Coacervation characteristics of recombinant human tropoelastin

Structural and physical basis for the elasticity of elastin

Elastin function is to endow vertebrate tissues with elasticity so that they can adapt to local mechanical constraints. The hydrophobicity and insolubility of the mature elastin polymer have hampered studies of its molecular organisation and structure-elasticity relationships. Nevertheless, a growing number of studies from a broad range of disciplines have provided invaluable insights, and several structural models of elastin have been proposed. However, many questions remain regarding how the primary sequence of elastin (and the soluble precursor tropoelastin) governs the molecular structure, its organisation into a polymeric network, and the mechanical properties of the resulting material. The elasticity of elastin is known to be largely entropic in origin, a property that is understood to arise from both its disordered molecular structure and its hydrophobic character. Despite a high degree of hydrophobicity, elastin does not form compact, water-excluding domains and remains highly disordered. However, elastin contains both stable and labile secondary structure elements. Current models of elastin structure and function are drawn from data collected on tropoelastin and on elastin-like peptides (ELPs) but at the tissue level, elasticity is only achieved after polymerisation of the mature elastin. In tissues, the reticulation of tropoelastin chains in water defines the polymer elastin that bears elasticity. Similarly, ELPs require polymerisation to become elastic. There is considerable interest in elastin especially in the biomaterials and cosmetic fields where ELPs are widely used. This review aims to provide an up-to-date survey of/perspective on current knowledge about the interplay between elastin structure, solvation, and entropic elasticity.

Physicochemical Characterization of a Biomimetic, Elastin-Inspired Polypeptide with Enhanced Thermoresponsive Properties and Improved Cell Adhesion

Genetic engineering allows fine-tuning and controlling protein properties, thus exploiting the new derivatives to obtain novel materials and systems with improved capacity to actively interact with biological systems. The elastin-like polypeptides are tunable recombinant biopolymers that have proven to be ideal candidates for realizing bioactive interfaces that can interact with biological systems. They are characterized by a thermoresponsive behavior that is strictly related to their peculiar amino acid sequence. We describe here the rational design of a new biopolymer inspired by elastin and the comparison of its physicochemical properties with those of another already characterized member of the same protein class. To assess the cytocompatibility, the behavior of cells of different origins toward these components was evaluated. Our study shows that the biomimetic strategy adopted to design new elastin-based recombinant polypeptides represents a versatile and valuable tool for the development of protein-based materials with improved properties and advanced functionality.

Structural Breakdown of Collagen Type I Elastin Blend Polymerization

Biopolymer blends are advantageous materials with novel properties that may show performances way beyond their individual constituents. Collagen elastin hybrid gels are a new representative of such materials as they employ elastin’s thermo switching behavior in the physiological temperature regime. Although recent studies highlight the potential applications of such systems, little is known about the interaction of collagen and elastin fibers during polymerization. In fact, the final network structure is predetermined in the early and mostly arbitrary association of the fibers. We investigated type I collagen polymerized with bovine neck ligament elastin with up to 33.3 weight percent elastin and showed, by using a plate reader, zeta potential and laser scanning microscopy (LSM) experiments, that elastin fibers bind in a lateral manner to collagen fibers. Our plate reader experiments revealed an elastin concentration-dependent increase in the polymerization rate, although the rate increase was greatest at intermediate elastin concentrations. As elastin does not significantly change the structural metrics pore size, fiber thickness or 2D anisotropy of the final gel, we are confident to conclude that elastin is incorporated homogeneously into the collagen fibers.

The "Elastic Perspective" of SARS-CoV-2 Infection and the Role of Intrinsic and Extrinsic Factors

Elastin represents the structural component of the extracellular matrix providing elastic recoil to tissues such as skin, blood vessels and lungs. Elastogenic cells secrete soluble tropoelastin monomers into the extracellular space where these monomers associate with other matrix proteins (e.g., microfibrils and glycoproteins) and are crosslinked by lysyl oxidase to form insoluble fibres. Once elastic fibres are formed, they are very stable, highly resistant to degradation and have an almost negligible turnover. However, there are circumstances, mainly related to inflammatory conditions, where increased proteolytic degradation of elastic fibres may lead to consequences of major clinical relevance. In severely affected COVID-19 patients, for instance, the massive recruitment and activation of neutrophils is responsible for the profuse release of elastases and other proteolytic enzymes which cause the irreversible degradation of elastic fibres. Within the lungs, destruction of the elastic network may lead to the permanent impairment of pulmonary function, thus suggesting that elastases can be a promising target to preserve the elastic component in COVID-19 patients. Moreover, intrinsic and extrinsic factors additionally contributing to damaging the elastic component and to increasing the spread and severity of SARS-CoV-2 infection are reviewed.

Synergistic phase separation of two pathways promotes integrin clustering and nascent adhesion formation

Integrin adhesion complexes (IACs) are integrin-based plasma-membrane-associated compartments where cells sense environmental cues. The physical mechanisms and molecular interactions that mediate initial IAC formation are unclear. We found that both p130Cas ('Cas') and Focal adhesion kinase ('FAK') undergo liquid-liquid phase separation in vitro under physiologic conditions. Cas- and FAK- driven phase separation is sufficient to reconstitute kindlin-dependent integrin clustering in vitro with recombinant mammalian proteins. In vitro condensates and IACs in mouse embryonic fibroblasts (MEFs) exhibit similar sensitivities to environmental perturbations including changes in temperature and pH. Furthermore, mutations that inhibit or enhance phase separation in vitro reduce or increase the number of IACs in MEFs, respectively. Finally, we find that the Cas and FAK pathways act synergistically to promote phase separation, integrin clustering, IAC formation and partitioning of key components in vitro and in cells. We propose that Cas- and FAK-driven phase separation provides an intracellular trigger for integrin clustering and nascent IAC formation.

