Hyaluronic Acid hydrogels support cord-like structures from endothelial colony-forming cells

Phototuning of Hyaluronic-Acid-Based Hydrogel Properties to Control Network Formation in Human Vascular Endothelial Cells

In vitro network formation by endothelial cells serves as a fundamental model for studies aimed at understanding angiogenesis. The morphogenesis of these cells to form a network is intricately regulated by the mechanical and biochemical properties of the extracellular matrix. Here the effects of modulating these properties in hydrogels derived from phenolated hyaluronic acid (HA-Ph) and phenolated gelatin (Gelatin-Ph) are presented. Visible-light irradiation in the presence of tris(2,2'-bipyridyl)ruthenium(II) chloride hexahydrate and sodium persulfate induces the crosslinking of these polymers, thereby forming a hydrogel and degrading HA-Ph. Human vascular endothelial cells form networks on the hydrogel prepared by visible-light irradiation for 45 min (42 W cm-2 at 450 nm) but not on the hydrogels prepared by irradiation for 15, 30, or 60 min. The irradiation time-dependent degradation of HA-Ph and the changes in the mechanical stiffness of the hydrogels, coupled with the expressions of RhoA and β-actin genes and CD44 receptors in the cells, reveal that the network formation is synergistically influenced by the hydrogel stiffness and HA-Ph degradation. These findings highlight the potential of tailoring HA-based hydrogel properties to modulate human vascular endothelial cell responses, which is critical for advancing their application in vascular tissue engineering.

Molecular weight of hyaluronic acid crosslinked into biomaterial scaffolds affects angiogenic potential

While hyaluronic acid (HA)-based hydrogels have been used clinically for decades, the mechanisms by which HA exerts molecular weight-dependent bioactivity and how chemical modification and crosslinking may affect molecular weight-dependent bioactivity remain poorly understood. This knowledge gap presents a significant barrier to designing HA hydrogels with predictable bioactivities. As HA has been widely reported to have molecular weight-dependent effects on endothelial cells (ECs), we investigated how the molecular weight of HA in either soluble or crosslinked forms affects angiogenesis and interrogated CD44 clustering on the surface of endothelial cells as a candidate mechanism for these affects. Using soluble HA, our results show high molecular weight (HMW) HA, but not low molecular weight (LMW) HA, increased viability and tube formation in cultured human cerebral microvascular ECs (HCMVECs). No size of HA affected proliferation. When HCMVECs were cultured with crosslinked HA of varying molecular weights in the form of HA-based microporous annealed particle scaffold (HMAPS), the cell response was comparable to when cultured with soluble HA. Similarly, when implanted subcutaneously, HMAPS with HMW HA were more vascularized than those with LMW HA. We also show that antibody-mediated CD44 clustering resulted in HCMVECs with increased viability and tube-like structure formation in a manner comparable to exposure to HMW HA, suggesting that HMW acts through CD44 clustering. STATEMENT OF SIGNIFICANCE: Biomaterials based on hyaluronic acid (HA), a bioactive extracellular matrix polysaccharide, have been used in clinical products for several years. Despite the knowledge that HA molecular weight heavily influences its bioactivity, molecular weight has been largely ignored in the development of HA-based biomaterials. Given the high viscosity of high molecular weight HA typically found in native tissues, lower molecular weight polysaccharides have been used most commonly for biomaterial fabrication. By comparing the ability of injectable, microporous annealed particle scaffolds (MAPS) fabricated from variably sized HA to promote angiogenesis, this study demonstrates that MAPS with high molecular weight HA better support vascularization, likely through an unique ability to induce clustering of CD44 receptors on endothelial cells.

Human Umbilical Vein Endothelial Cells Form a Network on a Hyaluronic Acid/Gelatin Composite Hydrogel Moderately Crosslinked and Degraded by Hydrogen Peroxide

The study of the capillary-like network formation of human umbilical vein endothelial cells (HUVECs) in vitro is important for understanding the factors that promote or inhibit angiogenesis. Here, we report the behavior of HUVECs on the composite hydrogels containing hyaluronic acid (HA) and gelatin with different degrees of degradation, inducing the different physicochemical properties of the hydrogels. The hydrogels were obtained through horseradish peroxidase (HRP)-catalyzed hydrogelation consuming hydrogen peroxide (H₂O₂, 16 ppm) supplied from the air, and the degradation degree was tuned by altering the exposure time to the air. The HUVECs on the composite hydrogel with intermediate stiffness (1.2 kPa) obtained through 120 min of the exposure were more elongated than those on the soft (0.4 kPa) and the stiff (2.4 kPa) composite hydrogels obtained through 15 min and 60 min of the exposure, respectively. In addition, HUVECs formed a capillary-like network only on the stiff composite hydrogel although those on the hydrogels with comparable stiffness but containing gelatin alone or alginate instead of HA did not form the network. These results show that the HA/gelatin composite hydrogels obtained through the H₂O₂-mediated crosslinking and degradation could be a tool for studies using HUVECs to understand the promotion and inhibition of angiogenesis.

