Use of proteolytic sequences with different cleavage kinetics as a way to generate hydrogels with preprogrammed cell-infiltration patterns imparted over their given 3D spatial structure

Contribution of the ELRs to the development of advanced in vitro models

Developing in vitro models that accurately mimic the microenvironment of biological structures or processes holds substantial promise for gaining insights into specific biological functions. In the field of tissue engineering and regenerative medicine, in vitro models able to capture the precise structural, topographical, and functional complexity of living tissues, prove to be valuable tools for comprehending disease mechanisms, assessing drug responses, and serving as alternatives or complements to animal testing. The choice of the right biomaterial and fabrication technique for the development of these in vitro models plays an important role in their functionality. In this sense, elastin-like recombinamers (ELRs) have emerged as an important tool for the fabrication of in vitro models overcoming the challenges encountered in natural and synthetic materials due to their intrinsic properties, such as phase transition behavior, tunable biological properties, viscoelasticity, and easy processability. In this review article, we will delve into the use of ELRs for molecular models of intrinsically disordered proteins (IDPs), as well as for the development of in vitro 3D models for regenerative medicine. The easy processability of the ELRs and their rational design has allowed their use for the development of spheroids and organoids, or bioinks for 3D bioprinting. Thus, incorporating ELRs into the toolkit of biomaterials used for the fabrication of in vitro models, represents a transformative step forward in improving the accuracy, efficiency, and functionality of these models, and opening up a wide range of possibilities in combination with advanced biofabrication techniques that remains to be explored.

Protease-Sensitive, VEGF-Mimetic Peptide, and IKVAV Laminin-Derived Peptide Sequences within Elastin-Like Recombinamer Scaffolds Provide Spatiotemporally Synchronized Guidance of Angiogenesis and Neurogenesis

Spatiotemporal control of vascularization and innervation is a desired hallmark in advanced tissue regeneration. For this purpose, we design a 3D model scaffold, based on elastin-like recombinamer (ELR) hydrogels. This contains two interior and well-defined areas, small cylinders, with differentiated bioactivities with respect to the bulk. Both are constructed on a protease sensitive ELR with a fast-proteolyzed domain, but one bears a VEGF-mimetic peptide (QK) and the other a laminin-derived pentapeptide (IKVAV), to promote angiogenesis and neurogenesis, respectively. The outer bulk is based on a slow proteolytic sequence and RGD cell adhesion domains. In vitro studies show the effect of QK and IKVAV peptides on the promotion of endothelial cell and axon spreading, respectively. The subcutaneous implantation of the final 3D scaffold demonstrates the ability to spatiotemporally control angiogenesis and neurogenesis in vivo. Specifically, the inner small cylinder containing the QK peptide promotes fast endothelialization, whereas the one with IKVAV peptide promotes fast neurogenesis. Both, vascularization and innervation take place in advance of the bulk scaffold infiltration. This scaffold shows that it is possible to induce vascularization and innervation in predetermined areas of the scaffold well ahead to the bulk infiltration. That significantly increases the efficiency of the regenerative activity.

Biohybrid elastin-like venous valve with potential for in situ tissue engineering

Chronic venous insufficiency (CVI) is a leading vascular disease whose clinical manifestations include varicose veins, edemas, venous ulcers, and venous hypertension, among others. Therapies targeting this medical issue are scarce, and so far, no single venous valve prosthesis is clinically available. Herein, we have designed a bi-leaflet transcatheter venous valve that consists of (i) elastin-like recombinamers, (ii) a textile mesh reinforcement, and (iii) a bioabsorbable magnesium stent structure. Mechanical characterization of the resulting biohybrid elastin-like venous valves (EVV) showed an anisotropic behavior equivalent to the native bovine saphenous vein valves and mechanical strength suitable for vascular implantation. The EVV also featured minimal hemolysis and platelet adhesion, besides actively supporting endothelialization in vitro, thus setting the basis for its application as an in situ tissue engineering implant. In addition, the hydrodynamic testing in a pulsatile bioreactor demonstrated excellent hemodynamic valve performance, with minimal regurgitation (<10%) and pressure drop (<5 mmHg). No stagnation points were detected and an in vitro simulated transcatheter delivery showed the ability of the venous valve to withstand the implantation procedure. These results present a promising concept of a biohybrid transcatheter venous valve as an off-the-shelf implant, with great potential to provide clinical solutions for CVI treatment.

