Tropoelastin modulates systemic and local tissue responses to enhance wound healing.

Wound healing is facilitated by biomaterials-based grafts and substantially impacted by orchestrated inflammatory responses that are essential to the normal repair process. Tropoelastin (TE) based materials are known to shorten the period for wound repair but the mechanism of anti-inflammatory performance is not known. To explore this, we compared the performance of the gold standard Integra Dermal Regeneration Template (Integra), polyglycerol sebacate (PGS), and TE blended with PGS, in a murine full-thickness cutaneous wound healing study. Systemically, blending with TE favorably increased the F4/80+ macrophage population by day 7 in the spleen and contemporaneously induced elevated plasma levels of anti-inflammatory IL-10. In contrast, the PGS graft without TE prompted prolonged inflammation, as evidenced by splenomegaly and greater splenic granulocyte and monocyte fractions at day 14. Locally, the inclusion of TE in the graft led to increased anti-inflammatory M2 macrophages and CD4+T cells at the wound site, and a rise in Foxp3+ regulatory T cells in the wound bed by day 7. We conclude that the TE-incorporated skin graft delivers a pro-healing environment by modulating systemic and local tissue responses. STATEMENT OF SIGNIFICANCE: Tropoelastin (TE) has shown significant benefits in promoting the repair and regeneration of damaged human tissues. In this study, we show that TE promotes an anti-inflammatory environment that facilitates cutaneous wound healing. In a mouse model, we find that inserting a TE-containing material into a full-thickness wound results in defined, pro-healing local and systemic tissue responses. These findings advance our understanding of TE's restorative value in tissue engineering and regenerative medicine, and pave the way for clinical applications.

Polyglycerol sebacate-based elastomeric materials for arterial regeneration.

Synthetic vascular grafts are commonly used in patients with severe occlusive arterial disease when autologous grafts are not an option. Commercially available synthetic grafts are confronted with challenging outcomes: they have a lower patency rate than autologous grafts and are currently unable to promote arterial regeneration. Polyglycerol sebacate (PGS), a non-toxic polymer with a tunable degradation profile, has shown promising results as a small-diameter vascular graft component that can support the formation of neoarteries. In this review, we first present an overview of the synthesis and modification of PGS followed by an examination of its mechanical properties. We then report on the performance, degradation, regeneration, and remodeling of PGS-based small-diameter vascular grafts, with a focus on efforts to reduce thrombosis, prevent dilation, and promote cellular residency and extracellular matrix regeneration that resembles the native artery in spatial distribution and organization. We also highlight recent advances in the incorporation of novel in situ cell sources for arterial regeneration and their potential application in PGS-based vascular grafts. Finally, we compare vascular grafts fabricated using PGS-based materials with other elastomeric alternatives.

Development of tropoelastin-functionalized anisotropic PCL scaffolds for musculoskeletal tissue engineering.

The highly organized extracellular matrix (ECM) of musculoskeletal tissues, encompassing tendons, ligaments and muscles, is structurally anisotropic, hierarchical and multi-compartmental. These features collectively contribute to their unique function. Previous studies have investigated the effect of tissue-engineered scaffold anisotropy on cell morphology and organization for musculoskeletal tissue repair and regeneration, but the hierarchical arrangement of ECM and compartmentalization are not typically replicated. Here, we present a method for multi-compartmental scaffold design that allows for physical mimicry of the spatial architecture of musculoskeletal tissue in regenerative medicine. This design is based on an ECM-inspired macromolecule scaffold. Polycaprolactone (PCL) scaffolds were fabricated with aligned fibers by electrospinning and mechanical stretching, and then surface-functionalized with the cell-supporting ECM protein molecule, tropoelastin (TE). TE was attached using two alternative methods that allowed for either physisorption or covalent attachment, where the latter was achieved by plasma ion immersion implantation (PIII). Aligned fibers stimulated cell elongation and improved cell alignment, in contrast to randomly oriented fibers. TE coatings bound by physisorption or covalently following 200 s PIII treatment promoted fibroblast proliferation. This represents the first cytocompatibility assessment of novel PIII-treated TE-coated PCL scaffolds. To demonstrate their versatility, these 2D anisotropic PCL scaffolds were assembled into 3D hierarchical constructs with an internally compartmentalized structure to mimic the structure of musculoskeletal tissue.

Tropoelastin Improves Post-Infarct Cardiac Function.

