Elastin: a representative ideal protein elastomer

Programmability and biomedical utility of intrinsically-disordered protein polymers

Intrinsically disordered proteins (IDPs) exhibit molecular-level conformational dynamics that are functionally harnessed across a wide range of fascinating biological phenomena. The low sequence complexity of IDPs has led to the design and development of intrinsically-disordered protein polymers (IDPPs), a class of engineered repeat IDPs with stimuli-responsive properties. The perfect repetitive architecture of IDPPs allows for repeat-level encoding of tunable protein functionality. Designer IDPPs can be modeled on endogenous IDPs or engineered de novo as protein polymers with dual biophysical and biological functionality. Their properties can be rationally tailored to access enigmatic IDP biology and to create programmable smart biomaterials. With the goal of inspiring the bioengineering of multifunctional IDP-based materials, here we synthesize recent multidisciplinary progress in programming and exploiting the bio-functionality of IDPPs and IDPP-containing proteins. Collectively, expanding beyond the traditional sequence space of extracellular IDPs, emergent sequence-level control of IDPP functionality is fueling the bioengineering of self-assembling biomaterials, advanced drug delivery systems, tissue scaffolds, and biomolecular condensates -genetically encoded organelle-like structures. Looking forward, we emphasize open challenges and emerging opportunities, arguing that the intracellular behaviors of IDPPs represent a rich space for biomedical discovery and innovation. Combined with the intense focus on IDP biology, the growing landscape of IDPPs and their biomedical applications set the stage for the accelerated engineering of high-value biotechnologies and biomaterials.

Recombinant fibrous protein biomaterials meet skin tissue engineering

Natural biomaterials, particularly fibrous proteins, are extensively utilized in skin tissue engineering. However, their application is impeded by batch-to-batch variance, limited chemical or physical versatility, and environmental concerns. Recent advancements in gene editing and fermentation technology have catalyzed the emergence of recombinant fibrous protein biomaterials, which are gaining traction in skin tissue engineering. The modular and highly customizable nature of recombinant synthesis enables precise control over biomaterial design, facilitating the incorporation of multiple functional motifs. Additionally, recombinant synthesis allows for a transition from animal-derived sources to microbial sources, thereby reducing endotoxin content and rendering recombinant fibrous protein biomaterials more amenable to scalable production and clinical use. In this review, we provide an overview of prevalent recombinant fibrous protein biomaterials (collagens, elastin, silk proteins and their chimeric derivatives) used in skin tissue engineering (STE) and compare them with their animal-derived counterparts. Furthermore, we discuss their applications in STE, along with the associated challenges and future prospects.

Understanding the Phase Behavior of a Multistimuli-Responsive Elastin-like Polymer: Insights from Dynamic Light Scattering Analysis

Elastin-like polymers are a class of stimuli-responsive protein polymers that hold immense promise in applications such as drug delivery, hydrogels, and biosensors. Yet, understanding the intricate interplay of factors influencing their stimuli-responsive behavior remains a challenging frontier. Using temperature-controlled dynamic light scattering and zeta potential measurements, we investigate the interactions between buffer, pH, salt, water, and protein using an elastin-like polymer containing ionizable lysine residues. We observed the elevation of transition temperature in the presence of the common buffering agent HEPES at low concentrations, suggesting a "salting-in" effect of HEPES as a cosolute through weak association with the protein. Our findings motivate a more comprehensive investigation of the influence of buffer and other cosolute molecules on elastin-like polymer behavior.

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.

Mimicking the Tendon Microenvironment to Enhance Skeletal Muscle Adhesion and Longevity in a Functional Microcantilever Platform