Nanofiber/hydrogel core-shell scaffolds with three-dimensional multilayer patterned structure for accelerating diabetic wound healing

Impaired angiogenesis is one of the predominant reasons for non-healing diabetic wounds. Herein, a nanofiber/hydrogel core–shell scaffold with three-dimensional (3D) multilayer patterned structure (3D-PT-P/GM) was introduced for promoting diabetic wound healing with improved angiogenesis. The results showed that the 3D-PT-P/GM scaffolds possessed multilayered structure with interlayer spacing of about 15–80 μm, and the hexagonal micropatterned structures were uniformly distributed on the surface of each layer. The nanofibers in the scaffold exhibited distinct core–shell structures with Gelatin methacryloyl (GelMA) hydrogel as the shell and Poly (d, l-lactic acid) (PDLLA) as the core. The results showed that the porosity, water retention time and water vapor permeability of the 3D-PT-P/GM scaffolds increased to 1.6 times, 21 times, and 1.9 times than that of the two-dimensional (2D) PDLLA nanofibrous scaffolds, respectively. The in vitro studies showed that the 3D-PT-P/GM scaffolds could significantly promote cell adhesion, proliferation, infiltration and migration throughout the scaffolds, and the expression of cellular communication protein-related genes, as well as angiogenesis-related genes in the same group, was remarkably upregulated. The in vivo results further demonstrated that the 3D-PT-P/GM scaffolds could not only effectively absorb exudate and provide a moist environment for the wound sites, but also significantly promote the formation of a 3D network of capillaries. As a result, the healing of diabetic wounds was accelerated with enhanced angiogenesis, granulation tissue formation, and collagen deposition. These results indicate that nanofiber/hydrogel core–shell scaffolds with 3D multilayer patterned structures could provide a new strategy for facilitating chronic wound healing.

Recombinant Proteins-Based Strategies in Bone Tissue Engineering

The increase in fracture rates and/or problems associated with missing bones due to accidents or various pathologies generates socio-health problems with a very high impact. Tissue engineering aims to offer some kind of strategy to promote the repair of damaged tissue or its restoration as close as possible to the original tissue. Among the alternatives proposed by this specialty, the development of scaffolds obtained from recombinant proteins is of special importance. Furthermore, science and technology have advanced to obtain recombinant chimera’s proteins. This review aims to offer a synthetic description of the latest and most outstanding advances made with these types of scaffolds, particularly emphasizing the main recombinant proteins that can be used to construct scaffolds in their own right, i.e., not only to impregnate them, but also to make scaffolds from their complex structure, with the purpose of being considered in bone regenerative medicine in the near future.

Combining tunable proteolytic sequences and a VEGF-mimetic peptide for the spatiotemporal control of angiogenesis within Elastin-Like Recombinamer scaffolds