Elastin Structure, Synthesis, Regulatory Mechanism and Relationship With Cardiovascular Diseases

As the primary component of elastic fibers, elastin plays an important role in maintaining the elasticity and tensile ability of cardiovascular, pulmonary and many other tissues and organs. Studies have shown that elastin expression is regulated by a variety of molecules that have positive and negative regulatory effects. However, the specific mechanism is unclear. Moreover, elastin is reportedly involved in the development and progression of many cardiovascular diseases through changes in its expression and structural modifications once deposited in the extracellular matrix. This review article summarizes the role of elastin in myocardial ischemia-reperfusion, atherosclerosis, and atrial fibrillation, with emphasis on the potential molecular regulatory mechanisms.

Autosomal Recessive Cutis Laxa 1C Mutations Disrupt the Structure and Interactions of Latent TGFβ Binding Protein-4

Latent TGFβ binding protein-4 (LTBP4) is a multi-domain glycoprotein, essential for regulating the extracellular bioavailability of TGFβ and assembly of elastic fibre proteins, fibrillin-1 and tropoelastin. LTBP4 mutations are linked to autosomal recessive cutis laxa type 1C (ARCL1C), a rare congenital disease characterised by high mortality and severely disrupted connective tissues. Despite the importance of LTBP4, the structure and molecular consequences of disease mutations are unknown. Therefore, we analysed the structural and functional consequences of three ARCL1C causing point mutations which effect highly conserved cysteine residues. Our structural and biophysical data show that the LTBP4 N- and C-terminal regions are monomeric in solution and adopt extended conformations with the mutations resulting in subtle changes to their conformation. Similar to LTBP1, the N-terminal region is relatively inflexible, whereas the C-terminal region is flexible. Interaction studies show that one C-terminal mutation slightly decreases binding to fibrillin-1. We also found that the LTBP4 C-terminal region directly interacts with tropoelastin which is perturbed by both C-terminal ARCL1C mutations, whereas an N-terminal mutation increased binding to fibulin-4 but did not affect the interaction with heparan sulphate. Our results suggest that LTBP4 mutations contribute to ARCL1C by disrupting the structure and interactions of LTBP4 which are essential for elastogenesis in a range of mammalian connective tissues.

Insights into Structure and Aggregation Behavior of Elastin-like Polypeptide Coacervates: All-Atom Molecular Dynamics Simulations

The stimuli-responsive character of elastin-like polypeptides (ELP) has led to their use in a wide range of applications. The temperature-triggered aggregation, or LCST behavior, of ELPs is a complex and multistep phenomenon, which proposed to include the structural transitions, loss of hydrophobic hydration, expulsion of water molecules and physical association of chains. Thus, the origin and detailed mechanism of LCST in ELPs is difficult to elucidate. Here, to gain insights into structure and dynamics of coacervates, we performed all-atom molecular dynamics simulations of 27 90-mer ELPs in explicit water at 350 K. Two sequences, poly(VGPVG)₁₈ and poly(VPGVG)₁₈, were examined due to their experimentally observed differences in thermal hysteresis albeit identical overall composition but different arrangement of amino acids. The simulation results indicate that surface hydrophobicity of poly(VGPVG) aggregate is less than that of the poly(VPGVG) aggregate, and there are marked changes in torsion angles and the propensities of secondary structural motifs during the aggregation process. Moreover, there are significant differences between structure of a single polypeptide in water and structure within the aggregate. Overall, the aggregation process is driven by the formation of peptide-peptide interactions whereas the average hydration of peptides remains almost the same between dissolved and aggregated states. Even though the aggregation is driven by the hydrophobic interactions, ELP coacervate has no hydrophobic core and contains many water molecules. Overall, our findings provide an insight into the sequence-dependent structure of coacervates and molecular behavior of individual peptides during aggregation.

Elastic fibers: formation, function, and fate during aging and disease

Elastic fibers are extracellular components of higher vertebrates and confer elasticity and resilience to numerous tissues and organs such as large blood vessels, lungs, and skin. Their formation and maturation take place in a complex multistage process called elastogenesis. It requires interactions between very different proteins but also other molecules and leads to the deposition and crosslinking of elastin's precursor on a scaffold of fibrillin-rich microfibrils. Mature fibers are exceptionally resistant to most influences and, under healthy conditions, retain their biomechanical function over the life of the organism. However, due to their longevity, they accumulate damages during aging. These are caused by proteolytic degradation, formation of advanced glycation end products, calcification, oxidative damage, aspartic acid racemization, lipid accumulation, carbamylation, and mechanical fatigue. The resulting changes can lead to diminution or complete loss of elastic fiber function and ultimately affect morbidity and mortality. Particularly, the production of elastokines has been clearly shown to influence several life-threatening diseases. Moreover, the structure, distribution, and abundance of elastic fibers are directly or indirectly influenced by a variety of inherited pathological conditions, which mainly affect organs and tissues such as skin, lungs, or the cardiovascular system. A distinction can be made between microfibril-related inherited diseases that are the result of mutations in diverse microfibril genes and indirectly affect elastogenesis, and elastinopathies that are linked to changes in the elastin gene. This review gives an overview on the formation, structure, and function of elastic fibers and their fate over the human lifespan in health and disease.