Strategies for Regenerative Vascular Tissue Engineering

Vascularization remains one of the key challenges in creating functional tissue-engineered constructs for therapeutic applications. This review aims to provide a developmental lens on the necessity of blood vessels in defining tissue function while exploring stem cells as a suitable source for vascular tissue engineering applications. The intersections of stem cell biology, material science, and engineering are explored as potential solutions for directing vascular assembly.

Biomimetic Hydrogels to Promote Wound Healing

Wound healing is a common physiological process which consists of a sequence of molecular and cellular events that occur following the onset of a tissue lesion in order to reconstitute barrier between body and external environment. The inherent properties of hydrogels allow the damaged tissue to heal by supporting a hydrated environment which has long been explored in wound management to aid in autolytic debridement. However, chronic non-healing wounds require added therapeutic features that can be achieved by incorporation of biomolecules and supporting cells to promote faster and better healing outcomes. In recent decades, numerous hydrogels have been developed and modified to match the time scale for distinct stages of wound healing. This review will discuss the effects of various types of hydrogels on wound pathophysiology, as well as the ideal characteristics of hydrogels for wound healing, crosslinking mechanism, fabrication techniques and design considerations of hydrogel engineering. Finally, several challenges related to adopting hydrogels to promote wound healing and future perspectives are discussed.

Matrix stiffness primes lymphatic tube formation directed by vascular endothelial growth factor-C

Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes. The restoration of dysfunctional lymphatic vessels has been hypothesized as an innovative method to rescue healthy phenotypes in diseased states including neurological conditions, metabolic syndromes, and cardiovascular disease. Compared to the vascular system, little is known about the molecular regulation that controls lymphatic tube morphogenesis. Using synthetic hyaluronic acid (HA) hydrogels as a chemically and mechanically tunable system to preserve lymphatic endothelial cell (LECs) phenotypes, we demonstrate that low matrix elasticity primes lymphatic cord-like structure (CLS) formation directed by a high concentration of vascular endothelial growth factor-C (VEGF-C). Decreasing the substrate stiffness results in the upregulation of key lymphatic markers, including PROX-1, lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1), and VEGFR-3. Consequently, higher levels of VEGFR-3 enable stimulation of LECs with VEGF-C which is required to both activate matrix metalloproteinases (MMPs) and facilitate LEC migration. Both of these steps are critical in establishing CLS formation in vitro. With decreases in substrate elasticity, we observe increased MMP expression and increased cellular elongation, as well as formation of intracellular vacuoles, which can further merge into coalescent vacuoles. RNAi studies demonstrate that MMP-14 is required to enable CLS formation and that LECs sense matrix stiffness through YAP/TAZ mechanosensors leading to the activation of their downstream target genes. Collectively, we show that by tuning both the matrix stiffness and VEGF-C concentration, the signaling pathways of CLS formation can be regulated in a synthetic matrix, resulting in lymphatic networks which will be useful for the study of lymphatic biology and future approaches in tissue regeneration.

Biomaterials for Bioprinting Microvasculature

Microvasculature functions at the tissue and cell level, regulating local mass exchange of oxygen and nutrient-rich blood. While there has been considerable success in the biofabrication of large- and small-vessel replacements, functional microvasculature has been particularly challenging to engineer due to its size and complexity. Recently, three-dimensional bioprinting has expanded the possibilities of fabricating sophisticated microvascular systems by enabling precise spatiotemporal placement of cells and biomaterials based on computer-aided design. However, there are still significant challenges facing the development of printable biomaterials that promote robust formation and controlled 3D organization of microvascular networks. This review provides a thorough examination and critical evaluation of contemporary biomaterials and their specific roles in bioprinting microvasculature. We first provide an overview of bioprinting methods and techniques that enable the fabrication of microvessels. We then offer an in-depth critical analysis on the use of hydrogel bioinks for printing microvascularized constructs within the framework of current bioprinting modalities. We end with a review of recent applications of bioprinted microvasculature for disease modeling, drug testing, and tissue engineering, and conclude with an outlook on the challenges facing the evolution of biomaterials design for bioprinting microvasculature with physiological complexity.

Engineering PEG-based hydrogels to foster efficient endothelial network formation in free-swelling and confined microenvironments

In vitro tissue engineered models are poised to have significant impact on disease modeling and preclinical drug development. Reliable methods to induce microvascular networks in such microphysiological systems are needed to improve the size and physiological function of these models. By systematically engineering several physical and biomolecular properties of the cellular microenvironment (including crosslinking density, polymer density, adhesion ligand concentration, and degradability), we establish design principles that describe how synthetic matrix properties influence vascular morphogenesis in modular and tunable hydrogels based on commercial 8-arm poly (ethylene glycol) (PEG8a) macromers. We apply these design principles to generate endothelial networks that exhibit consistent morphology throughout depths of hydrogel greater than 1 mm. These PEG8a-based hydrogels have relatively high volumetric swelling ratios (>1.5), which limits their utility in confined environments such as microfluidic devices. To overcome this limitation, we mitigate swelling by incorporating a highly functional PEG-grafted alpha-helical poly (propargyl-l-glutamate) (PPLGgPEG) macromer along with the canonical 8-arm PEG8a macromer in gel formation. This hydrogel platform supports enhanced endothelial morphogenesis in neutral-swelling environments. Finally, we incorporate PEG8a-PPLGgPEG gels into microfluidic devices and demonstrate improved diffusion kinetics and microvascular network formation in situ compared to PEG8a-based gels.