Microcantilever platforms are functional models for studying skeletal muscle force dynamics in vitro. However, the contractile force generated by the myotubes can cause them to detach from the cantilevers, especially during long-term experiments, thus impeding the chronic investigations of skeletal muscles for drug efficacy and toxicity. To improve the integration of myotubes with microcantilevers, we drew inspiration from the elastomeric proteins, elastin and resilin, that are present in the animal and insect worlds, respectively. The spring action of these proteins plays a critical role in force dampening in vivo. In animals, elastin is present in the collagenous matrix of the tendon which is the attachment point of muscles to bones. The tendon microenvironment consists of elastin, collagen, and an aqueous jelly-like mass of proteoglycans. In an attempt to mimic this tendon microenvironment, elastin, collagen, heparan sulfate proteoglycan, and hyaluronic acid were deposited on a positively charged silane substrate. This enabled the long-term survival of mechanically active myotubes on glass and silicon microcantilevers for over 28 days. The skeletal muscle cultures were derived from both primary and induced pluripotent stem cell (iPSC)-derived human skeletal muscles. Both types of myoblasts formed myotubes which survived for five weeks. Primary skeletal muscles and iPSC-derived skeletal muscles also showed a similar trend in fatigue index values. Upon integration with the microcantilever system, the primary muscle and iPSC-derived myotubes were tested successively over a one month period, thus paving the way for long-term chronic experiments on these systems for both drug efficacy and toxicity studies.

Programming protein phase-separation employing a modular library of intrinsically disordered precision block copolymer-like proteins creating dynamic cytoplasmatic compartmentalization

The control of supramolecular complexes in living systems at the molecular level is an important goal in life-sciences. Spatiotemporal organization of molecular distribution & flow of such complexes are essential physicochemical processes in living cells and important for pharmaceutical processes. Membraneless organelles (MO) found in eukaryotic cells, formed by liquid-liquid phase-separation (LLPS) of intrinsically disordered proteins (IDPs) control and adjust intracellular organization. Artificially designed compartments based on LLPS open up a novel pathway to control chemical flux and partition in vitro and in vivo. We designed a library of chemically precisely defined block copolymer-like proteins based on elastin-like proteins (ELPs) with defined charge distribution and type, as well as polar and hydrophobic block domains. This enables the programmability of physicochemical properties and to control adjustable LLPS in vivo attaining control over intracellular partitioning and flux as role model for in vitro and in vivo applications. Tailor-made ELP-like block copolymer proteins exhibiting IDP-behavior enable LLPS formation in vitro and in vivo allowing the assembly of membrane-based and membraneless superstructures via protein phase-separation in E. coli. Subsequently, we demonstrate the responsiveness of protein phase-separated spaces (PPSSs) to environmental physicochemical triggers and their selective, charge-dependent and switchable interaction with DNA or extrinsic and intrinsic molecules enabling their selective shuttling across semipermeable phase boundaries including (cell)membranes. This paves the road for adjustable artificial PPSS-based storage and reaction spaces and the specific transport across phase boundaries for applications in pharmacy and synthetic biology.

Mechanical Properties and Functions of Elastin: An Overview

Human tissues must be elastic, much like other materials that work under continuous loads without losing functionality. The elasticity of tissues is provided by elastin, a unique protein of the extracellular matrix (ECM) of mammals. Its function is to endow soft tissues with low stiffness, high and fully reversible extensibility, and efficient elastic-energy storage. Depending on the mechanical functions, the amount and distribution of elastin-rich elastic fibers vary between and within tissues and organs. The article presents a concise overview of the mechanical properties of elastin and its role in the elasticity of soft tissues. Both the occurrence of elastin and the relationship between its spatial arrangement and mechanical functions in a given tissue or organ are overviewed. As elastin in tissues occurs only in the form of elastic fibers, the current state of knowledge about their mechanical characteristics, as well as certain aspects of degradation of these fibers and their mechanical performance, is presented. The overview also outlines the latest understanding of the molecular basis of unique physical characteristics of elastin and, in particular, the origin of the driving force of elastic recoil after stretching.