Tropoelastin and Elastin Assembly

Elastic fibers are an important component of the extracellular matrix, providing stretch, resilience, and cell interactivity to a broad range of elastic tissues. Elastin makes up the majority of elastic fibers and is formed by the hierarchical assembly of its monomer, tropoelastin. Our understanding of key aspects of the assembly process have been unclear due to the intrinsic properties of elastin and tropoelastin that render them difficult to study. This review focuses on recent developments that have shaped our current knowledge of elastin assembly through understanding the relationship between tropoelastin’s structure and function.

Biomolecular Condensates and Cancer

Malignant transformation is characterized by dysregulation of diverse cellular processes that have been the subject of detailed genetic, biochemical, and structural studies, but only recently has evidence emerged that many of these processes occur in the context of biomolecular condensates. Condensates are membrane-less bodies, often formed by liquid-liquid phase separation, that compartmentalize protein and RNA molecules with related functions. New insights from condensate studies portend a profound transformation in our understanding of cellular dysregulation in cancer. Here we summarize key features of biomolecular condensates, note where they have been implicated-or will likely be implicated-in oncogenesis, describe evidence that the pharmacodynamics of cancer therapeutics can be greatly influenced by condensates, and discuss some of the questions that must be addressed to further advance our understanding and treatment of cancer.

Transglutaminase-Mediated Cross-Linking of Tropoelastin to Fibrillin Stabilises the Elastin Precursor Prior to Elastic Fibre Assembly

Elastic fibres are essential components of all mammalian elastic tissues such as blood vessels, lung and skin, and are critically important for the mechanical properties they endow. The main components of elastic fibres are elastin and fibrillin, where correct formation of elastic fibres requires a fibrillin microfibril scaffold for the deposition of elastin. It has been demonstrated previously that the interaction between fibrillin and tropoelastin, the elastin precursor, increases the rate of assembly of tropoelastin. Furthermore, tropoelastin and fibrillin can be cross-linked by transglutaminase-2, but the function of cross-linking on their elastic properties is yet to be elucidated. Here we show that transglutaminase cross-linking supports formation of a 1:1 stoichiometric fibrillin-tropoelastin complex. SAXS data show that the complex retains features of the individual proteins but is elongated supporting end-to-end assembly. Elastic network models were constructed to compare the dynamics of tropoelastin and fibrillin individually as well as in the cross-linked complex. Normal mode analysis was performed to determine the structures' most energetically favourable, biologically accessible motions which show that within the complex, tropoelastin is less mobile and this molecular stabilisation extends along the length of the tropoelastin molecule to regions remote from the cross-linking site. Together, these data suggest a long-range stabilising effect of cross-linking that occurs due to the covalent linkage of fibrillin to tropoelastin. This work provides insight into the interactions of tropoelastin and fibrillin and how cross-link formation stabilises the elastin precursor so it is primed for elastic fibre assembly.

A Two-State Model Describes the Temperature-Dependent Conformational Equilibrium in the Alanine-Rich Domains in Elastin

Elastin is the insoluble elastomeric protein that provides extensibility and resilience to vertebrate tissues. Limited high-resolution structural data for elastin are notably complex. To access this information, this protein is considered in the simplified context of its two general domain types, that is, hydrophobic (HP) and crosslinking (CL). The question of elastin's structure-function has directed the focus of nearly all previous studies in the literature to the unique repeating sequences characteristic of this protein, found primarily in the HP domains. The CL domains were assumed to play a very limited role in biological elasticity due in part to the significant α-helical character that was (incorrectly) predicted for these regions. In this study, the conformational heterogeneity of alanines in native elastin's CL domains is examined in the context of helix-coil transition theory (HCTT) using solid-state nuclear magnetic resonance (SSNMR) spectroscopy in tandem with strategic isotopic labeling. Helix and coil populations are observed at all temperatures, but the former increases significantly at lower temperatures. Below the glass transition temperature (T_g), two major populations of alanines in the CL regions are resolved by two-dimensional SSNMR; one-dimensional methods are used for characterization in nativelike conditions. The spectra of ¹³CO-Ala in the CL regions are simulated using an HCTT-based statistical mechanical representation. Below T_g, longer segments with significant helical probabilities are consistent with the experimental data. At higher temperatures, the SSNMR lineshapes are best fit with a distribution of shorter (Ala)_n segments, most in random coil. These results are used to refine a structure-function model for elastin in the context of HCTT, redirecting attention to the CL domains and their role in elasticity.

Magnetically responsive peptide coacervates for dual hyperthermia and chemotherapy treatments of liver cancer

Liver cancer is an aggressive malignancy associated with high levels of mortality and morbidity. Doxorubicin (Dox) is often used to slow down liver cancer progression; however its efficacy is limited, and its severe side effects prevent its routine use at therapeutic concentrations. We present a biomimetic peptide that coacervates into micro-droplets, within which both Dox and magnetic nanoparticles (MNPs) can be sequestered. These Dox-loaded Magnetic Coacervates (DMCs) can be used for thermo-chemotherapy, with the controlled release of Dox triggered by an external Alternating Magnetic Field (AMF). The DMCs are internalized by the cells via an energy-independent mechanism which is not based on endocytosis. Application of AMF generates a temperature of 45 °C within the DMCs, triggering their disassembly and the simultaneous release of Dox, thereby resulting in dual hyperthermia and chemotherapy for more efficient cancer therapy. In vitro studies conducted under AMF reveal that DMCs are cytocompatible and effective in inducing HepG2 liver cancer cell death. Thermo-chemotherapy treatment against HepG2 cells is also shown to be more effective compared to either hyperthermia or chemotherapy treatments alone. Thus, our novel peptide DMCs can open avenues in theranostic strategies against liver cancer through programmable, wireless, and remote control of Dox release. STATEMENT OF SIGNIFICANCE: Simultaneous administration of chemical and thermal therapy (thermo-chemotherapy) is more effective in inducing liver cancer cell death and improving survival rate. Thus, there is a keen interest in developing suitable carriers for thermo-chemotherapy. Coacervate micro-droplets display significant advantages, including high loading capacity, fast self-assembly in aqueous environments, and liquid-like behavior. However, they have not yet been explored as carriers for thermo-chemotherapy. Here, we demonstrate that peptide coacervate micro-droplets can co-encapsulate Dox and magnetic nanoparticles and cross the cell membrane. Applying an alternating magnetic field to cells containing drug-loaded coacervates triggers the release of Dox as well as the localized heating by magnetic hyperthermia, resulting in efficient liver cancer cell death by dual thermo-chemotherapy.