BACKGROUND: Myocardial infarction (MI) is among the leading causes of death worldwide. Following MI, necrotic cardiomyocytes are replaced by a stiff collagen-rich scar. Compared to collagen, the extracellular matrix protein elastin has high elasticity and may have more favorable properties within the cardiac scar. We sought to improve post-MI healing by introducing tropoelastin, the soluble subunit of elastin, to alter scar mechanics early after MI. METHODS AND RESULTS: We developed an ultrasound-guided direct intramyocardial injection method to administer tropoelastin directly into the left ventricular anterior wall of rats subjected to induced MI. Experimental groups included shams and infarcted rats injected with either PBS vehicle control or tropoelastin. Compared to vehicle treated controls, echocardiography assessments showed tropoelastin significantly improved left ventricular ejection fraction (64.7±4.4% versus 46.0±3.1% control) and reduced left ventricular dyssynchrony (11.4±3.5 ms versus 31.1±5.8 ms control) 28 days post-MI. Additionally, tropoelastin reduced post-MI scar size (8.9±1.5% versus 20.9±2.7% control) and increased scar elastin (22±5.8% versus 6.2±1.5% control) as determined by histological assessments. RNA sequencing (RNAseq) analyses of rat infarcts showed that tropoelastin injection increased genes associated with elastic fiber formation 7 days post-MI and reduced genes associated with immune response 11 days post-MI. To show translational relevance, we performed immunohistochemical analyses on human ischemic heart disease cardiac samples and showed an increase in tropoelastin within fibrotic areas. Using RNA-seq we also demonstrated the tropoelastin gene ELN is upregulated in human ischemic heart disease and during human cardiac fibroblast-myofibroblast differentiation. Furthermore, we showed by immunocytochemistry that human cardiac fibroblast synthesize increased elastin in direct response to tropoelastin treatment. CONCLUSIONS: We demonstrate for the first time that purified human tropoelastin can significantly repair the infarcted heart in a rodent model of MI and that human cardiac fibroblast synthesize elastin. Since human cardiac fibroblasts are primarily responsible for post-MI scar synthesis, our findings suggest exciting future clinical translation options designed to therapeutically manipulate this synthesis.

Rapid Regeneration of a Neoartery with Elastic Lamellae.

Native arteries contain a distinctive intima-media composed of organized elastin and an adventitia containing mature collagen fibrils. In contrast, implanted biodegradable small-diameter vascular grafts do not present spatially regenerated, organized elastin. The elastin-containing structures within the intima-media region encompass the elastic lamellae (EL) and internal elastic lamina (IEL) and are crucial for normal arterial function. Here, the development of a novel electrospun small-diameter vascular graft that facilitates de novo formation of a structurally appropriate elastin-containing intima-media region following implantation is described. The graft comprises a non-porous microstructure characterized by tropoelastin fibers that are embedded in a PGS matrix. After implantation in mouse abdominal aorta, the graft develops distinct cell and extracellular matrix profiles that approximate the native adventitia and intima-media by 8 weeks. Within the newly formed intima-media region there are circumferentially aligned smooth muscle cell layers that alternate with multiple EL similar to that found in the arterial wall. By 8 months, the developed adventitia region contains mature collagen fibrils and the neoartery presents a distinct IEL with thickness comparable to that in mouse abdominal aorta. It is proposed that this new class of material can generate the critically required, organized elastin needed for arterial regeneration.

Extracellular Vesicles: Interplay with the Extracellular Matrix and Modulated Cell Responses.

The discovery that cells secrete extracellular vesicles (EVs), which carry a variety of regulatory proteins, nucleic acids, and lipids, has shed light on the sophisticated manner by which cells can communicate and accordingly function. The bioactivity of EVs is not only defined by their internal content, but also through their surface associated molecules, and the linked downstream signaling effects they elicit in target cells. The extracellular matrix (ECM) contains signaling and structural molecules that are central to tissue maintenance and repair. Recently, a subset of EVs residing within the extracellular matrix has been identified. Although some roles have been proposed for matrix-bound vesicles, their role as signaling molecules within the ECM is yet to be explored. Given the close association of EVs and the ECM, it is not surprising that EVs partly mediate repair and regeneration by modulating matrix deposition and degradation through their cellular targets. This review addresses unique EV features that allow them to interact with and navigate through the ECM, describes how their release and content is influenced by the ECM, and emphasizes the emerging role of stem-cell derived EVs in tissue repair and regeneration through their matrix-modulating properties.