One of the main challenges in regenerative medicine is the spatiotemporal control of angiogenesis, which is key for the successful repair of many tissues, and determines the proper integration of the implant through the generation of a functional vascular network. To this end, we have designed a three-dimensional (3D) model consisting of a coaxial binary elastin-like recombinamer (ELR) tubular construct. It displays fast and slow proteolytic hydrogels on its inner and outer part, respectively, both sensitive to the urokinase plasminogen activator protease. The ELRs used to build the scaffold included crosslinkable domains to stabilize the structure and a conjugated VEGF-derived peptide (QK) to induce angiogenesis. The mechanical and morphological evaluation of the ELR hydrogels proved their suitability for soft tissue regeneration. In addition, in vitro studies evidenced the effect of the QK peptide on endothelial cell spreading and anastomosis. Moreover, immunohistochemical analyses after subcutaneous implantation of the ELR hydrogels in mice showed the induction of a low macrophage response that resolved over time. The implantation of the 3D model constructs evidenced the ability of the fast proteolytic sequence and the QK peptide to guide cell infiltration and capillary formation in the pre-designed arrangement of the constructs. These results set the basis for the application of this type of scaffolds in regenerative medicine, where spatiotemporally controlled vascularization will help in the promotion of an optimal tissue repair. STATEMENT OF SIGNIFICANCE: Herein, we show the spatiotemporal control of angiogenesis in vivo by the combination of proteolytic sequences, with fast and slow degradation kinetics, and VEGF-mimetic peptide (QK) in a coaxial binary elastin-like recombinamer (ELR) tubular scaffold. These two bioactivities have been previously described for angiogenesis purposes, but have never been combined. This work demonstrates that the bioactivities act synergistically in promoting cell infiltration and subsequent vascularization, thus leading to a controlled evolution in space and time of the vascular microstructure within the hydrogel-like tubular scaffold. This effect has not been showed before and holds great potential for future vascular applications, which might be of great interest for a substantial part of Acta Biomaterialia readership.

Elastin-like recombinamers-based hydrogel modulates post-ischemic remodeling in a non-transmural myocardial infarction in sheep

Ischemic heart disease is a leading cause of mortality due to irreversible damage to cardiac muscle. Inspired by the post-ischemic microenvironment, we devised an extracellular matrix (ECM)-mimicking hydrogel using catalyst-free click chemistry covalent bonding between two elastin-like recombinamers (ELRs). The resulting customized hydrogel included functional domains for cell adhesion and protease cleavage sites, sensitive to cleavage by matrix metalloproteases overexpressed after myocardial infarction (MI). The scaffold permitted stromal cell invasion and endothelial cell sprouting in vitro. The incidence of non-transmural infarcts has increased clinically over the past decade, and there is currently no treatment preventing further functional deterioration in the infarcted areas. Here, we have developed a clinically relevant ovine model of non-transmural infarcts induced by multiple suture ligations. Intramyocardial injections of the degradable ELRs-hydrogel led to complete functional recovery of ejection fraction 21 days after the intervention. We observed less fibrosis and more angiogenesis in the ELRs-hydrogel-treated ischemic core region compared to the untreated animals, as validated by the expression, proteomic, glycomic, and histological analyses. These findings were accompanied by enhanced preservation of GATA4⁺ cardiomyocytes in the border zone of the infarct. We propose that our customized ECM favors cardiomyocyte preservation in the border zone by modulating the ischemic core and a marked functional recovery. The functional benefits obtained by the timely injection of the ELRs-hydrogel in a clinically relevant MI model support the potential utility of this treatment for further clinical translation.

Biocasting of an elastin-like recombinamer and collagen bi-layered model of the tunica adventitia and external elastic lamina of the vascular wall

The development of techniques for fabricating vascular wall models will foster the development of preventive and therapeutic therapies for treating cardiovascular diseases. However, the physical and biological complexity of vascular tissue represents a major challenge, especially for the design and the production of off-the-shelf biomimetic vascular replicas. Herein, we report the development of a biocasting technique that can be used to replicate the tunica adventitia and the external elastic lamina of the vascular wall. Type I collagen embedded with neonatal human dermal fibroblast (HDFn) and an elastic click cross-linkable, cell-adhesive and protease-sensitive elastin-like recombinamer (ELR) hydrogel were investigated as readily accessible and tunable layers to the envisaged model. Mechanical characterization confirmed that the viscous and elastic attributes predominated in the collagen and ELR layers, respectively. In vitro maturation confirmed that the collagen and ELR provided a favorable environment for the HDFn viability, while histology revealed the wavy and homogenous morphology of the ELR and collagen layer respectively, the cell polarization towards the cell-attachment sites encoded on the ELR, and the enhanced expression of glycosaminoglycan-rich extracellular matrix and differentiation of the embedded HDFn into myofibroblasts. As a complementary assay, 30% by weight of the collagen layer was substituted with the ELR. This model proved the possibility to tune the composition and confirm the versatile character of the technology developed, while revealing no significant differences with respect to the original construct. On-demand modification of the model dimensions, number and composition of the layers, as well as the type and density of the seeded cells, can be further envisioned, thus suggesting that this bi-layered model may be a promising platform for the fabrication of biomimetic vascular wall models.