Engineering the Architecture of Elastin-Like Polypeptides: From Unimers to Hierarchical Self-Assembly

Well-defined tunable nanostructures formed through the hierarchical self-assembly of peptide building blocks have drawn significant attention due to their potential applications in biomedical science. Artificial protein polymers derived from elastin-like polypeptides (ELPs), which are based on the repeating sequence of tropoelastin (the water-soluble precursor to elastin), provide a promising platform for creating nanostructures due to their biocompatibility, ease of synthesis, and customizable architecture. By designing the sequence and composition of ELPs at the gene level, their physicochemical properties can be controlled to a degree that is unmatched by synthetic polymers. A variety of ELP-based nanostructures are designed, inspired by the self-assembly of elastin and other proteins in biological systems. The choice of building blocks determines not only the physical properties of the nanostructures, but also their self-assembly into architectures ranging from spherical micelles to elongated nanofibers. This review focuses on the molecular determinants of ELP and ELP-hybrid self-assembly and formation of spherical, rod-like, worm-like, fibrillar, and vesicle architectures. A brief discussion of the potential biomedical applications of these supramolecular assemblies is also included.

Elastin in the Tumor Microenvironment

Elastic fibers are found in the extracellular matrix (ECM) of tissues requiring resilience and depend on elasticity. Elastin and its degradation products have multiple roles in the oncologic process. In many malignancies, the remodeled ECM expresses high levels of the elastin protein which may have either positive or negative effects on tumor growth. Elastin cross-linking with other ECM components and the enzymes governing this process all have effects on tumorigenesis. Elastases, and specifically neutrophil elastase, are key drivers of invasion and metastasis and therefore are important targets for inhibition. Elastin degradation leads to the generation of bioactive fragments and elastin-derived peptides that further modulate tumor growth and spread. Interestingly, elastin-like peptides (ELP) and elastin-derived peptides (EDP) may also be utilized as nano-carriers to combat tumor growth. EDPs drive tumor development in a variety of ways, and specifically targeting EDPs and their binding proteins are major objectives for ongoing and future anti-cancer therapies. Research on both the direct anti-cancer activity and the drug delivery capabilities of ELPs is another area likely to result in novel therapeutic agents in the near future.

Elastic fiber ultrastructure and assembly

Studies over the years have described a filamentous structure to mature elastin that suggests a complicated packing arrangement of tropoelastin subunits. The currently accepted mechanism for tropoelastin assembly requires microfibrils to serve as a physical extracellular scaffold for alignment of tropoelastin monomers during and before crosslinking. However, recent evidence suggests that the initial stages of tropoelastin assembly occur within the cell or at unique assembly sites on the plasma membrane where tropoelastin self assembles to form elastin aggregates. Outside the cell, elastin aggregates transfer to growing elastic fibers in the extracellular matrix where tensional forces on microfibrils generated through cell movement help shape the growing fiber. Overall, these observations challenge the widely held idea that interaction between monomeric tropoelastin and microfibrils is a requirement for elastin assembly, and point to self-assembly of tropoelastin as a driving force in elastin maturation.

Avidity and Cell Uptake of Integrin-Targeting Polypeptide Micelles is Strongly Shape-Dependent

We describe a genetically encoded micelle for targeted delivery consisting of a diblock polypeptide with segments derived from repetitive protein motifs inspired by Drosophila melanogaster Rec-1 resilin and human tropoelastin with a C-terminal fusion of an integrin-targeting fibronectin type III domain. By systematically varying the weight fraction of the hydrophilic elastin-like polypeptide (ELP) block and molecular weight of the diblock polypeptide, we designed micelles of different morphologies that modulate the binding avidity of the human wild-type 10th fibronectin domain (Fn3) as a function of shape. We show that wormlike micelles that present the Fn3 domain have a 1000-fold greater avidity for the α_vβ₃ receptor compared to the monomer ligand and an avidity that is greater than a clinically relevant antibody that is driven by their multivalency. The amplified avidity of these micelles leads to significantly increased cellular internalization, a feature that may have utility for the intracellular delivery of drugs that are loaded into the core of these micelles.