Lymphatic Tissue Engineering and Regeneration

The lymphatic system is a major circulatory system within the body, responsible for the transport of interstitial fluid, waste products, immune cells, and proteins. Compared to other physiological systems, the molecular mechanisms and underlying disease pathology largely remain to be understood which has hindered advancements in therapeutic options for lymphatic disorders. Dysfunction of the lymphatic system is associated with a wide range of disease phenotypes and has also been speculated as a route to rescue healthy phenotypes in areas including cardiovascular disease, metabolic syndrome, and neurological conditions. This review will discuss lymphatic system functions and structure, cell sources for regenerating lymphatic vessels, current approaches for engineering lymphatic vessels, and specific therapeutic areas that would benefit from advances in lymphatic tissue engineering and regeneration.

Gellan Gum Hydrogels with Enzyme-Sensitive Biodegradation and Endothelial Cell Biorecognition Sites

The survival of a biomaterial or tissue engineered construct is mainly hampered by the deficient microcirculation in its core, and limited nutrients and oxygen availability to the implanted or colonizing host cells. Aiming to address these issues, we herein propose bioresponsive gellan gum (GG) hydrogels that are biodegradable by metalloproteinase 1 (MMP-1) and enable endothelial cells adhesion and proliferation. GG is chemically functionalized with divinyl sulfone (DVS) and then biofunctionalized with thiol cell-adhesive peptides (T1 or C16) to confer GG endothelial cell biorecognition cues. Biodegradable hydrogels are then formed by Michael type addition of GGDVS or/and peptide-functionalized GGDVS with a dithiol peptide crosslinker sensitive to MMP-1. The mechanical properties (6 to 5580 Pa), swelling (17 to 11), MMP-1-driven degradation (up to 70%), and molecules diffusion coefficients of hydrogels are tuned by increasing the polymer amount and crosslinking density. Human umbilical cord vein endothelial cells depict a polarized elongated morphology when encapsulated within T1-containing hydrogels, in contrast to the round morphology observed in C16-containing hydrogels. Cell organization is favored as early as 1 d of cell culture within the T1-modified hydrogels with higher concentration of peptide, while cell proliferation is higher in T1-modified hydrogels with higher modulus. In conclusion, biodegradable and bioresponsive GGDVS hydrogels are promising endothelial cell responsive materials that can be used for vascularization strategies.

Endothelial Progenitor Cells for the Vascularization of Engineered Tissues

Self-assembled microvasculature from cocultures of endothelial cells (ECs) and stromal cells has significantly advanced efforts to vascularize engineered tissues by enhancing perfusion rates in vivo and producing investigative platforms for microvascular morphogenesis in vitro. However, to clinically translate prevascularized constructs, the issue of EC source must be resolved. Endothelial progenitor cells (EPCs) can be noninvasively supplied from the recipient through adult peripheral and umbilical cord blood, as well as derived from induced pluripotent stem cells, alleviating antigenicity issues. EPCs can also differentiate into all tissue endothelium, and have demonstrated potential for therapeutic vascularization. Yet, EPCs are not the standard EC choice to vascularize tissue constructs in vitro. Possible reasons include unresolved issues with EPC identity and characterization, as well as uncertainty in the selection of coculture, scaffold, and culture media combinations that promote EPC microvessel formation. This review addresses these issues through a summary of EPC vascular biology and the effects of tissue engineering design parameters upon EPC microvessel formation. Also included are perspectives to integrate EPCs with emerging technologies to produce functional, organotypic vascularized tissues.

Directing three-dimensional multicellular morphogenesis by self-organization of vascular mesenchymal cells in hyaluronic acid hydrogels

BACKGROUND

Physical scaffolds are useful for supporting cells to form three-dimensional (3D) tissue. However, it is non-trivial to develop a scheme that can robustly guide cells to self-organize into a tissue with the desired 3D spatial structures. To achieve this goal, the rational regulation of cellular self-organization in 3D extracellular matrix (ECM) such as hydrogel is needed.

RESULTS

In this study, we integrated the Turing reaction-diffusion mechanism with the self-organization process of cells and produced multicellular 3D structures with the desired configurations in a rational manner. By optimizing the components of the hydrogel and applying exogenous morphogens, a variety of multicellular 3D architectures composed of multipotent vascular mesenchymal cells (VMCs) were formed inside hyaluronic acid (HA) hydrogels. These 3D architectures could mimic the features of trabecular bones and multicellular nodules. Based on the Turing reaction-diffusion instability of morphogens and cells, a theoretical model was proposed to predict the variations observed in 3D multicellular structures in response to exogenous factors. It enabled the feasibility to obtain diverse types of 3D multicellular structures by addition of Noggin and/or BMP2.