Tropoelastin Promotes the Formation of Dense, Interconnected Endothelial Networks.

Tropoelastin, the soluble precursor of elastin, has been used for regenerative and wound healing purposes and noted for its ability to accelerate wound repair by enhancing vascularization at the site of implantation. However, it is not clear whether these effects are directly due to the interaction of tropoelastin with endothelial cells or communicated to endothelial cells following interactions between tropoelastin and neighboring cells, such as mesenchymal stem cells (MSCs). We adapted an endothelial tube formation assay to model in vivo vascularization with the goal of exploring the stimulatory mechanism of tropoelastin. In the presence of tropoelastin, endothelial cells formed less tubes, with reduced spreading into capillary-like networks. In contrast, conditioned media from MSCs that had been cultured on tropoelastin enhanced the formation of more dense, complex, and interconnected endothelial tube networks. This pro-angiogenic effect of tropoelastin is mediated indirectly through the action of tropoelastin on co-cultured cells. We conclude that tropoelastin inhibits endothelial tube formation, and that this effect is reversed by pro-angiogenic crosstalk from tropoelastin-treated MSCs. Furthermore, we find that the known in vivo pro-angiogenic effects of tropoelastin can be modeled in vitro, highlighting the value of tropoelastin as an indirect mediator of angiogenesis.

A step closer to elastogenesis on demand; Inducing mature elastic fibre deposition in a natural biomaterial scaffold.

Elastic fibres play a key role in bodily functions where fatigue resistance and elastic recovery are necessary while regulating phenotype, proliferation and migration in cells. While in vivo elastic fibres are created at a late foetal stage, a major obstacle in the development of engineered tissue is that human vascular smooth muscle cells (hVSMCs), one of the principal elastogenic cells, are unable to spontaneously promote elastogenesis in vitro. Therefore, the overall aim of this study was to activate elastogenesis in vitro by hVSMCs seeded in fibrin, collagen, glycosaminoglycan (FCG) scaffolds, following the addition of recombinant human tropoelastin. This combination of scaffold, tropoelastin and cells induced the deposition of elastin and formation of lamellar maturing elastic fibres, similar to those found in skin, blood vessels and heart valves. Furthermore, higher numbers of maturing branched elastic fibres were synthesised when a higher cell density was used and by drop-loading tropoelastin onto cell-seeded FCG scaffolds prior to adding growth medium. The addition of tropoelastin showed no effect on cell proliferation or mechanical properties of the scaffold which remained dimensionally stable throughout. With these results, we have established a natural biomaterial scaffold that can undergo controlled elastogenesis on demand, suitable for tissue engineering applications.

Fabricating Organized Elastin in Vascular Grafts.

Surgically bypassing or replacing a severely damaged artery using a biodegradable synthetic vascular graft is a promising treatment that allows for the remodeling and regeneration of the graft to form a neoartery. Elastin-based structures, such as elastic fibers, elastic lamellae, and laminae, are key functional components in the arterial extracellular matrix. In this review, we identify the lack of elastin in vascular grafts as a key factor that prevents their long-term success. We further summarize advances in vascular tissue engineering that are focused on either de novo production of organized elastin or incorporation of elastin-based biomaterials within vascular grafts to mitigate failure and enhance enduring in vivo performance.

Novel Recombinant Tropoelastin Implants Restore Skin Extracellular Matrix.

BACKGROUND: Elastin is an essential component of the dermis, providing skin with elasticity and integrity. Elastin and other dermal components are gradually lost through aging, sun damage, and following injury, highlighting a need to replace these components to repair the skin. Tropoelastin (TE) in monomeric form was previously shown to be utilized as a substrate by dermal fibroblasts during the production of elastin fibers in vitro. OBJECTIVE: To analyze coaccumulation of elastin and collagen and gene expression of biomarkers associated with elastin production, examine the ex vivo effects of recombinant human TE (rhTE) and hyaluronic acid (HA) on epidermal and dermal structures, and evaluate the in vivo response following intradermal injections of rhTE and HA. METHODS: Human dermal fibroblasts and 3-D skin patch models were cultured for in vitro analysis. Ex vivo analysis was performed using skin explants. In vivo studies were done in 6-week-old male CD Hairless rats. Different formulations of rhTE, soluble or crosslinked using derivatized HA (dHA), were tested and analyzed. RESULTS: rhTE in monomeric form was utilized as a substrate by dermal fibroblasts during the production of branched elastin and fibrous collagen networks in vitro. Formulations of rhTE crosslinked with dHA demonstrated increased expression of hyaluronic acid synthase 1 and ex vivo results revealed increased moisture content and glycosaminoglycan (GAG) deposition versus dermal filler control. Intradermal rhTE‒dHA injection produced colocalized human‒rat elastin fibers in vivo. CONCLUSIONS: These results suggest that the novel rhTE‒dHA matrix is an attractive material to support skin tissue repair.J Drugs Dermatol. 2020;19(12): doi:10.36849/JDD.2020.5375.