A comprehensive map of human elastin cross-linking during elastogenesis

Elastin is an essential structural protein in the extracellular matrix of vertebrates. It is the core component of elastic fibers, which enable connective tissues such as those of the skin, lungs or blood vessels to stretch and recoil. This function is provided by elastin's exceptional properties, which mainly derive from a unique covalent cross-linking between hydrophilic lysine-rich motifs of units of the monomeric precursor tropoelastin. To date, elastin's cross-linking is poorly investigated. Here, we purified elastin from human tissue and cleaved it into soluble peptides using proteases with different specificities. We then analyzed elastin's molecular structure by identifying unmodified residues, post-translational modifications and cross-linked peptides by high-resolution mass spectrometry and amino acid analysis. The data revealed the presence of multiple isoforms in parallel and a complex and heterogeneous molecular interconnection. We discovered that the same lysine residues in different monomers were simultaneously involved in various cross-link types or remained unmodified. Furthermore, both types of cross-linking domains, Lys-Pro and Lys-Ala domains, participate not only in bifunctional inter- but also in intra-domain cross-links. We elucidated the sequences of several desmosine-containing peptides and the contribution of distinct domains such as 6, 14 and 25. In contrast to earlier assumptions proposing that desmosine cross-links are formed solely between two domains, we elucidated the structure of a peptide that proves a desmosine formation with participation of three Lys-Ala domains. In summary, these results provide new and detailed insights into the cross-linking process, which takes place within and between human tropoelastin units in a stochastic manner.

Coarse-grained model of tropoelastin self-assembly into nascent fibrils

Elastin is the dominant building block of elastic fibers that impart structural integrity and elasticity to a range of important tissues, including the lungs, blood vessels, and skin. The elastic fiber assembly process begins with a coacervation stage where tropoelastin monomers reversibly self-assemble into coacervate aggregates that consist of multiple molecules. In this paper, an atomistically based coarse-grained model of tropoelastin assembly is developed. Using the previously determined atomistic structure of tropoelastin, the precursor molecule to elastic fibers, as the basis for coarse-graining, the atomistic model is mapped to a MARTINI-based coarse-grained framework to account for chemical details of protein-protein interactions, coupled to an elastic network model to stabilize the structure. We find that self-assembly of monomers generates up to ∼70 ​nm of dense aggregates that are distinct at different temperatures, displaying high temperature sensitivity. Resulting assembled structures exhibit a combination of fibrillar and globular substructures within the bulk aggregates. The results suggest that the coalescence of tropoelastin assemblies into higher order structures may be reinforced in the initial stages of coacervation by directed assembly, supporting the experimentally observed presence of heterogeneous cross-linking. Self-assembly of tropoelastin is driven by interactions of specific hydrophobic domains and the reordering of water molecules in the system. Domain pair orientation analysis throughout the self-assembly process at different temperatures suggests coacervation is a driving force to orient domains for heterogeneous downstream cross-linking. The model provides a framework to characterize macromolecular self-assembly for elastin, and the formulation could easily be adapted to similar assembly systems.

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.

Physical Chemistry of Cellular Liquid-Phase Separation

Compartmentalization of biochemical processes is essential for cell function. Although membrane-bound organelles are well studied in this context, recent work has shown that phase separation is a key contributor to cellular compartmentalization through the formation of liquid-like membraneless organelles (MLOs). In this Minireview, the key mechanistic concepts that underlie MLO dynamics and function are first briefly discussed, including the relevant noncovalent interaction chemistry and polymer physical chemistry. Next, a few examples of MLOs and relevant proteins are given, along with their functions, which highlight the relevance of the above concepts. The developing area of active matter and non-equilibrium systems, which can give rise to unexpected effects in fluctuating cellular conditions, are also discussed. Finally, our thoughts for emerging and future directions in the field are discussed, including in vitro and in vivo studies of MLO physical chemistry and function.

Upper Critical Solution Temperature (UCST) Behavior of Coacervate of Cationic Protamine and Multivalent Anions

Complex coacervation is an emerging liquid/liquid phase separation (LLPS) phenomenon that behaves as a membrane-less organelle in living cells. Yet while one of the critical factors for complex coacervation is temperature, little analysis and research has been devoted to the temperature effect on complex coacervation. Here, we performed a complex coacervation of cationic protamine and multivalent anions (citrate and tripolyphosphate (TPP)). Both mixtures (i.e., protamine/citrate and protamine/TPP) underwent coacervation in an aqueous solution, while a mixture of protamine and sodium chloride did not. Interestingly, the complex coacervation of protamine and multivalent anions showed upper critical solution temperature (UCST) behavior, and the coacervation of protamine and multivalent anions was reversible with solution temperature changes. The large asymmetry in molecular weight between positively charged protamine (~4 kDa) and the multivalent anions (<0.4 kDa) and strong electrostatic interactions between positively charged guanidine residues in protamine and multivalent anions were likely to contribute to UCST behavior in this coacervation system.

Elastic fibers and biomechanics of the aorta: Insights from mouse studies

Elastic fibers are major components of the extracellular matrix (ECM) in the aorta and support a life-long cycling of stretch and recoil. Elastic fibers are formed from mid-gestation throughout early postnatal development and the synthesis is regulated at multiple steps, including coacervation, deposition, cross-linking, and assembly of insoluble elastin onto microfibril scaffolds. To date, more than 30 molecules have been shown to associate with elastic fibers and some of them play a critical role in the formation and maintenance of elastic fibers in vivo. Because the aorta is subjected to high pressure from the left ventricle, elasticity of the aorta provides the Windkessel effect and maintains stable blood flow to distal organs throughout the cardiac cycle. Disruption of elastic fibers due to congenital defects, inflammation, or aging dramatically reduces aortic elasticity and affects overall vessel mechanics. Another important component in the aorta is the vascular smooth muscle cells (SMCs). Elastic fibers and SMCs alternate to create a highly organized medial layer within the aortic wall. The physical connections between elastic fibers and SMCs form the elastin-contractile units and maintain cytoskeletal organization and proper responses of SMCs to mechanical strain. In this review, we revisit the components of elastic fibers and their roles in elastogenesis and how a loss of each component affects biomechanics of the aorta. Finally, we discuss the significance of elastin-contractile units in the maintenance of SMC function based on knowledge obtained from mouse models of human disease.