CONCLUSIONS

The morphological consistency between the simulation prediction and experimental results probably revealed a Turing-type mechanism underlying the 3D self-organization of VMCs in HA hydrogels. Our study has provided new ways to create a variety of self-organized 3D multicellular architectures for regenerating biomaterial and tissues in a Turing mechanism-based approach.

Impact of immobilizing of low molecular weight hyaluronic acid within gelatin-based hydrogel through enzymatic reaction on behavior of enclosed endothelial cells

The hydrogels having the ability to promote migration and morphogenesis of endothelial cells (ECs) are useful for fabricating vascularized dense tissues in vitro. The present study explores the immobilization of low molecular weight hyaluronic acid (LMWHA) derivative within gelatin-based hydrogel to stimulate migration of ECs. The LMWHA derivative possessing phenolic hydroxyl moieties (LMWHA-Ph) was bound to gelatin-based derivative hydrogel through the horseradish peroxidase-catalyzed reaction. The motility of ECs was analyzed by scratch migration assay and microparticle-based cell migration assay. The incorporated LMWHA-Ph molecules within hydrogel was found to be preserved stably through covalent bonds during incubation. The free and immobilized LMWHA-Ph did not lose an inherent stimulatory effect on human umbilical vein endothelial cells (HUVECs). The immobilized LMWHA-Ph within gelatin-based hydrogel induced the high motility of HUVECs, accompanied by robust cytoskeleton extension, and cell subpopulation expressing CD44 cell receptor. In the presence of immobilized LMWHA-Ph, the migration distance and the number of existing HUVECs were demonstrated to be encouraged in dose-dependent and time-dependent manners. Based on the results obtained in this work, it was concluded that the enzymatic immobilization of LMWHA-Ph within gelatin-based hydrogel represents a promising approach to promote ECs' motility and further exploitation for vascular tissue engineering applications.

Injectable scaffolds: Preparation and application in dental and craniofacial regeneration

Injectable scaffolds are appealing for tissue regeneration because they offer many advantages over pre-formed scaffolds. This article provides a comprehensive review of the injectable scaffolds currently being investigated for dental and craniofacial tissue regeneration. First, we provide an overview of injectable scaffolding materials, including natural, synthetic, and composite biomaterials. Next, we discuss a variety of characteristic parameters and gelation mechanisms of the injectable scaffolds. The advanced injectable scaffolding systems developed in recent years are then illustrated. Furthermore, we summarize the applications of the injectable scaffolds for the regeneration of dental and craniofacial tissues that include pulp, dentin, periodontal ligament, temporomandibular joint, and alveolar bone. Finally, our perspectives on the injectable scaffolds for dental and craniofacial tissue regeneration are offered as signposts for the future advancement of this field.

Neovascularization Induced by the Hyaluronic Acid-Based Spongy-Like Hydrogels Degradation Products

Neovascularization has been a major challenge in many tissue regeneration strategies. Hyaluronic acid (HA) of 3-25 disaccharides is known to be angiogenic due to its interaction with endothelial cell receptors. This effect has been explored with HA-based structures but a transitory response is observed due to HA burst biodegradation. Herein we developed gellan gum (GG)-HA spongy-like hydrogels from semi-interpenetrating network hydrogels with different HA amounts. Enzymatic degradation was more evident in the GG-HA with high HA amount due to their lower mechanical stability, also resulting from the degradation itself, which facilitated the access of the enzyme to the HA in the bulk. GG-HA spongy-like hydrogels hyaluronidase-mediated degradation lead to the release of HA oligosaccharides of different amounts and sizes in a HA content-dependent manner which promoted in vitro proliferation of human umbilical cord vein endothelial cells (HUVECs) but not their migration. Although no effect was observed in human dermal microvascular endothelial cells (hDMECs) in vitro, the implantation of GG-HA spongy-like hydrogels in an ischemic hind limb mice model promoted neovascularization in a material-dependent manner, consistent with the in vitro degradation profile. Overall, GG-HA spongy-like hydrogels with a sustained release of HA oligomers are valuable options to improve tissue vascularization, a critical issue in several applications in the tissue engineering and regenerative medicine field.