Fabrication of Tunable Mechanical Gradients by Mussels via Bottom-Up Self-Assembly of Collagenous Precursors

Functionally graded interfaces are prominent in biological tissues and are used to mitigate stress concentrations at junctions between mechanically dissimilar components. Biological mechanical gradients serve as important role models for bioinspired design in technically and biomedically relevant applications. However, this necessitates elucidating exactly how natural gradients mitigate mechanical mismatch and how such gradients are fabricated. Here, we applied a cross-disciplinary experimental approach to understand structure, function, and formation of mechanical gradients in byssal threads─collagen-based fibers used by marine mussels to anchor on hard surfaces. The proximal end of threads is approximately 50-fold less stiff and twice as extensible as the distal end. However, the hierarchical structure of the distal-proximal junction is still not fully elucidated, and it is unclear how it is formed. Using tensile testing coupled with video extensometry, confocal Raman spectroscopy, and transmission electron microscopy on native threads, we identified a continuous graded transition in mechanics, composition, and nanofibrillar morphology, which extends several hundreds of microns and which can vary significantly between individual threads. Furthermore, we performed in vitro fiber assembly experiments using purified secretory vesicles from the proximal and distal regions of the secretory glands (which contain different precursor proteins), revealing spontaneous self-assembly of distinctive distal- and proximal-like fiber morphologies. Aside from providing fundamental insights into the byssus structure, function, and fabrication, our findings reveal key design principles for bioinspired design of functionally graded polymeric materials.

The coexistence region in the Van der Waals fluid and the liquid-liquid phase transitions

Cellular membraneless organelles are thought to be droplets formed within the two-phase region corresponding to proteinaceous systems endowed with the liquid-liquid transition. However, their metastability requires an additional constraint-they arise in a certain region of density and temperature between the spinodal and binodal lines. Here, we consider the well-studied van der Waals fluid as a test model to work out criteria to determine the location of the spinodal line for situations in which the equation of state is not known. Our molecular dynamics studies indicate that this task can be accomplished by considering the specific heat, the surface tension and characteristics of the molecular clusters, such as the number of component chains and radius of gyration.

Fast and reversible crosslinking of a silk elastin-like polymer

Elastin-like polymers (ELPs) and their chimeric subfamily the silk elastin-like polymers (SELPs) exhibit a lower critical solvation temperature (LCST) behavior in water which has been extensively studied from theoretical, computational and experimental perspectives. The inclusion of silk domains in the backbone of the ELPs effects the molecular dynamics of the elastin-like domains in response to increased temperature above its transition temperature and confers gelation ability. This response has been studied in terms of initial and long-term changes in structures, however, intermediate transition states have been less investigated. Moreover, little is known about the effects of reversible hydration on the elastin versus silk domains in the physical crosslinks. We used spectroscopic techniques to analyze initial, intermediate and long-term states of the crosslinks in SELPs. A combination of thermoanalytical and rheological measurements demonstrated that the fast reversible rehydration of the elastin motifs adjacent to the relatively small silk domains was capable of breaking the silk physical crosslinks. This feature can be exploited to tailor the dynamics of these types of crosslinks in SELPs. STATEMENT OF SIGNIFICANCE: The combination of silk and elastin in a single molecule results in synergy via their interactions to impact the protein polymer properties. The ability of the silk domains to crosslink affects the thermoresponsive properties of the elastin domains. These interactions have been studied at early and late states of the physical crosslinking, while the intermediate states were the focus of the present study to understand the reversible phase-transitions of the elastin domains over the silk physical crosslinking. The thermoresponsive properties of the elastin domains at the initial, intermediate and late states of silk crosslinking were characterized to demonstrate that reversible hydration of the elastin domains influenced the reversibility of the silk crosslinks.

Metal ion scavenging activity of elastin-like peptide analogues containing a cadmium ion binding sequence

The development of simple and safe methods for recovering environmental pollutants, such as heavy metals, is needed for sustainable environmental management. Short elastin-like peptide (ELP) analogues conjugated with metal chelating agents are considered to be useful as metal sequestering agents as they are readily produced, environment friendly, and the metal binding domain can be selected based on any target metal of interest. Due to the temperature dependent self-assembly of ELP, the peptide-based sequestering agents can be transformed from the solution state into the particles that chelate metal ions, which can then be collected as precipitates. In this study, we developed a peptide-based sequestering agent, AADAAC-(FPGVG)₄, by introducing the metal-binding sequence AADAAC on the N-terminus of a short ELP, (FPGVG)₄. In turbidity measurements, AADAAC-(FPGVG)₄ revealed strong self-assembling ability in the presence of metal ions such as Cd²⁺ and Zn²⁺. The results from colorimetric analysis indicated that AADAAC-(FPGVG)₄ could capture Cd²⁺ and Zn²⁺. Furthermore, AADAAC-(FPGVG)₄ that bound to metal ions could be readily recycled by treatment with acidic solution without compromising its metal binding affinity. The present study indicates that the fusion of the metal-binding sequence and ELP is a useful and powerful strategy to develop cost-effective heavy metal scavenging agents with low environmental impacts.