Allysine modifications perturb tropoelastin structure and mobility on a local and global scale

Elastin provides elastic tissues with resilience through stretch and recoil cycles, and is primarily made of its extensively cross-linked monomer, tropoelastin. Here, we leverage the recently published full atomistic model of tropoelastin to assess how allysine modifications, which are essential to cross-linking, contribute to the dynamics and structural changes that occur in tropoelastin in the context of elastin assembly. We used replica exchange molecular dynamics to generate structural ensembles of allysine containing tropoelastin. We conducted principal component analysis on these ensembles and found that the molecule departs from the canonical structural ensemble. Furthermore, we showed that, while the canonical scissors-twist movement was retained, new movements emerged that deviated from those of the wild type protein, providing evidence for the involvement of a variety of molecular motions in elastin assembly. Additionally, we highlighted secondary structural changes and linked these perturbations to the longevity of specific salt bridges. We propose a model where allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics.

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converting lysine to allysine by lysyl oxidases is needed to generate crosslinks between tropoelastin molecules in order to make elastin

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structural changes in the intact tropoelastin molecule ensue where modified tropoelastin molecules structurally depart from the canonical ensemble

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new molecular motions deviate from those of unmodified tropoelastin

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persistence times of specific salt bridges contribute to these perturbations

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allysines in tropoelastin contribute to hierarchical elastin assembly through global and local perturbations to molecular structure and dynamics

Lysyl oxidase-like 2 (LOXL2)-mediated cross-linking of tropoelastin

Lysyl oxidases (LOXs) play a central role in extracellular matrix remodeling during development and tumor growth and fibrosis through cross-linking of collagens and elastin. We have limited knowledge of the structure and substrate specificity of these secreted enzymes. LOXs share a conserved C-terminal catalytic domain but differ in their N-terminal region, which is composed of 4 repeats of scavenger receptor cysteine-rich (SRCR) domains in LOX-like (LOXL) 2. We investigated by X-ray scattering and electron microscopy the low-resolution structure of the full-length enzyme and the structure of a shorter form lacking the catalytic domain. Our data demonstrate that LOXL2 has a rod-like structure with a stalk composed of the SRCR domains and the catalytic domain at its tip. We detected direct interaction between LOXL2 and tropoelastin (TE) and also LOXL2-mediated deamination of TE. Using proteomics, we identified several allysines together with cross-linked TE peptides. The elastin-like material generated was resistant to trypsin proteolysis and displayed mechanical properties similar to mature elastin. Finally, we detected the codistribution of LOXL2 and elastin in the vascular wall. Altogether, these data suggest that LOXL2 could participate in elastogenesis in vivo and could be used as a means of cross-linking TE in vitro for biomimetic and cell-compatible tissue engineering purposes.-Schmelzer, C. E. H., Heinz, A., Troilo, H., Lockhart-Cairns, M.-P., Jowitt, T. A., Marchand, M. F., Bidault, L., Bignon, M., Hedtke, T., Barret, A., McConnell, J. C., Sherratt, M. J., Germain, S., Hulmes, D. J. S., Baldock, C., Muller, L. Lysyl oxidase-like 2 (LOXL2)-mediated cross-linking of tropoelastin.

Intrinsically disordered proteins in crowded milieu: when chaos prevails within the cellular gumbo

Effects of macromolecular crowding on structural and functional properties of ordered proteins, their folding, interactability, and aggregation are well documented. Much less is known about how macromolecular crowding might affect structural and functional behaviour of intrinsically disordered proteins (IDPs) or intrinsically disordered protein regions (IDPRs). To fill this gap, this review represents a systematic analysis of the available literature data on the behaviour of IDPs/IDPRs in crowded environment. Although it was hypothesized that, due to the excluded-volume effects present in crowded environments, IDPs/IDPRs would invariantly fold in the presence of high concentrations of crowding agents or in the crowded cellular environment, accumulated data indicate that, based on their response to the presence of crowders, IDPs/IDPRs can be grouped into three major categories, foldable, non-foldable, and unfoldable. This is because natural cellular environment is not simply characterized by the presence of high concentration of "inert" macromolecules, but represents an active milieu, components of which are engaged in direct physical interactions and soft interactions with target proteins. Some of these interactions with cellular components can cause (local) unfolding of query proteins. In other words, since crowding can cause both folding and unfolding of an IDP or its regions, the outputs of the placing of a query protein to the crowded environment would depend on the balance between these two processes. As a result, and because of the spatio-temporal heterogeneity in structural organization of IDPs, macromolecular crowding can differently affect structures of different IDPs. Recent studies indicate that some IDPs are able to undergo liquid-liquid-phase transitions leading to the formation of various proteinaceous membrane-less organelles (PMLOs). Although interiors of such PMLOs are self-crowded, being characterized by locally increased concentrations of phase-separating IDPs, these IDPs are minimally foldable or even non-foldable at all (at least within the physiologically safe time-frame of normal PMLO existence).