Tubular Fibrous Scaffolds Functionalized with Tropoelastin as a Small-Diameter Vascular Graft.

Cardiovascular disorders are a healthcare problem in today's society. The clinically available synthetic vascular grafts are thrombogenic and could induce intimal hyperplasia. Rapid endothelialization and matched mechanical properties are two major requirements to be considered when designing functional vascular grafts. Herein, an electrospun tubular fibrous (eTF) scaffold was biofunctionalized with tropoelastin at the luminal surface. The luminal surface functionalization was confirmed by an increase of the zeta potential and by the insertion of NH2 groups. Tropoelastin was immobilized via its -NH2 or -COOH groups at the activated or aminolysed eTF scaffolds, respectively, to study the effect of exposed functional groups on human endothelial cells (ECs) behavior. Tensile properties demonstrated that functionalized eTF scaffolds presented strength and stiffness within the range of those of native blood vessels. Tropoelastin immobilized on activated eTF scaffolds promoted higher metabolic activity and proliferation of ECs, whereas when immobilized on aminolysed eTF scaffolds, significantly higher protein synthesis was observed. These biofunctional eTF scaffolds are a promising small-diameter vascular graft that promote rapid endothelialization and have compatible mechanical properties.

Elastin Biomaterials in Dermal Repair.

Wound healing has historically relied on endogenous processes, but engineered materials are increasingly being used to assist tissue repair. Elastin is an essential functional component of the dermal extracellular matrix and is an important part of skin wound repair that encompasses an elastic dermis. Advances in modern technology have better elucidated the specific signaling factors and cells that contribute to the physiological process and have led to new developments in wound care technology. We review elastin-based materials that are used to encourage wound repair. Elastin-related biomaterials, particularly those based on tropoelastin, are particularly promising because tropoelastin is assembled to make elastin. We present insights into the roles of elastin-related biomaterials and their associated in vitro and in vivo benefits on wound healing.

Fabrication Techniques for Vascular and Vascularized Tissue Engineering.

Impaired or damaged blood vessels can occur at all levels in the hierarchy of vascular systems from large vasculatures such as arteries and veins to meso- and microvasculatures such as arterioles, venules, and capillary networks. Vascular tissue engineering has become a promising approach for fabricating small-diameter vascular grafts for occlusive arteries. Vascularized tissue engineering aims to fabricate meso- and microvasculatures for the prevascularization of engineered tissues and organs. The ideal small-diameter vascular graft is biocompatible, bridgeable, and mechanically robust to maintain patency while promoting tissue remodeling. The desirable fabricated meso- and microvasculatures should rapidly integrate with the host blood vessels and allow nutrient and waste exchange throughout the construct after implantation. A number of techniques used, including engineering-based and cell-based approaches, to fabricate these synthetic vasculatures are herein explored, as well as the techniques developed to fabricate hierarchical structures that comprise multiple levels of vasculature.

Elastin architecture.

Elastic fibers are an essential component of the extracellular matrix where they provide structural integrity and elastic recoil in a number of important tissues. A major constituent of these fibers is elastin, an insoluble metabolically stable polymer formed via extensive crosslinking of the monomeric precursor tropoelastin. Research over the past few decades has shown that tropoelastin possesses unique structural features that differ from both intrinsically disordered and globular proteins. This review details the advances in our understanding of tropoelastin's structural properties and illustrates how these dictate its biological function.

Tropoelastin-Coated Tendon Biomimetic Scaffolds Promote Stem Cell Tenogenic Commitment and Deposition of Elastin-Rich Matrix.