Fibronectin Deposition Participates in Extracellular Matrix Assembly and Vascular Morphogenesis

The extracellular matrix (ECM) has been demonstrated to facilitate angiogenesis. In particular, fibronectin has been documented to activate endothelial cells, resulting in their transition from a quiescent state to an active state in which the cells exhibit enhanced migration and proliferation. The goal of this study is to examine the role of polymerized fibronectin during vascular tubulogenesis using a 3 dimensional (3D) cell-derived de-cellularized matrix. A fibronectin-rich 3D de-cellularized ECM was used as a scaffold to study vascular morphogenesis of endothelial cells (ECs). Confocal analyses of several matrix proteins reveal high intra- and extra-cellular deposition of fibronectin in formed vascular structures. Using a small peptide inhibitor of fibronectin polymerization, we demonstrate that inhibition of fibronectin fibrillogenesis in ECs cultured atop de-cellularized ECM resulted in decreased vascular morphogenesis. Further, immunofluorescence and ultrastructural analyses reveal decreased expression of stromal matrix proteins in the absence of polymerized fibronectin with high co-localization of matrix proteins found in association with polymerized fibronectin. Evaluating vascular kinetics, live cell imaging showed that migration, migration velocity, and mean square displacement, are disrupted in structures grown in the absence of polymerized fibronectin. Additionally, vascular organization failed to occur in the absence of a polymerized fibronectin matrix. Consistent with these observations, we tested vascular morphogenesis following the disruption of EC adhesion to polymerized fibronectin, demonstrating that block of integrins α₅β₁ and α_vβ_3, abrogated vascular morphogenesis. Overall, fibronectin deposition in a 3D cell-derived de-cellularized ECM appears to be imperative for matrix assembly and vascular morphogenesis.

Hydrogels for therapeutic cardiovascular angiogenesis

Acute myocardial infarction (MI) caused by ischemia is the most common cause of cardiac dysfunction. While growth factor or cell therapy is promising, the retention of bioactive agents in the highly vascularized myocardium is limited and prevents sustained activation needed for adequate cellular responses. Various types of biomaterials with different physical and chemical properties have been developed to improve the localized delivery of growth factor and/or cells for therapeutic angiogenesis in ischemic tissues. Hydrogels are particularly advantageous as carrier systems because they are structurally similar to the tissue extracellular matrix (ECM), they can be processed under relatively mild conditions and can be delivered in a minimally invasive manner. Moreover, hydrogels can be designed to degrade in a timely fashion that coincides with the angiogenic process. For these reasons, hydrogels have shown great potential as pro-angiogenic matrices. This paper reviews a few of the hydrogel systems currently being applied together with growth factor delivery and/or cell therapy to promote therapeutic angiogenesis in ischemic tissues, with emphasis on myocardial applications.

Vascularization strategies of engineered tissues and their application in cardiac regeneration

The primary function of vascular networks is to transport blood and deliver oxygen and nutrients to tissues, which occurs at the interface of the microvasculature. Therefore, the formation of the vessels at the microcirculatory level, or angiogenesis, is critical for tissue regeneration and repair. Current strategies for vascularization of engineered tissues have incorporated multi-disciplinary approaches including engineered biomaterials, cells and angiogenic factors. Pre-vascularization of scaffolds composed of native matrix, synthetic polymers, or other biological materials can be achieved through the use of single cells in mono or co-culture, in combination or not with angiogenic factors or by the use of isolated vessels. The advance of these methods, together with a growing understanding of the biology behind vascularization, has facilitated the development of vascularization strategies for engineered tissues with therapeutic potential for tissue regeneration and repair. Here, we review the different cell-based strategies utilized to pre-vascularize engineered tissues and in making more complex vascularized cardiac tissues for regenerative medicine applications.

Scarring, stem cells, scaffolds and skin repair

The treatment of full thickness skin loss, which can be extensive in the case of large burns, continues to represent a challenging clinical entity. This is due to an on-going inability to produce a suitable tissue engineered substrate that can satisfactorily replicate the epidermal and dermal in vivo niches to fulfil both aesthetic and functional demands. The current gold standard treatment of autologous skin grafting is inadequate because of poor textural durability, scarring and associated contracture, and because of a paucity of donor sites in larger burns. Tissue engineering has seen exponential growth in recent years with a number of 'off-the-shelf' dermal and epidermal substitutes now available. Each has its own limitations. In this review, we examine normal wound repair in relation to stem/progenitor cells that are intimately involved in this process within the dermal niche. Endothelial precursors, in particular, are examined closely and their phenotype, morphology and enrichment from multiple sources are described in an attempt to provide some clarity regarding the controversy surrounding their classification and role in vasculogenesis. We also review the role of the next generation of cellularized scaffolds and smart biomaterials that attempt to improve the revascularisation of artificial grafts, the rate of wound healing and the final cosmetic and functional outcome.