Biophysical Approaches for Applying and Measuring Biological Forces

Over the past decades, increasing evidence has indicated that mechanical loads can regulate the morphogenesis, proliferation, migration, and apoptosis of living cells. Investigations of how cells sense mechanical stimuli or the mechanotransduction mechanism is an active field of biomaterials and biophysics. Gaining a further understanding of mechanical regulation and depicting the mechanotransduction network inside cells require advanced experimental techniques and new theories. In this review, the fundamental principles of various experimental approaches that have been developed to characterize various types and magnitudes of forces experienced at the cellular and subcellular levels are summarized. The broad applications of these techniques are introduced with an emphasis on the difficulties in implementing these techniques in special biological systems. The advantages and disadvantages of each technique are discussed, which can guide readers to choose the most suitable technique for their questions. A perspective on future directions in this field is also provided. It is anticipated that technical advancement can be a driving force for the development of mechanobiology.

Elastin-like Polymers as Nanovaccines: Protein Engineering of Self-Assembled, Epitope-Exposing Nanoparticles

In this chapter we describe two unconventional strategies for the formulation of new nanovaccines. Both strategies are based on obtaining chimeric genes that code for proteins in which the major antigens of the pathogens are fused to an elastin-like recombinamer (ELR) as carrier. ELRs are a family of synthetic protein biopolymers obtained using DNA recombinant techniques. The ELRs employed in the present chapter are block copolymers that are able to assemble, under controlled conditions, into nanoparticles similar to virus-like particles and to provoke an immune response. We describe the biosynthesis of ELRs genetically fused to an antigenic sequence from Mycobacterium tuberculosis and a simple procedure for obtaining stable nanoparticles displaying the antigen in the first strategy. The second approach describes the production of a DNA vaccine library consisting of plasmids codifying for major antigens from Rift Valley fever virus fused to different ELR-based block copolymer architectures.The procedures described can be adapted for the production of other chimeric DNA-protein vaccines based on protein polymer carriers.

De Novo Synthesis of Elastin-like Polypeptides (ELPs): An Applied Overview on the Current Experimental Techniques

Elastin-like polypeptides (ELPs) are protein-based biopolymers genetically produced from polypeptides composed of a repeating pentapeptide sequence V-P-G-X-G. The inherent properties of recombinant ELPs, such as smart nature, controlled sequence complexity, physicochemical properties, and biocompatibility, make these polymers suitable for use in nanobiotechnological applications, as biofunctionalized scaffolds for tissue-engineering purposes and drug delivery. In this work, we report the design and synthesis of two elastomeric self-assembling polypeptides (ELPs) that mimic the endogenous human tropoelastin. Using molecular biology techniques, two artificial genes that encode two ELP concatemers of approximate molecular mass 60 kDa, one of them carrying biotin-binding peptide motifs, were constructed. These motifs could facilitate biofunctionalization of the ELPs through tethering biotinylated factors, such as growth factors. The ELPs were heterologously overexpressed in E. coli and subsequently purified in two steps: a nonchromatographic technique by organic solvent extraction, followed by nickel-nitrilotriacetic acid (Ni-NTA) affinity chromatography. The characterization of the biochemical properties and biocompatibility of ELPs was also performed in this study. The ELP carrying the biotin-binding motifs was tested for its capability to bind biotin, and indeed, it was observed that it can bind biotinylated proteins specifically. Additionally, results concerning the cytotoxicity of the ELPs exhibited excellent compatibility of the ELPs with mammalian cells in vitro. We anticipate that these ELPs can be used as components of a scaffold that mimics the extracellular matrix (ECM) for the regeneration of endogenously highly elastic tissues.

