Heparin-modified alginate microspheres enhance neovessel formation in hiPSC-derived endothelial cells and heterocellular in vitro models by controlled release of vascular endothelial growth factor

A formidable challenge in regenerative medicine is the development of stable microvascular networks to restore adequate blood flow or to sustain graft viability and long-term function in implanted or ischemic tissues. In this work, we develop a biomimetic approach to increase the binding affinity of the extracellular matrix for the class of heparin-binding growth factors to localize and control the release of proangiogenic cues while maintaining their bioactivity. Sulfate and heparin moieties are covalently coupled to alginate, and alginate microspheres are produced and used as local delivery depots for vascular endothelial growth factor (VEGF). Release of VEGF from sulfate-alginate and heparin-alginate bulk hydrogels and microspheres was sustained over 14 days. In vitro evaluation with human induced pluripotent stem cell (hiPSC)-derived endothelial cells and aortic ring assay in a chemically defined hydrogel demonstrates development of primitive three-dimensional vessel-like networks in the presence of VEGF released from the chemically modified alginate microspheres. Furthermore, our results suggest that the sulfate groups available on the chemically modified alginate microspheres promote some new vessel formation even in VEGF-free samples. Based on this evidence, we conclude that sulfate- and heparin-alginate hydrogels are adaptive and bioactive delivery systems for revascularization therapy and translational vascular tissue engineering.

Assessing the Angiogenic Efficacy of Pleiotrophin Released from Injectable Heparin-Alginate Gels

With this work, we design alginate-based hydrogels for therapeutically directing revascularization and repair processes in vivo. We immobilize pleiotrophin (PTN) in injectable hydrogel formulations as the target factor to stimulate proangiogenic responses in endothelial cells. The optimized heparin-alginate/chitosan hydrogels, produced by internal crosslinking with calcium carbonate, show good biocompatibility and injectability and allow controlling the release of immobilized proteins in the subcutaneous tissue over a period of 7 days. In vitro assays, performed with translational human induced pluripotent stem cell-derived endothelial cells, and the in vivo Matrigel plug assay are conducted to demonstrate the angiogenic effects of PTN on endothelial cells. Our results indicate that PTN stimulates endothelial cell morphogenesis in vitro and the migration of endothelial cells and macrophages as soon as 4 days after injections of the developed hydrogels, promoting the formation of structures similar to the healthy granulation tissue, which is an indicator of healing in ischemic wounds. These studies provide the rationale for further investigating this novel therapeutic for pursuing increased vascular density for efficient regeneration of ischemic tissues, by leveraging the host endothelial cell population to initiate angiogenic and reparative processes in vivo. Impact statement Localized, sustained, and controlled delivery of angiogenic factors is crucial for enabling the formation of novel vascular networks in ischemic tissues. This study describes the development of an injectable heparin-alginate/collagen hydrogel for controlling the in vivo release and bioactivity of pleiotrophin (PTN), a heparin-binding factor with significant angiogenic activity. We demonstrate that PTN promotes angiogenesis in an in vitro model of hypoxia and in preclinical subcutaneous models. These results advance our understanding of PTN function in guiding therapeutic angiogenesis and are critical to inform the development of novel translational strategies for ischemic tissue repair and regeneration.

Engineered human myocardium with local release of angiogenic proteins improves vascularization and cardiac function in injured rat hearts

Heart regeneration after myocardial infarction requires new cardiomyocytes and a supportive vascular network. Here, we evaluate the efficacy of localized delivery of angiogenic factors from biomaterials within the implanted muscle tissue to guide growth of a more dense, organized, and perfused vascular supply into implanted engineered human cardiac tissue on an ischemia/reperfusion injured rat heart. We use large, aligned 3-dimensional engineered tissue with cardiomyocytes derived from human induced pluripotent stem cells in a collagen matrix that contains dispersed alginate microspheres as local protein depots. Release of angiogenic growth factors VEGF and bFGF in combination with morphogen sonic hedgehog from the microspheres into the local microenvironment occurs from the epicardial implant site. Analysis of the 3D vascular network in the engineered tissue via Microfil® perfusion and microCT imaging at 30 days shows increased volumetric network density with a wider distribution of vessel diameters, proportionally increased branching and length, and reduced tortuosity. Global heart function is increased in the angiogenic factor-loaded cardiac implants versus sham. These findings demonstrate for the first time the efficacy of a combined remuscularization and revascularization therapy for heart regeneration after myocardial infarction.