Umbilical Cord Blood-Derived Mononuclear Cells Exhibit Pericyte-Like Phenotype and Support Network Formation of Endothelial Progenitor Cells In Vitro

Umbilical cord blood represents a promising cell source for pro-angiogenic therapies. The present study examined the potential of mononuclear cells (MNCs) from umbilical cord blood to support endothelial progenitor cell (EPC) microvessel formation. MNCs were isolated from the cord blood of 20 separate donors and selected for further characterization based upon their proliferation potential and morphological resemblance to human vascular pericytes (HVPs). MNCs were screened for their ability to support EPC network formation using an in vitro assay (Matrigel™) as well as a reductionist, coculture system consisting of no additional angiogenic cytokines beyond those present in serum. In less than 15% of the isolations, we identified a population of highly proliferative MNCs that phenotypically resembled HVPs as assessed by expression of PDGFR-β, NG2, α-SMA, and ephrin-B2. Within a Matrigel™ system, MNCs demonstrated pericyte-like function through colocalization to EPC networks and similar effects as HVPs upon total EPC tubule length (p = 0.95) and number of branch points (p = 0.93). In a reductionist coculture system, MNCs served as pro-angiogenic mural cells by supporting EPC network formation to a significantly greater extent than HVP cocultures, by day 14 of coculture, as evidenced through EPC total tubule length (p < 0.0001) and number of branch points (p < 0.0001). Our findings are significant as we demonstrate mural cell progenitors can be isolated from umbilical cord blood and develop culture conditions to support their use in microvascular tissue engineering applications.

Harnessing developmental processes for vascular engineering and regeneration

The formation of vasculature is essential for tissue maintenance and regeneration. During development, the vasculature forms via the dual processes of vasculogenesis and angiogenesis, and is regulated at multiple levels: from transcriptional hierarchies and protein interactions to inputs from the extracellular environment. Understanding how vascular formation is coordinated in vivo can offer valuable insights into engineering approaches for therapeutic vascularization and angiogenesis, whether by creating new vasculature in vitro or by stimulating neovascularization in vivo. In this Review, we will discuss how the process of vascular development can be used to guide approaches to engineering vasculature. Specifically, we will focus on some of the recently reported approaches to stimulate therapeutic angiogenesis by recreating the embryonic vascular microenvironment using biomaterials for vascular engineering and regeneration.

Enzymatic conjugation of a bioactive peptide into an injectable hyaluronic acid-tyramine hydrogel system to promote the formation of functional vasculature

In this study, one-step enzyme-mediated preparation of a multi-functional injectable hyaluronic-acid-based hydrogel system is reported. Hydrogel was formed through the in situ coupling of phenol moieties by horseradish peroxidase (HRP) and hydrogen peroxide (H2O2), and bioactive peptides were simultaneously conjugated into the hydrogel during the gel formation process. The preparation of this multi-functional hydrogel was made possible by synthesizing peptides containing phenols which could couple with the phenol moieties of hyaluronic-acid-tyramine (HA-Tyr) during the HRP-mediated crosslinking reaction. Preliminary studies demonstrated that two phenol moieties per molecule resulted in a consistently high degree of conjugation into the HA-Tyr hydrogel network, unlike the one modified with one phenol moiety per molecule. Therefore, an Arg-Gly-Asp (RGD) peptide bearing two phenol moieties (phenol2-poly(ethylene glycol)-RGD) was designed for conjugation to endow the HA-Tyr hydrogel with adhesion signals and enhance its bioactivities. Human umbilical vein endothelial cells (HUVECs) cultured on or within the RGD-modified hydrogels showed significantly different adhesion behavior, from non-adherence on the HA-Tyr hydrogel to strong adhesion on hydrogels modified with phenol2-poly(ethylene glycol)-RGD. This altered cell adhesion behavior led to improved cell proliferation, migration and formation of capillary-like network in the hydrogel in vitro. More importantly, when HUVECs and human fibroblasts (HFF1) were encapsulated together in the RGD-modified HA-Tyr hydrogel, functional vasculature was observed inside the cell-laden gel after 2weeks in the subcutaneous tissue. Taken together, the in situ conjugation of phenol2-poly(ethylene glycol)-RGD into HA-Tyr hydrogel system, coupled with the ease of incorporating cells, offers a simple and effective means to introduce biological signals for preparation of multi-functional injectable hydrogels for tissue engineering application.

Design of cell-matrix interactions in hyaluronic acid hydrogel scaffolds

The design of hyaluronic acid (HA)-based hydrogel scaffolds to elicit highly controlled and tunable cell response and behavior is a major field of interest in developing tissue engineering and regenerative medicine applications. This review will begin with an overview of the biological context of HA, which is needed to better understand how to engineer cell-matrix interactions in the scaffolds via the incorporation of different types of signals in order to direct and control cell behavior. Specifically, recent methods of incorporating various bioactive, mechanical and spatial signals are reviewed, as well as novel HA modifications and crosslinking schemes with a focus on specificity.