Dynamics in natural and designed elastins and their relation to elastic fiber structure and recoil

Elastin fibers assemble in the extracellular matrix from the precursor protein tropoelastin and provide the flexibility and spontaneous recoil required for arterial function. Unlike many proteins, a structure-function mechanism for elastin has been elusive. We have performed detailed NMR relaxation studies of the dynamics of the minielastins 24x' and 20x' using solution NMR, and of purified bovine elastin fibers in the presence and absence of mechanical stress using solid state NMR. The low sequence complexity of the minielastins enables us to determine average dynamical timescales and degrees of local ordering in the cross-link and hydrophobic modules separately using NMR relaxation by taking advantage of their residue-specific resolution. We find an extremely high degree of disorder, with order parameters for the entirety of the hydrophobic domains near zero, resembling that of simple chemical polymers and less than the order parameters that have been observed in other intrinsically disordered proteins. We find that average backbone order parameters in natural, purified elastin fibers are comparable to those found in 24x' and 20x' in solution. The difference in dynamics, compared with the minielastins, is that backbone correlation times are significantly slowed in purified elastin. Moreover, when elastin is mechanically stretched, the high chain disorder in purified elastin is retained, showing that any change in local ordering is below that detectable in our experiment. Combined with our previous finding of a 10-fold increase in the ordering of water when fully hydrated elastin fibers are stretched by 50%, these results support the hypothesis that stretch induced solvent ordering, i.e., the hydrophobic effect, is a key player in the elastic recoil of elastin as opposed to configurational entropy loss.

A coarse-grained mechanical model for folding and unfolding of tropoelastin with possible mutations

We propose a simple general framework to predict folding, native states, energy barriers, protein unfolding, as well as mutation induced diseases and other protein structural analyses. The model should not be considered as an alternative to classical approaches (Molecular Dynamics or Monte Carlo) because it neglects low scale details and rather focuses on global features of proteins and structural information. We aim at the description of phenomena that are out of the range of classical molecular modeling approaches due to the large computational cost: multimolecular interactions, cyclic behavior under variable external interactions, and similar. To demonstrate the effectiveness of the approach in a real case, we focus on the folding and unfolding behavior of tropoelastin and its mutations. Specifically, we derive a discrete mechanical model whose structure is deduced based on a coarse graining approach that allows us to group the amino acids sequence in a smaller number of `equivalent' masses. Nearest neighbor energy terms are then introduced to reproduce the interaction of such amino acid groups. Nearest and non-nearest neighbor energy terms, inter and intra functional blocks are phenomenologically added in the form of Morse potentials. As we show, the resulting system reproduces important properties of the folding-unfolding mechanical response, including the monotonic and cyclic force-elongation behavior, representing a physiologically important information for elastin. The comparison with the experimental behavior of mutated tropoelastin confirms the predictivity of the model. STATEMENT OF SIGNIFICANCE: Classical approaches to the study of phenomena at the molecular scale such as Molecular Dynamics (MD) represent an incredible tool to unveil mechanical and conformational properties of macromolecules, in particular for biological and medical applications. On the other hand, due to the computational cost, the time and spatial scales are limited. Focusing of the real case of tropoelastin, we propose a new approach based on a careful coarse graining of the system, able to describe the overall properties of the macromolecule and amenable of extension to larger scale effects (protein bundles, protein-protein interactions, cyclic loading). The comparison with tropoelastin behavior, also for mutations, is very promising.

Structure of Elastin

As the extracellular matrix protein, elastin is a crucial component of connective tissue in life. It is responsible for the structural integrity and function of tissues undergoing reversible extensibility or deformability, even though it may make up only a small percentage of a tissue. The structure stability, elastic resilience, bioactivity, and ability of self-assembly make elastin a highly desirable candidate for the fabrication of biomaterials. Elastin's properties mainly depend on their special structure. As elastin can be obtained by the assembly and cross-linking of its soluble precursor, tropoelastin. This chapter centers on introducing the structure of those two materials.