Engineering Immunomodulatory Biomaterials for Regenerating the Infarcted Myocardium

Coronary artery disease is a severe ischemic condition characterized by the reduction of blood flow in the arteries of the heart that results in the dysfunction and death of cardiac tissue. Despite research over several decades on how to reduce long-term complications and promote angiogenesis in the infarct, the medical field has yet to define effective treatments for inducing revascularization in the ischemic tissue. With this work, we have developed functional biomaterials for the controlled release of immunomodulatory cytokines to direct immune cell fate for controlling wound healing in the ischemic myocardium. The reparative effects of colony-stimulating factor (CSF-1), and anti-inflammatory interleukins 4/6/13 (IL4/6/13) have been evaluated in vitro and in a predictive in vivo model of ischemia (the skin flap model) to optimize a new immunomodulatory biomaterial that we use for treating infarcted rat hearts. Alginate hydrogels have been produced by internal gelation with calcium carbonate (CaCO₃) as carriers for the immunomodulatory cues, and their stability, degradation, rheological properties and release kinetics have been evaluated in vitro. CD14 positive human peripheral blood monocytes treated with the immunomodulatory biomaterials show polarization into pro-healing macrophage phenotypes. Unloaded and CSF-1/IL4 loaded alginate gel formulations have been implanted in skin flap ischemic wounds to test the safety and efficacy of the delivery system in vivo. Faster wound healing is observed with the new therapeutic treatment, compared to the wounds treated with the unloaded controls at day 14. The optimized therapy has been evaluated in a rat model of myocardial infarct (ischemia/reperfusion). Macrophage polarization toward healing phenotypes and global cardiac function measured with echocardiography and immunohistochemistry at 4 and 15 days demonstrate the therapeutic potential of the proposed immunomodulatory treatment in a clinically relevant infarct model.

Abstract 716: 2D and 3D Assessment of Angiogenesis in Cardiac Engineered Tissues Implanted on Infarcted Rat Hearts

A lack of efficient vessel perfusion in cardiac engineered tissues affects cell viability and limits the success of implanted cardiac tissues. To address the need for vascular infiltration, we have developed a collagen-based cardiac construct releasing pro-angiogenic growth factors. The constructs were implanted in a rodent model of myocardial infarction four days after ischemia/reperfusion injury. Angiogenesis assessment was performed 4 weeks after implantation by 2D histological analysis with RECA-1 antibody and with Microfil® (Flow Tech, MA) perfusion followed by 3D microCT analysis (voxel dimensions 10x10x10 μm). Cross-registration of microCT z-stacks with histological images allows segmentation of remote, infarct and implant regions. The heart vasculature was reconstructed in 3D with ImageJ and Imaris software, and Image J volume calculator and skeletonization algorithm have been used to measure vessels volume, length and number of branch points in each region of interest. Results from 2D histological analysis confirmed the development of capillary-like structures in the implants, visible in both cross and longitudinal sections in the histological slides. The density of cross-sectional vessels measured in the cardiac tissues was comparable to the density of vessels present in the infarct region (118 ± 77 and 93 ±74 vessels/mm 2 counted in infarct and implant regions, respectively). Furthermore, a 1.5-fold increase in vessels density was detected in the infarct zone of the treatment group with respect to the sham rats at 4 weeks. 3D reconstruction of rat heart vasculature showed evidence of the formation of new, perfused vessels in cardiac implants and presumably host-to graft connections. However, perfused vessels volume was not very different in the implants compared to the infarct regions. To the best of our knowledge, this is the first study where a 3D reconstruction of the perfused vasculature of a repaired rat heart has been conducted with a resolution of 10 μm, and that evidence of host-to-graft connections and vascular integration were shown 4 weeks after implantation. The automated quantification proposed is this study represents a non-biased method that can be further adopted by other groups to assess angiogenesis in cardiac implants.