Effects of Doxorubicin on the Liquid-Liquid Phase Change Properties of Elastin-Like Polypeptides

The lower critical solution temperature (LCST) of the thermo-responsive engineered elastin-like polypeptide (ELP) biopolymer is being exploited for the thermal targeted delivery of doxorubicin (Dox) to solid tumors. We examine the impact of Dox labeling on the thermodynamic and hydrodynamic behavior of an ELP drug carrier and how Dox influences the liquid-liquid phase separation (LLPS). Turbidity, dynamic light scattering (DLS), and differential scanning calorimetry measurements show that ELP undergoes a cooperative liquid-liquid phase separation from a soluble to insoluble coacervated state that is enhanced by Dox labeling. Circular dichroism measurements show that below the LCST ELP consists of both random coils and temperature-dependent β-turn structures. Labeling with Dox further enhances β-turn formation. DLS measurements reveal a significant increase in the hydrodynamic radius of ELP below the LCST consistent with weak self-association. Dox-labeled SynB1-ELP1 (Dox-ELP) has a significant increase in the hydrodynamic radius by DLS measurements that is consistent with stable oligomers and, at high Dox-ELP concentrations, micelle structures. Enhanced association by Dox-ELP is confirmed by sedimentation velocity analytical ultracentrifugation measurements. Both ELP self-association and the ELP inverse phase transition are entropically driven with positive changes in enthalpy and entropy. We show by turbidity and DLS that the ELP phase transition is monophasic, whereas mixtures of ELP and Dox-ELP are biphasic, with Dox-labeled ELP phase changing first and unlabeled ELP partitioning into the coacervate as the temperature is raised. DLS reveals a complex growth in droplet sizes consistent with coalescence and fusion of liquid droplets. Differential scanning calorimetry measurements show a -11 kcal/mol change in enthalpy for Dox-ELP coacervation relative to the unlabeled ELP, consistent with droplet formation being stabilized by favorable enthalpic interactions. We propose that the ELP phase change is initiated by ELP self-association, enhanced by increased Dox-ELP oligomer and micelle formation and stabilized by favorable enthalpic interactions in the liquid droplets.

Coacervation of the Recombinant Mytilus galloprovincialis Foot Protein-3b

The underwater adhesion of marine mussels is a fascinating example of how proteinaceous adhesives, although water-soluble to begin with, can be used in seawater. Marine mussels adhere to the substrate via adhesive plaques, where the adhesive proteins are located especially at the substratum's interface. One major compound of the adhesives in Mytilidae is the mussel foot protein 3b (mfp-3b). Here, recombinant mfp-3b (rmfp-3b) was produced in Escherichia coli. rmfp-3b showed upper critical solution temperature (UCST) mediated complex coacervation at pH 3.0 in the presence of citrate yielding a liquid-liquid phase separation. Further, the rmfp-3b coacervation could also be induced in seawater conditions such as the respective pH and ionic strength, but without UCST behavior. In particular, sulfate and citrate anions could significantly induce complex coacervation. This study provides insights into the molecular behavior of one of the key proteins of mussels involved in underwater adhesion and may inspire new applications of bioadhesives using recombinant mussel foot proteins.

Coacervation of resilin fusion proteins containing terminal functionalities

Liquid-liquid phase transition known as coacervation of resilin-like-peptide fusion proteins containing different terminal domains were investigated. Two different modular proteins were designed and produced and their behavior were compared to a resilin-like-peptide without terminal domains. The size of the particle-like coacervates was modulated by the protein concentration, pH and temperature. The morphology and three-dimensional (3D) structural details of the coacervate particles were investigated by cryogenic transmission electron microscopy (cryo-TEM) and tomography (cryo-ET) reconstruction. Selective adhesion of the coacervates on cellulose and graphene surfaces was demonstrated.

Order, Disorder, and Temperature-Driven Compaction in a Designed Elastin Protein

Artificial minielastin constructs have been designed that replicate the structure and function of natural elastins in a simpler context, allowing the NMR observation of structure and dynamics of elastin-like proteins with complete residue-specific resolution. We find that the alanine-rich cross-linking domains of elastin have a partially helical structure, but only when capped by proline-rich hydrophobic domains. We also find that the hydrophobic domains, composed of prominent 6-residue repeats VPGVGG and APGVGV found in natural elastins, appear random coil by both NMR chemical shift analysis and circular dichroism. However, these elastin hydrophobic domains exhibit structural bias for a dynamically disordered conformation that is neither helical nor β sheet with a degree of nonrandom structural bias which is dependent on residue type and position in the sequence. Another nonrandom-coil aspect of hydrophobic domain structure lies in the fact that, in contrast to other intrinsically disordered proteins, these hydrophobic domains retain a relatively condensed conformation whether attached to cross-linking domains or not. Importantly, these domains and the proteins containing them constrict with increasing temperature by up to 30% in volume without becoming more ordered. This property is often observed in nonbiological polymers and suggests that temperature-driven constriction is a new type of protein structural change that is linked to elastin's biological functions of coacervation-driven assembly and elastic recoil.

Stepwise Mechanism of Temperature-Dependent Coacervation of the Elastin-like Peptide Analogue Dimer, (C(WPGVG)3)2

Elastin-like peptides (ELPs) are distinct, repetitive, hydrophobic sequences, such as (VPGVG) _n, that exhibit coacervation, the property of reversible, temperature-dependent self-association and dissociation. ELPs can be found in elastin and have been developed as new scaffold biomaterials. However, the detailed relationship between their amino acid sequences and coacervation properties remains obscure because of the structural flexibility of ELPs. In this study, we synthesized a novel, dimeric ELP analogue (H-C(WPGVG)₃-NH₂)₂, henceforth abbreviated (CW3)2, and analyzed its self-assembly properties and structural factors as indicators of coacervation. Turbidity measurements showed that (CW3)2 demonstrated coacervation at a concentration much lower than that of its monomeric form and another ELP. In addition, the coacervate held water-soluble dye molecules. Thus, potent and distinct coacervation was obtained with a remarkably short sequence of (CW3)2. Furthermore, fluorescence microscopy, dynamic light scattering, and optical microscopy revealed that the coacervation of (CW3)2 was a stepwise process. The structural factors of (CW3)2 were analyzed by molecular dynamics simulations and circular dichroism spectroscopy. These measurements indicated that helical structures primarily consisting of proline and glycine became more disordered at high temperatures with concurrent, significant exposure of their hydrophobic surfaces. This extreme change in the hydrophobic surface contributes to the potent coacervation observed for (CW3)2. These results provide important insights into more efficient applications of ELPs and their analogues, as well as the coacervation mechanisms of ELP and elastin.