Recombination and Purification of Elastin-Like Polypeptides

Elastin, as an extracellular matrix protein, has inherent advantages for biomedical applications. For example, it is highly extensible and biocompatible, biodegradable, and has no immunogenicity. However, directly extracting elastin from biological tissues remains challenging because they usually coexist with other proteins such as collagen. Therefore, an effective strategy to produce elastin is to transfer the elastin's target gene into other expression hosts and synthesize the resultant polypeptides using chemical biology methods. The polypeptides and proteins produced using these methods are usually referred to as elastin-like peptides (ELPs), which have received intensive interests in drug delivery and release, tissue engineering, implanted devices, and so on. Therefore, this chapter introduces the detailed protocol for the preparation of ELPs using genetic recombination, including DNA recombination, expression, and purification. The methods presented here are expected to provide methodological guidance for preparation and application of ELP materials.

Anisotropy in mechanical unfolding of protein upon partner-assisted pulling and handle-assisted pulling

Proteins as force-sensors respond to mechanical cues and regulate signaling in physiology. Proteins commonly connect the source and response points of mechanical cues in two conformations, independent proteins in end-to-end geometry and protein complexes in handshake geometry. The force-responsive property of independent proteins in end-to-end geometry is studied extensively using single-molecule force spectroscopy (SMFS). The physiological significance of the complex conformations in force-sensing is often disregarded as mere surge protectors. However, with the potential of force-steering, protein complexes possess a distinct mechano-responsive property over individual force-sensors. To decipher, we choose a force-sensing protein, cadherin-23, from tip-link complex and perform SMFS using end-to-end geometry and handshake complex geometry. We measure higher force-resilience of cadherin-23 with preferential shorter extensions in handshake mode of pulling over the direct mode. The handshake geometry drives the force-response of cadherin-23 through different potential-energy landscapes than direct pulling. Analysis of the dynamic network structure of cadherin-23 under tension indicates narrow force-distributions among residues in cadherin-23 in direct pulling, resulting in low force-dissipation paths and low resilience to force. Overall, the distinct and superior mechanical responses of cadherin-23 in handshake geometry than single protein geometry highlight a probable evolutionary drive of protein-protein complexes as force-conveyors over independent ones.

Emerging Role of Elastin-Like Polypeptides in Regenerative Medicine

Significance: Wound dressing based on naturally derived polymer provides a useful platform for treatment of skin injuries. Owing to the high mechanical strength and tunable structural and physicochemical properties of human elastin-like polypeptides (ELPs), they may be used as excellent materials for fabricating biocompatible scaffolds and other products for wound management. Recent Advances: Designing recombinant ELPs mimicking natural elastin to fabricate synthetic polymers suitable for human health care has generated significant interest. ELP-based cell-adhesive biopolymers have been used as an alternative for successful sutureless wound closure due to the physicochemical characteristics of the extracellular matrix. Critical Issues: Different systems of ELPs are being developed in the form of scaffolds, films, hydrogels, photo-linkable sheets, and composites linked with various types of growth factors for wound healing application. However, optimizing the quality and safety attributes for specific application needs designing of recombinant ELPs with structural and functional modifications as needed for the intervention. Future Direction: Chronic wounds are difficult to treat as the wound repair process is interrupted by conditions such as excessive inflammation, impaired extracellular matrix formation, and persistent infections. Conventional therapies such as skin substitutes or autologous skin grafts, in many cases, are unable to reestablish tissue homeostasis and proper healing. The development of innovative materials could induce a better regenerative healing response. In this study, we are reviewing different types of elastin-based materials for wound care application and their future prospects in regenerative medicine.

Elastin-inspired supramolecular hydrogels: a multifaceted extracellular matrix protein in biomedical engineering

The phenomenal advancement in regenerative medicines has led to the development of bioinspired materials to fabricate a biomimetic artificial extracellular matrix (ECM) to support cellular survival, proliferation, and differentiation. Researchers have diligently developed protein polymers consisting of functional sequences of amino acids evolved in nature. Nowadays, certain repetitive bioinspired polymers are treated as an alternative to synthetic polymers due to their unique properties like biodegradability, easy scale-up, biocompatibility, and non-covalent molecular associations which imparts tunable supramolecular architecture to these materials. In this direction, elastin has been identified as a potential scaffold that renders extensibility and elasticity to the tissues. Elastin-like polypeptides (ELPs) are artificial repetitive polymers that exhibit lower critical solution temperature (LCST) behavior in a particular environment than synthetic polymers and hence have gained extensive interest in the fabrication of stimuli-responsive biomaterials. This review discusses in detail the unique structural aspects of the elastin and its soluble precursor, tropoelastin. Furthermore, the versatility of elastin-like peptides is discussed through numerous examples that bolster the significance of elastin in the field of regenerative medicines such as wound care, cardiac tissue engineering, ocular disorders, bone tissue regeneration, etc. Finally, the review highlights the importance of exploring short elastin-mimetic peptides to recapitulate the structural and functional aspects of elastin for advanced healthcare applications.