Hyaluronic acid hydrogel stiffness and oxygen tension affect cancer cell fate and endothelial sprouting

Three-dimensional (3D) tissue culture models may recapitulate aspects of the tumorigenic microenvironment in vivo, enabling the study of cancer progression in vitro. Both hypoxia and matrix stiffness are known to regulate tumor growth. Using a modular culture system employing an acrylated hyaluronic acid (AHA) hydrogel, three hydrogel matrices with distinctive degrees of viscoelasticity - soft (78±16 Pa), medium (309± 57 Pa), and stiff (596± 73 Pa) - were generated using the same concentration of adhesion ligands. Oxygen levels within the hydrogel in atmospheric (21 %), hypoxic (5 %), and severely hypoxic (1 %) conditions were assessed with a mathematical model. HT1080 fibrosarcoma cells, encapsulated within the AHA hydrogels in high densities, generated nonuniform oxygen distributions, while lower cell densities resulted in more uniform oxygen distributions in the atmospheric and hypoxic environments. When we examined how varying viscoelasticity in atmospheric and hypoxic environments affects cell cycles and the expression of BNIP3 and BNIP3L (autophagy and apoptosis genes), and GLUT-1 (a glucose transport gene), we observed that HT1080 cells in 3D hydrogel adapted better to hypoxic conditions than those in a Petri dish, with no obvious correlation to matrix viscoelasticity, by recovering rapidly from possible autophagy/apoptotic events and alternating metabolism mechanisms. Further, we examined how HT1080 cells cultured in varying viscoelasticity and oxygen tension conditions affected endothelial sprouting and invasion. We observed that increased matrix stiffness reduced endothelial sprouting and invasion in atmospheric conditions; however, we observed increased endothelial sprouting and invasion under hypoxia at all levels of matrix stiffness with the upregulation of vascular endothelial growth factor (VEGF) and angiopoeitin-1 (ANG-1). Overall, HT1080 cells encapsulated in the AHA hydrogels under hypoxic stress recovered better from apoptosis and demonstrated greater angiogenic induction. Thus, we propose that oxygen tension more profoundly influences cell fate and the angiogenic potential of 3D cultured HT1080 fibrosarcoma cells than does matrix stiffness.

A novel in vitro model for microvasculature reveals regulation of circumferential ECM organization by curvature

In microvascular vessels, endothelial cells are aligned longitudinally whereas several components of the extracellular matrix (ECM) are organized circumferentially. While current three-dimensional (3D) in vitro models for microvasculature have allowed the study of ECM-regulated tubulogenesis, they have limited control over topographical cues presented by the ECM and impart a barrier for the high-resolution and dynamic study of multicellular and extracellular organization. Here we exploit a 3D fibrin microfiber scaffold to develop a novel in vitro model of the microvasculature that recapitulates endothelial alignment and ECM deposition in a setting that also allows the sequential co-culture of mural cells. We show that the microfibers' nanotopography induces longitudinal adhesion and alignment of endothelial colony-forming cells (ECFCs), and that these deposit circumferentially organized ECM. We found that ECM wrapping on the microfibers is independent of ECFCs' actin and microtubule organization, but it is dependent on the curvature of the microfiber. Microfibers with smaller diameters (100–400 µm) guided circumferential ECM deposition, whereas microfibers with larger diameters (450 µm) failed to support wrapping ECM. Finally, we demonstrate that vascular smooth muscle cells attached on ECFC-seeded microfibers, depositing collagen I and elastin. Collectively, we establish a novel in vitro model for the sequential control and study of microvasculature development and reveal the unprecedented role of the endothelium in organized ECM deposition regulated by the microfiber curvature.

Spatial control of cell-mediated degradation to regulate vasculogenesis and angiogenesis in hyaluronan hydrogels

Matrix remodeling is crucial for neovascularization, however its utilization to control this process in synthetic biomaterials has been limited. Here, we utilized hyaluronic acid (HA) hydrogels to spatially control cellular remodeling during vascular network formation. Specifically, we exploited a secondary radical polymerization to alter the ability of cells to degrade the hydrogel and utilized it to create spatial patterning using light initiation. We first demonstrated the ability of the hydrogel to either support or inhibit in vitro vasculogenesis of endothelial colony-forming cells (ECFCs) or angiogenesis from ex ovo chorioallantoic membranes. We showed that vascular tube branching and sprouting, which required matrix metalloproteinases (MMPs)-dependent remodeling, could be achieved in hydrogels formed by primary addition-crosslinking only. Although ECFCs expressed higher levels of MMPs in the hydrogels with the secondary radical crosslinking, the generated kinetic chains disabled cell-mediated remodeling and therefore vascular formation was arrested at the vacuole and lumen stage. We then patterned hydrogels to have regions that either permitted or inhibited cell-mediated degradation during in vitro vasculogenesis or angiogenesis. Our ability to control degradation cues that regulate vascular tube formation is important for the study of vascular biology and the application of synthetic biomaterials in tissue regeneration.