Chemoenzymatic synthesis of polypeptides consisting of periodic di- and tri-peptide motifs similar to elastin

Elastin-like polypeptides containing proline were synthesized via chemoenzymatic polymerization and exhibited a temperature-dependent structural transition.

Genipin Cross-Linking of Elastin and Elastin-Based Proteins

Genipin is a natural plant-derived compound that covalently cross-links biopolymers into lattice networks with good biocompatibility, controllable swelling, and mechanical properties. This protocol describes the genipin cross-linking of elastic proteins, including tropoelastin and elastin-based polypeptides, through steps of elastin phase-separation upon addition of salt and heat, centrifugation to rapidly concentrate the dense protein phase, and incubation. This method is applicable for the fabrication of elastic materials suitable for use as scaffolds for biomedical applications.

Targeted Modulation of Tropoelastin Structure and Assembly

Tropoelastin, as the monomer unit of elastin, assembles into elastic fibers that impart strength and resilience to elastic tissues. Tropoelastin is also widely used to manufacture versatile materials with specific mechanical and biological properties. The assembly of tropoelastin into elastic fibers or biomaterials is crucially influenced by key submolecular regions and specific residues within these domains. In this work, we identify the functional contributions of two rarely occurring negatively charged residues, glutamate 345 in domain 19 and glutamate 414 in domain 21, in jointly maintaining the native conformation of the tropoelastin hinge, bridge and foot regions. Alanine substitution of E345 and/or E414 variably alters the positioning and interactive accessibility of these regions, as illustrated by nanostructural studies and detected by antibody and cell probes. These structural changes are associated with a lower propensity for monomer coacervation, cross-linking into morphologically and functionally atypical hydrogels, and markedly impaired and abnormal elastic fiber formation. Our work indicates the crucial significance of both E345 and E414 residues in modulating specific local structure and higher-order assembly of human tropoelastin.

Direct observation of structure and dynamics during phase separation of an elastomeric protein

Despite its growing importance in biology and in biomaterials development, liquid-liquid phase separation of proteins remains poorly understood. In particular, the molecular mechanisms underlying simple coacervation of proteins, such as the extracellular matrix protein elastin, have not been reported. Coacervation of the elastin monomer, tropoelastin, in response to heat and salt is a critical step in the assembly of elastic fibers in vivo, preceding chemical cross-linking. Elastin-like polypeptides (ELPs) derived from the tropoelastin sequence have been shown to undergo a similar phase separation, allowing formation of biomaterials that closely mimic the material properties of native elastin. We have used NMR spectroscopy to obtain site-specific structure and dynamics of a self-assembling elastin-like polypeptide along its entire self-assembly pathway, from monomer through coacervation and into a cross-linked elastic material. Our data reveal that elastin-like hydrophobic domains are composed of transient β-turns in a highly dynamic and disordered chain, and that this disorder is retained both after phase separation and in elastic materials. Cross-linking domains are also highly disordered in monomeric and coacervated ELP3 and form stable helices only after chemical cross-linking. Detailed structural analysis combined with dynamic measurements from NMR relaxation and diffusion data provides direct evidence for an entropy-driven mechanism of simple coacervation of a protein in which transient and nonspecific intermolecular hydrophobic contacts are formed by disordered chains, whereas bulk water and salt are excluded.

Collagen and Elastin Biomaterials for the Fabrication of Engineered Living Tissues

Collagen and elastin represent the two most predominant proteins in the body and are responsible for modulating important biological and mechanical properties. Thus, the focus of this review is the use of collagen and elastin as biomaterials for the fabrication of living tissues. Considering the importance of both biomaterials, we first propose the notion that many tissues in the human body represent a reinforced composite of collagen and elastin. In the rest of the review, collagen and elastin biosynthesis and biophysics, as well as molecular sources and biomaterial fabrication methodologies, including casting, fiber spinning, and bioprinting, are discussed. Finally, we summarize the current attempts to fabricate a subset of living tissues and, based on biochemical and biomechanical considerations, suggest that future tissue-engineering efforts consider direct incorporation of collagen and elastin biomaterials.

Aggregation behaviour of biohybrid microgels from elastin-like recombinamers

Investigation of the aggregation behavior of biohybrid microgels, which can potentially be used as drug carriers, is an important topic, because aggregation not only causes loss of activity, but also toxicity and immunogenicity. To study this effect we synthesized microgels from elastin-like recombinamers (ELRs) using the miniemulsion technique. The existence of aggregation for such biohybrid microgels at different concentrations and temperatures was studied by different methods which include dynamic light scattering (DLS), (1)H high-resolution magic angle sample spinning (HRMAS) NMR spectroscopy, relaxometry and diffusometry. A hysteresis effect was detected in the process of aggregation by DLS as a function of temperature that strongly depends on ELR microgel concentration. The aggregation process was further quantitatively analyzed by the concentration dependence of the (1)H amino-acid residue chemical shifts and microgel diffusivity measured by NMR methods using the population balance kinetic aggregation model.