Quasielastic neutron scattering studies on couplings of protein and water dynamics in hydrated elastin

Performing quasielastic neutron scattering measurements and analyzing both elastic and quasielasic contributions, we study protein and water dynamics of hydrated elastin. At low temperatures, hydration-independent methyl group rotation dominates the findings. It is characterized by a Gaussian distribution of activation energies centered at about E_m = 0.17 eV. At ∼195 K, coupled protein-water motion sets in. The hydration water shows diffusive motion, which is described by a Gaussian distribution of activation energies with E_m = 0.57 eV. This Arrhenius behavior of water diffusion is consistent with previous results for water reorientation, but at variance with a fragile-to-strong crossover at ∼225 K. The hydration-related elastin backbone motion is localized and can be attributed to the cage rattling motion. We speculate that its onset at ∼195 K is related to a secondary glass transition, which occurs when a β relaxation of the protein has a correlation time of τ_β ∼ 100 s. Moreover, we show that its temperature-dependent amplitude has a crossover at the regular glass transition T_g = 320 K of hydrated elastin, where the α relaxation of the protein obeys τ_α ∼ 100 s. By contrast, we do not observe a protein dynamical transition when water dynamics enters the experimental time window at ∼240 K.

Adjustable Bioorthogonal Conjugation Platform for Protein Studies in Live Cells Based on Artificial Compartments

The investigation of complex biological processes in vivo often requires defined multiple bioconjugation and positioning of functional entities on 3D structures. Prominent examples include spatially defined protein complexes in nature, facilitating efficient biocatalysis of multistep reactions. Mimicking natural strategies, synthetic scaffolds should comprise bioorthogonal conjugation reactions and allow for absolute stoichiometric quantification as well as facile scalability through scaffold reproduction. Existing in vivo scaffolding strategies often lack covalent conjugations on geometrically confined scaffolds or precise quantitative characterization. Addressing these shortcomings, we present a bioorthogonal dual conjugation platform based on genetically encoded artificial compartments in vivo, comprising two distinct genetically encoded covalent conjugation reactions and their precise stoichiometric quantification. The SpyTag/SpyCatcher (ST/SC) bioconjugation and the controllable strain-promoted azide-alkyne cycloaddition (SPAAC) were implemented on self-assembled protein membrane-based compartments (PMBCs). The SPAAC reaction yield was quantified to be 23% ± 3% and a ST/SC surface conjugation yield of 82% ± 9% was observed, while verifying the compatibility of both chemical reactions as well as enhanced proteolytic stability. Using tandem mass spectrometry, absolute concentrations of the proteinaceous reactants were calculated to be 0.11 ± 0.05 attomol/cell for PMBC surface-tethered mCherry-ST-His and 0.22 ± 0.09 attomol/cell for PMBC-constituting pAzF-SC-E20F20-His. The established in vivo conjugation platform enables quantifiable protein-protein interaction studies on geometrically defined scaffolds and paves the road to investigate effects of scaffold-tethering on enzyme activity.

Job Opening for Nucleosome Mechanic: Flexibility Required

The nucleus has been studied for well over 100 years, and chromatin has been the intense focus of experiments for decades. In this review, we focus on an understudied aspect of chromatin biology, namely the chromatin fiber polymer’s mechanical properties. In recent years, innovative work deploying interdisciplinary approaches including computational modeling, in vitro manipulations of purified and native chromatin have resulted in deep mechanistic insights into how the mechanics of chromatin might contribute to its function. The picture that emerges is one of a nucleus that is shaped as much by external forces pressing down upon it, as internal forces pushing outwards from the chromatin. These properties may have evolved to afford the cell a dynamic and reversible force-induced communication highway which allows rapid coordination between external cues and internal genomic function.