Breast cancer cell-derived matrix supports vascular morphogenesis

The extracellular matrix (ECM), important for maintaining tissue homeostasis, is abnormally expressed in mammary tumors and additionally plays a crucial role in angiogenesis. We hypothesize that breast cancer cells (BCCs) deposit ECM that supports unique patterns of vascular morphogenesis of endothelial cells (ECs). Evaluation of ECM expression revealed that a nontumorigenic cell line (MCF10A), a tumorigenic cell line (MCF7), and a metastatic cell line (MDA-MB-231) express collagens I and IV, fibronectin, and laminin, with tenascin-C limited to MCF10A and MCF7. The amount of ECM deposited by BCCs was found to be higher in MCF10A compared with MCF7 and MDA231, with all ECM differing in their gross structure but similar in mean fiber diameter. Nonetheless, deposition of ECM from BCC lines was overall difficult to detect and insufficient to support capillary-like structure (CLS) formation of ECs. Therefore, a coculture approach was undertaken in which individual BCC lines were cocultured with fibroblasts. Variation in abundance of deposited ECM, deposition of ECM proteins, such as absent collagen I deposition from MDA231-fibroblast cocultures, and fibril organization was found. Deposited ECM from fibroblasts and each coculture supported rapid CLS formation of ECs. Evaluation of capillary properties revealed that CLS grown on ECM deposited from MDA231-fibroblast cocultures possessed significantly larger lumen diameters, occupied the greatest percentage of area, expressed the highest levels of von Willebrand factor, and expressed the greatest amount of E-selectin, which was upregulated independent of exposure to TNF-α. To our knowledge, this is the first study to report tumor cell ECM-mediated differences in vascular capillary features, and thus offers the framework for future investigations interrogating the role of the tumor ECM in supporting vascular morphogenesis.

Development of a biodegradable, temperature-sensitive dextran-based polymer as a cell-detaching substrate

A biodegradable, temperature-sensitive dextran-allyl isocyanate-ethylamine (TSDAIE) as a nonenzymatic cell detachment polymeric substrate for human endothelial progenitor cells (EPCs) is developed and examined. The lower critical solution temperature of TSDAIE is determined; its phase transition occurrs at 18 to 22 °C. For EPC culture, cell culture flasks are coated with TSDAIE and type I collagen. The TSDAIE coating enables EPC detachment when the culture is cooled to 4 °C. The concentration of TSDAIE affects EPC attachment, which is thereby used to optimize the concentration of TSDAIE for coating. At the determined optimal concentration, TSDAIE is found to be compatible for use in EPC culture as revealed by cell attachment, spreading, proliferation, and phenotype. Overall, biodegradable TSDAIE shows promise for applications that culture and expand EPCs including vascular regenerative medicine and tissue engineering.

Regulation of hepatic stem/progenitor phenotype by microenvironment stiffness in hydrogel models of the human liver stem cell niche

Human livers have maturational lineages of cells within liver acini, beginning periportally in stem cell niches, the canals of Hering, and ending in polyploid hepatocytes pericentrally and cholangiocytes in bile ducts. Hepatic stem cells (hHpSCs) in vivo are partnered with mesenchymal precursors to endothelia (angioblasts) and stellate cells, and reside in regulated microenvironments, stem cell niches, containing hyaluronans (HA). The in vivo hHpSC niche is modeled in vitro by growing hHpSC in two-dimensional (2D) cultures on plastic. We investigated effects of 3D microenvironments, mimicking the liver's stem cell niche, on these hHpSCs by embedding them in HA-based hydrogels prepared with Kubota's Medium (KM), a serum-free medium tailored for endodermal stem/progenitors. The KM-HA hydrogels mimicked the niches, matched diffusivity of culture medium, exhibited shear thinning and perfect elasticity under mechanical loading, and had predictable stiffness depending on their chemistry. KM-HA hydrogels, which supported cell attachment, survival and expansion of hHpSC colonies, induced transition of hHpSC colonies towards stable heterogeneous populations of hepatic progenitors depending on KM-HA hydrogel stiffness, as shown by both their gene and protein expression profile. These acquired phenotypes did not show morphological evidence of fibrotic responses. In conclusion, this study shows that the mechanical properties of the microenvironment can regulate differentiation in endodermal stem cell populations.

Controlled activation of morphogenesis to generate a functional human microvasculature in a synthetic matrix

Understanding the role of the extracellular matrix (ECM) in vascular morphogenesis has been possible using natural ECMs as in vitro models to study the underlying molecular mechanisms. However, little is known about vascular morphogenesis in synthetic matrices where properties can be tuned toward both the basic understanding of tubulogenesis in modular environments and as a clinically relevant alternative to natural materials for regenerative medicine. We investigated synthetic, tunable hyaluronic acid (HA) hydrogels and determined both the adhesion and degradation parameters that enable human endothelial colony-forming cells (ECFCs) to form efficient vascular networks. Entrapped ECFCs underwent tubulogenesis dependent on the cellular interactions with the HA hydrogel during each stage of vascular morphogenesis. Vacuole and lumen formed through integrins α(5)β(1) and α(V)β(3), while branching and sprouting were enabled by HA hydrogel degradation. Vascular networks formed within HA hydrogels containing ECFCs anastomosed with the host's circulation and supported blood flow in the hydrogel after transplantation. Collectively, we show that the signaling pathways of vascular morphogenesis of ECFCs can be precisely regulated in a synthetic matrix, resulting in a functional microvasculature useful for the study of 3-dimensional vascular biology and toward a range of vascular disorders and approaches in tissue regeneration.