Biomaterial-directed cell behavior for tissue engineering

Successful tissue regeneration strategies focus on the use of novel biomaterials, structures, and a variety of cues to control cell behavior and promote regeneration. Studies discovered how biomaterial/ structure cues in the form of biomaterial chemistry, material stiffness, surface topography, pore, and degradation properties play an important role in controlling cellular events in the contest of in vitro and in vivo tissue regeneration. Advanced biomaterials structures and strategies are developed to focus on the delivery of bioactive factors, such as proteins, peptides, and even small molecules to influence cell behavior and regeneration. The present article is an effort to summarize important findings and further discuss biomaterial strategies to influence and control cell behavior directly via physical and chemical cues. This article also touches on various modern methods in biomaterials processing to include bioactive factors as signaling cues to program cell behavior for tissue engineering and regenerative medicine.

Controlled Production of Elastin-like Recombinamer Polymer-Based Membranes at a Liquid-Liquid Interface by Click Chemistry

Diffusion of organic and inorganic molecules controls most industrial and biological processes that occur in a liquid phase. Although significant efforts have been devoted to the design and operation of large-scale purification systems, diffusion devices with adjustable biochemical characteristics have remained difficult to achieve. In this regard, micrometer-scale, bioinspired membranes with tunable diffusion properties have been engineered by covalent cross-linking of two elastin-like recombinamers (ELRs) at a liquid-liquid interface. The covalent approach selected provides the desired ELR-based membranes with structural support, and modulation of the concentration of the polypeptides employed confers direct control of the thickness, pore size, and diffusive properties over a broad range of molecular weights (4-150 kDa). The recombinant and versatile nature of the proteinaceous building blocks employed further paves the way to engineering bioactive motifs within the membrane scaffold, thereby widening their applicability in the biological field.

Functional characterization of an enzymatically degradable multi-bioactive elastin-like recombinamer

One of the main goals in both tissue engineering and regenerative medicine is to design innovative synthetic scaffolds that can simulate and control the communication pathways between cells and the extracellular matrix (ECM). In this context, we describe herein the characterization of protein polymer, a recombinant elastin-like recombinamer (ELR) designed for developing tissue-engineered devices for use in vascular regeneration. This ELR is composed of an elastin-like backbone that contains a fibronectin domain, which provides specific, endothelial cell adhesion, and a protease target domain directed towards specific proteases involved in ECM remodeling. We also compare the specific response of endothelial and fibroblast cells to ELR scaffolds and show that cell adhesion and spreading on this ELR is significantly higher for endothelial cells than for fibroblasts. The reactivity of this polymer and its hydrogels to specific enzymatic degradation is demonstrated in vitro. As with natural elastin, enzymatic hydrolysis of the ELR produces elastin-derived peptides, or "matrikines", which, in turn, are potentially able to regulate important cell activities.