Tendon tissue engineering strategies that recreate the biophysical and biochemical native microenvironment have a greater potential to achieve regeneration. Here, we developed tendon biomimetic scaffolds using mechanically competent yarns of poly-ε-caprolactone, chitosan, and cellulose nanocrystals to recreate the inherent tendon hierarchy from a nano-to-macro scale. These were then coated with tropoelastin (TROPO) through polydopamine (PDA) linking, to mimic the native extracellular matrix (ECM) composition and elasticity. Both PDA and TROPO coatings decreased surface stiffness without masking the underlying substrate. We found that human adipose-derived stem cells (hASCs) seeded onto these TROPO biomimetic scaffolds more rapidly acquired their spindle-shape morphology and high aspect ratio characteristic of tenocytes. Immunocytochemistry shows that the PDA and TROPO-coated surfaces boosted differentiation of hASCs toward the tenogenic lineage, with sustained expression of the tendon-related markers scleraxis and tenomodulin up to 21 days of culture. Furthermore, these surfaces enabled the deposition of a tendon-like ECM, supported by the expression of collagens type I and III, tenascin, and decorin. Gene expression analysis revealed a downregulation of osteogenic and fibrosis markers in the presence of TROPO when compared with the control groups, suggesting proper ECM deposition. Remarkably, differentiated cells exposed to TROPO acquired an elastogenic profile due to the evident elastin synthesis and deposition, contributing to the formation of a more mimetic matrix in comparison with the PDA-coated and uncoated conditions. In summary, our biomimetic substrates combining biophysical and biological cues modulate stem cell behavior potentiating their long-term tenogenic commitment and the production of an elastin-rich ECM.

Fabricated tropoelastin-silk yarns and woven textiles for diverse tissue engineering applications.

Electrospun yarns offer substantial opportunities for the fabrication of elastic scaffolds for flexible tissue engineering applications. Currently available yarns are predominantly made of synthetic elastic materials. Thus scaffolds made from these yarns typically lack cell signaling cues. This can result in poor integration or even rejection on implantation, which drive demands for a new generation of yarns made from natural biologically compatible materials. Here, we present a new type of cell-attractive, highly twisted protein-based yarns made from blended tropoelastin and silk fibroin. These yarns combine physical and biological benefits by being rendered elastic and bioactive through the incorporation of tropoelastin and strengthened through the presence of silk fibroin. Remarkably, the process delivered multi-meter long yarns of tropoelastin-silk mixture that were conducive to fabrication of meshes on hand-made frames. The resulting hydrated meshes are elastic and cell interactive. Furthermore, subcutaneous implantation of the meshes in mice demonstrates their tolerance and persistence over 8 weeks. This combination of mechanical properties, biocompatibility and processability into diverse shapes and patterns underscores the value of these materials and platform technology for tissue engineering applications. STATEMENT OF SIGNIFICANCE: Synthetic yarns are used to fabricate textile materials for various applications such as surgical meshes for hernia repair and pelvic organ prolapse. However, synthetic materials lack the attractive biological and physical cues characteristic of extracellular matrix and there is a demand for materials that can minimize postoperative complications. To address this need, we made yarns from a combination of recombinant human tropoelastin and silk fibroin using a modified electrospinning approach that blended these proteins into functional yarns. Prior to this study, no protein-based yarns using tropoelastin were available for the fabrication of functional textile materials. Multimeter-long, uniform and highly twisted yarns based on these proteins were elastic and cell interactive and demonstrated processing to yield textile fabrics. By using these yarns to weave fabrics, we demonstrate that an elastic human matrix protein blend can deliver a versatile platform technology to make textiles that can be explored for efficacy in tissue repair.

Freestanding hierarchical vascular structures engineered from ice.

The ability to engineer a synthetic hierarchical vascular network is one of the most demanding and unaddressed challenges in tissue engineering and regenerative medicine. A material that is both structurally rigid and biocompatible is needed to fabricate freestanding hierarchical vascular structures with defined dimensions and geometry. This is particularly important for creating commercially viable and easily suturable synthetic vasculature. Here, we present the surprising discovery that ice is a versatile material which satisfies these requirements. We demonstrate utilizing ice as a sacrificial scaffold, onto which a diverse range of materials were coated, including tropoelastin, polycaprolactone (PCL), silk, and polydimethylsiloxane (PDMS). We present ice facilitating the fabrication of freestanding hierarchical vascular structures with variable lumen dimensions, and validate the vascular application of these vessels by demonstrating their mechanical tunability, biocompatibility, and permeability to nutrient diffusion. This adaptable technology delivers engineered synthetic vasculature and has potential wider applications encompassing tissue engineering bespoke structures.