Custom Engineered Tissue Culture Molds from Laser-etched Masters

As the field of tissue engineering has continued to mature, there has been increased interest in a wide range of tissue parameters, including tissue shape. Manipulating tissue shape on the micrometer to centimeter scale can direct cell alignment, alter effective mechanical properties, and address limitations related to nutrient diffusion. In addition, the vessel in which a tissue is prepared can impart mechanical constraints on the tissue, resulting in stress fields that can further influence both the cell and matrix structure. Shaped tissues with highly reproducible dimensions also have utility for in vitro assays in which sample dimensions are critical, such as whole tissue mechanical analysis. This manuscript describes an alternative fabrication method utilizing negative master molds prepared from laser etched acrylic: these molds perform well with polydimethylsiloxane (PDMS), permit designs with dimensions on the centimeter scale and feature sizes smaller than 25 µm, and can be rapidly designed and fabricated at a low cost and with minimal expertise. The minimal time and cost requirements allow for laser etched molds to be rapidly iterated upon until an optimal design is determined, and to be easily adapted to suit any assay of interest, including those beyond the field of tissue engineering.

Laser-Etched Designs for Molding Hydrogel-Based Engineered Tissues

Rapid prototyping and fabrication of elastomeric molds for sterile culture of engineered tissues allow for the development of tissue geometries that can be tailored to different in vitro applications and customized as implantable scaffolds for regenerative medicine. Commercially available molds offer minimal capabilities for adaptation to unique conditions or applications versus those for which they are specifically designed. Here we describe a replica molding method for the design and fabrication of poly(dimethylsiloxane) (PDMS) molds from laser-etched acrylic negative masters with ∼0.2 mm resolution. Examples of the variety of mold shapes, sizes, and patterns obtained from laser-etched designs are provided. We use the patterned PDMS molds for producing and culturing engineered cardiac tissues with cardiomyocytes derived from human-induced pluripotent stem cells. We demonstrate that tight control over tissue morphology and anisotropy results in modulation of cell alignment and tissue-level conduction properties, including the appearance and elimination of reentrant arrhythmias, or circular electrical activation patterns. Techniques for handling engineered cardiac tissues during implantation in vivo in a rat model of myocardial infarction have been developed and are presented herein to facilitate development and adoption of surgical techniques for use with hydrogel-based engineered tissues. In summary, the method presented herein for engineered tissue mold generation is straightforward and low cost, enabling rapid design iteration and adaptation to a variety of applications in tissue engineering. Furthermore, the burden of equipment and expertise is low, allowing the technique to be accessible to all.

Immunological and Differentiation Properties of Amniotic Cells Are Retained After Immobilization in Pectin Gel

Mesenchymal stromal cells from the human amniotic membrane (i.e., human amniotic mesenchymal stromal cells [hAMSCs]) of term placenta are increasingly attracting attention for their applications in regenerative medicine. Osteochondral defects represent a major clinical problem with lifelong chronic pain and compromised quality of life. Great promise for osteochondral regeneration is held in hydrogel-based constructs that have a flexible composition and mimic the physiological structure of cartilage. Cell loading within a hydrogel represents an advantage for regenerative purposes, but the encapsulation steps can modify cell properties. As pectin gels have also been explored as cell vehicles on 3D scaffolds, the aim of this study was to explore the possibility to include hAMSCs in pectin gel. Immobilization of hAMSCs into pectin gels could expand their application in cell-based bioengineering strategies. hAMSCs were analyzed for their viability and recovery from the pectin gel and for their ability to differentiate toward the osteogenic lineage and to maintain their immunological characteristics. When treated with a purposely designed pectin/hydroxyapatite gel biocomposite, hAMSCs retained their ability to differentiate toward the osteogenic lineage, did not induce an immune response, and retained their ability to reduce T cell proliferation. Taken together, these results suggest that hAMSCs could be used in combination to pectin gels for the study of novel osteochondral regeneration strategies.