Elastin-Based Materials: Promising Candidates for Cardiac Tissue Regeneration

Stroke and cardiovascular episodes are still some of the most common diseases worldwide, causing millions of deaths and costing billions of Euros to healthcare systems. The use of new biomaterials with enhanced biological and physical properties has opened the door to new approaches in cardiovascular applications. Elastin-based materials are biomaterials with some of the most promising properties. Indeed, these biomaterials have started to yield good results in cardiovascular and angiogenesis applications. In this review, we explore the latest trends in elastin-derived materials for cardiac regeneration and the different possibilities that are being explored by researchers to regenerate an infarcted muscle and restore its normal function. Elastin-based materials can be processed in different manners to create injectable systems or hydrogel scaffolds that can be applied by simple injection or as patches to cover the damaged area and regenerate it. Such materials have been applied to directly regenerate the damaged cardiac muscle and to create complex structures, such as heart valves or new bio-stents that could help to restore the normal function of the heart or to minimize damage after a stroke. We will discuss the possibilities that elastin-based materials offer in cardiac tissue engineering, either alone or in combination with other biomaterials, in order to illustrate the wide range of options that are being explored. Moreover, although tremendous advances have been achieved with such elastin-based materials, there is still room for new approaches that could trigger advances in cardiac tissue regeneration.

Elastin-Like Recombinamer Hydrogels for Improved Skeletal Muscle Healing Through Modulation of Macrophage Polarization

Large skeletal muscle injuries, such as a volumetric muscle loss (VML), often result in an incomplete regeneration due to the formation of a non-contractile fibrotic scar tissue. This is, in part, due to the outbreak of an inflammatory response, which is not resolved over time, meaning that type-1 macrophages (M1, pro-inflammatory) involved in the initial stages of the process are not replaced by pro-regenerative type-2 macrophages (M2). Therefore, biomaterials that promote the shift from M1 to M2 are needed to achieve optimal regeneration in VML injuries. In this work, we used elastin-like recombinamers (ELRs) as biomaterials for the formation of non- (physical) and covalently (chemical) crosslinked bioactive and biodegradable hydrogels to fill the VML created in the tibialis anterior (TA) muscles of rats. These hydrogels promoted a higher infiltration of M2 within the site of injury in comparison to the non-treated control after 2 weeks (p<0.0001), indicating that the inflammatory response resolves faster in the presence of both types of ELR-based hydrogels. Moreover, there were not significant differences in the amount of collagen deposition between the samples treated with the chemical ELR hydrogel at 2 and 5 weeks, and this same result was found upon comparison of these samples with healthy tissue after 5 weeks, which implies that this treatment prevents fibrosis. The macrophage modulation also translated into the formation of myofibers that were morphologically more similar to those present in healthy muscle. Altogether, these results highlight that ELR hydrogels provide a friendly niche for infiltrating cells that biodegrades over time, leaving space to new muscle tissue. In addition, they orchestrate the shift of macrophage population toward M2, which resulted in the prevention of fibrosis in the case of the chemical hydrogel treatment and in a more healthy-like myofiber phenotype for both types of hydrogels. Further studies should focus in the assessment of the regeneration of skeletal muscle in larger animal models, where a more critical defect can be created and additional methods can be used to evaluate the functional recovery of skeletal muscle.

Elastin-like recombinamers in collagen-based tubular gels improve cell-mediated remodeling and viscoelastic properties

The partial substitution of collagen with elastin-like recombinamers in tubular gels improves cell-mediated remodeling, elastic moduli and strength during maturation.

Trends in the design and use of elastin-like recombinamers as biomaterials

Elastin-like recombinamers (ELRs), which derive from one of the repetitive domains found in natural elastin, have been intensively studied in the last few years from several points of view. In this mini review, we discuss all the recent works related to the investigation of ELRs, starting with those that define these polypeptides as model intrinsically disordered proteins or regions (IDPs or IDRs) and its relevance for some biomedical applications. Furthermore, we summarize the current knowledge on the development of drug, vaccine and gene delivery systems based on ELRs, while also emphasizing the use of ELR-based hydrogels in tissue engineering and regenerative medicine (TERM). Finally, we show different studies that explore applications in other fields, and several examples that describe biomaterial blends in which ELRs have a key role. This review aims to give an overview of the recent advances regarding ELRs and to encourage further investigation of their properties and applications.