main vascularization strategies in the tissue engineering field. Culturing cells within a tissue-engineered construct (TEC) prior to implantation provides researchers with a greater degree of control over the fate of the cells. However, balancing the diverse range of different cell culture parameters in vitro is seldom easy and in most cases, especially in highly vascularized tissues, more than one cell type will reside within the cell culture system. Culturing multiple cell types in the same construct presents its own unique challenges and pitfalls. The following review examines endothelial-driven vascularization and evaluates the direct and indirect role other cell types have in vessel and capillary formation. The article then analyses the different parameters researchers can modulate in a co-culture system in order to design optimal tissue-engineered constructs to match desired clinical applications.

Synergy of cell-cell repulsion and vacuolation in a computational model of lumen formation

A key step in blood vessel development (angiogenesis) is lumen formation: the hollowing of vessels for blood perfusion. Two alternative lumen formation mechanisms are suggested to function in different types of blood vessels. The vacuolation mechanism is suggested for lumen formation in small vessels by coalescence of intracellular vacuoles, a view that was extended to extracellular lumen formation by exocytosis of vacuoles. The cell–cell repulsion mechanism is suggested to initiate extracellular lumen formation in large vessels by active repulsion of adjacent cells, and active cell shape changes extend the lumen. We used an agent-based computer model, based on the cellular Potts model, to compare and study both mechanisms separately and combined. An extensive sensitivity analysis shows that each of the mechanisms on its own can produce lumens in a narrow region of parameter space. However, combining both mechanisms makes lumen formation much more robust to the values of the parameters, suggesting that the mechanisms may work synergistically and operate in parallel, rather than in different vessel types.

Procedure for the development of multi-depth circular cross-sectional endothelialized microchannels-on-a-chip

Efforts have been focused on developing in vitro assays for the study of microvessels because in vivo animal studies are more time-consuming, expensive, and observation and quantification are very challenging. However, conventional in vitro microvessel assays have limitations when representing in vivo microvessels with respect to three-dimensional (3D) geometry and providing continuous fluid flow. Using a combination of photolithographic reflowable photoresist technique, soft lithography, and microfluidics, we have developed a multi-depth circular cross-sectional endothelialized microchannels-on-a-chip, which mimics the 3D geometry of in vivo microvessels and runs under controlled continuous perfusion flow. A positive reflowable photoresist was used to fabricate a master mold with a semicircular cross-sectional microchannel network. By the alignment and bonding of the two polydimethylsiloxane (PDMS) microchannels replicated from the master mold, a cylindrical microchannel network was created. The diameters of the microchannels can be well controlled. In addition, primary human umbilical vein endothelial cells (HUVECs) seeded inside the chip showed that the cells lined the inner surface of the microchannels under controlled perfusion lasting for a time period between 4 days to 2 weeks.

ADAMTS-4 and biglycan are expressed at high levels and co-localize to podosomes during endothelial cell tubulogenesis in vitro

Proteolysis of the extracellular matrix influences vascular growth. We examined the expression of ADAMTS-1, -4, and -5 metalloproteinases and their proteoglycan substrates versican, decorin, and biglycan as human umbilical vein endothelial cells (HUVECs) formed tubes within type I collagen gels in vitro. Tubulogenic and control HUVEC cultures expressed low levels of ADAMTS-1 and -5 mRNAs, but ADAMTS-4 mRNA was relatively abundant and was significantly elevated (as was ADAMTS-4 protein) in tubulogenic cultures versus controls. Immunocytochemistry revealed ADAMTS-4 in f-actin- and cortactin-positive podosome-like puncta in single cells and mature tubes. Tubulogenic and control cultures expressed low levels of versican and decorin mRNAs; however, peak levels of biglycan mRNA were 400- and 16,000-fold that of versican and decorin, respectively. Biglycan mRNA was highest at 3 hr, declined steadily through day 7 and, at 12 hr and beyond, was significantly lower in tubulogenic cultures than in controls. Western blots of extracellular matrix from tubulogenic cultures contained bands corresponding to biglycan and its cleavage products. By immunocytochemistry, biglycan was found in the pericellular matrix surrounding endothelial tubes and in cell-associated puncta that co-localized with ADAMTS-4 and cortactin. Collectively, our results suggest that ADAMTS-4 and its substrate biglycan are involved in tubulogenesis by endothelial cells.

Tubulogenesis

Metazoans require epithelial and endothelial tubes to transport liquids and gasses throughout their bodies. Although biological tubes may look relatively similar at first glance, there are multiple and distinct mechanisms by which tubes form and even more regulatory events driving the cell shape changes that produce tubes of specific dimensions. An overview of the current understanding of the molecular processes and physical forces involved in tubulogenesis is presented in this review and the accompanying poster.

Oxidized low-density lipoprotein alters the effect of matrix stiffness on the formation of endothelial networks and capillary lumens

Abstract Formation of new blood vessels is essential for vascular repair and remodeling, and it is known that biomechanical properties of extracellular matrix play a major role in this process. Our earlier studies have also shown that exposing endothelial cells to oxidized modification of low-density lipoproteins (oxLDL) increases endothelial stiffness and facilitates their ability to form cellular networks, suggesting that it facilitates endothelial angiogenic potential. The goal of this study, therefore, was to test the interrelationship between matrix stiffness and oxLDL in the regulation of angiogenesis. Our results show that, as expected, an increase in matrix stiffness inhibited endothelial network formation and that exposure to oxLDL significantly facilitated this process. We also show, however, that oxLDL-induced facilitation of endothelial networks was observed only in stiff (3 mg/mL) but not in soft (1 mg/mL) collagen gels, resulting in blunting the effect of matrix stiffness. Also unexpectedly, we show that an increase in matrix stiffness results in a significant increase in the number of capillary lumens that are formed by single cells or pairs of cells, suggesting that while endothelial connectivity is impaired, formation of single-cell lumens is facilitated. oxLDL facilitates lumen formation, but this effect is also matrix dependent and is observed only in soft gels and not in stiff gels. Finally, an increase in both matrix stiffness and oxLDL exposure results in changes in capillary morphology, with the formation of larger capillary lumens. Overall, our study suggests that oxLDL plays an important role in formation of new capillaries and their morphology and that this effect is critically dependent on the extracellular environment's compliance, thereby underlining the importance of the interdependence of these parameters.