A novel 3-dimensional approach for cardiac regeneration

Ischemic heart diseases, such as coronary artery disease and microvascular disease, are cardiovascular pathologies that cause reduced blood supply to the heart muscle. Acute and chronic ischemia cause cardiomyocytes to die, and these cells are not naturally replaced as part of the wound healing process in the heart. To promote neovascularization in the wound bed and in implanted engineered tissues, we have developed a collagen-alginate microspheres scaffold intended for local release of drugs and growth factors in order to recruit host endothelial cells to the area and provide them with geometrical cues to form new vessels. Optimization of alginate microspheres included modulation of nitrogen pressure, alginate and CaCl2 concentrations, nozzle size, and velocity of extrusion to achieve monodisperse populations of 100 μm diameter microspheres with protein release over 3 days. In vitro incorporation of fibroblasts in the bulk collagen demonstrated cellular compatibility with embedded alginate microspheres. An in vitro vessel formation assay, performed with human umbilical vein endothelial cells (HUVECs) immobilized in the collagen phase of the collagen-alginate microspheres scaffolds, showed that HUVECs formed networks following the 3-dimensional pattern of the microspheres even in the absence of growth factor. Implantation of acellular collagen-alginate microspheres scaffolds onto healthy rat hearts confirmed the invasion of host cells at one week. Together, these results suggest that the collagen-alginate microspheres scaffold is a viable, tunable therapeutic approach for directing neovascularization in engineered tissues and in the heart after ischemic events.

Cross-linked poly(acrylic acids) microgels and agarose as semi-interpenetrating networks for resveratrol release

Carbomers, cross-linked poly(acrylic acid) microgels, have been widely used in pharmaceutical formulations as swollen hydrogels. Agarose, whose thermoreversibility may be exploited for drug loading, forms a gel with a mechanism involving coil-helix transition at about 36 °C. In this work carbomer microgels were combined with agarose networks in a semi-interpenetrating polymer network structure, aiming at obtaining suitable delivery systems for the loading and release of molecules with poor bioavailability but high therapeutic interest, like resveratrol. The rheological properties of the formulations and their in vitro cytocompatibility were studied and optimized acting on the neutralizing agent (triethylamine (N,N-diethylethanamine), triethanolamine (tris(2-hydroxyethyl)amine) and sodium hydroxide) and amount of OH donors (1,2-propanediol and glycerol). As a preparation method, autoclaving was introduced to simultaneously obtain heating and sterilising. Among the different neutralizing agents, NaOH was chosen to avoid the use of amines, considering the final application. Without the addition of alcohols as typical OH donors to induce Carbomer gelification, gels with appropriate rheological properties and stability were produced. For this formulation, the release of resveratrol after 7 days was about 80 % of the loaded mass, suggesting it is an interesting approach to be exploited for the development of innovative resveratrol delivery systems.

External and internal gelation of pectin solutions: microscopic dynamics versus macroscopic rheology

Pectin is a natural biopolymer that forms, in the presence of divalent cations, ionic-bound gels typifying a large class of biological gels stabilized by non-covalent cross-links. We investigate and compare the kinetics of formation and aging of pectin gels obtained either through external gelation via perfusion of free Ca(2+) ions, or by internal gelation due to the supply of the same ions from the dissolution of CaCO3 nanoparticles. The microscopic dynamics obtained with photon correlation imaging, a novel optical technique that allows obtaining the microscopic dynamics of the sample while retaining the spatial resolution of imaging techniques, is contrasted with macroscopic rheological measurements at constant strain. Pectin gelation is found to display peculiar two-stage kinetics, highlighted by non-monotonic growth in time of both microscopic correlations and gel mechanical strength. These results are compared to those found for alginate, another biopolymer extensively used in food formulation.

Structural properties of polysaccharide-based microcapsules for soft tissue regeneration

Autologous and eterologous cell encapsulation has been extensively studied for clinical application in functional organs substitution, recombinant cell transplantation in gene therapy or in muscle and cartilage regeneration to treat degenerative pathologies. In this work, calcium alginate, calcium alginate/chitosan, calcium alginate/gelatin and pectin/chitosan microcapsules were prepared to be used as innovative injectable scaffolds for soft issue regeneration by a simple extrusion method from aqueous solutions. Prepared microcapsules had spherical morphology, whereas their size was deeply influenced by the polymeric composition. When incubated in a physiological-like environment up to 30 days, they underwent an initial swelling, followed by weight loss at different rates, depending on the microcapsules formulation. The encapsulation of mouse myoblast cells (C2C12 cell line) was obtained in calcium alginate, calcium alginate/chitosan, calcium alginate/gelatin microcapsules. Cells were alive throughout the encapsulation procedure, and were recovered by a mechanical rupture of the microcapsules. After 7 days, fractured microcapsules led cells to migrate gradually out.