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.

Advances in Plant-Derived Scaffold Proteins

Scaffold proteins form critical biomatrices that support cell adhesion and proliferation for regenerative medicine and drug screening. The increasing demand for such applications urges solutions for cost effective and sustainable supplies of hypoallergenic and biocompatible scaffold proteins. Here, we summarize recent efforts in obtaining plant-derived biosynthetic spider silk analogue and the extracellular matrix protein, collagen. Both proteins are composed of a large number of tandem block repeats, which makes production in bacterial hosts challenging. Furthermore, post-translational modification of collagen is essential for its function which requires co-transformation of multiple copies of human prolyl 4-hydroxylase. We discuss our perspectives on how the GAANTRY system could potentially assist the production of native-sized spider dragline silk proteins and prolyl hydroxylated collagen. The potential of recombinant scaffold proteins in drug delivery and drug discovery is also addressed.

Minimalist Protocell Design: A Molecular System Based Solely on Proteins that Form Dynamic Vesicular Membranes Embedding Enzymatic Functions

Life in its molecular context is characterized by the challenge of orchestrating structure, energy and information processes through compartmentalization and chemical transformations amenable to mimicry of protocell models. Here we present an alternative protocell model incorporating dynamic membranes based on amphiphilic elastin-like proteins (ELPs) rather than phospholipids. For the first time we demonstrate the feasibility of combining vesicular membrane formation and biocatalytic activity with molecular entities of a single class: proteins. The presented self-assembled protein-membrane-based compartments (PMBCs) accommodate either an anabolic reaction, based on free DNA ligase as an example of information transformation processes, or a catabolic process. We present a catabolic process based on a single molecular entity combining an amphiphilic protein with tobacco etch virus (TEV) protease as part of the enclosure of a reaction space and facilitating selective catalytic transformations. Combining compartmentalization and biocatalytic activity by utilizing an amphiphilic molecular building block with and without enzyme functionalization enables new strategies in bottom-up synthetic biology, regenerative medicine, pharmaceutical science and biotechnology.

Prebiotic Protocell Model Based on Dynamic Protein Membranes Accommodating Anabolic Reactions

The nature of the first prebiotic compartments and their possible minimal molecular composition is of great importance in the origin of life scenarios. Current protocell model membranes are proposed to be lipid-based. This paradigm has several shortcomings such as limited membrane stability of monoacyl lipid-based membranes (e.g., fatty acids), missing pathways to synthesize protocell membrane components (e.g., phospholipids) under early earth conditions, and the requirement for different classes of molecules for the formation of compartments and the catalysis of reactions. Amino acids on the other hand are known to arise and persist with remarkable abundance under early earth conditions since the fundamental Miller-Urey experiments. They were also postulated early to form protocellular structures, for example, proteinoid capsules. Here, we present a protocell model constituted by membranes assembled from amphiphilic proteins based on prebiotic amino acids. Self-assembled dynamic protein membrane-based compartments (PMBCs) are impressively stable and compatible with prevalent cellular membrane constituents forming protein-only or protein-lipid hybrid membranes. They can embed processes essential for extant living cells, such as enclosure of molecules, membrane fusion, phase separation, and complex biosynthetic elements from modern cells demonstrating "upward" compatibility. Our findings suggest that prebiotic PMBCs represent a new type of protocell as a possible ancestor of current lipid-based cells. The presented prebiotic PMBC model can be used to design artificial cells, important for the study of structural, catalytic, and evolutionary pathways related to the emergence of life.

Self-Assembly Toolbox of Tailored Supramolecular Architectures Based on an Amphiphilic Protein Library

The molecular structuring of complex architectures and the enclosure of space are essential requirements for technical and living systems. Self-assembly of supramolecular structures with desired shape, size, and stability remains challenging since it requires precise regulation of physicochemical and conformational properties of the components. Here a general platform for controlled self-assembly of tailored amphiphilic elastin-like proteins into desired supramolecular protein assemblies ranging from spherical coacervates over molecularly defined twisted fibers to stable unilamellar vesicles is introduced. The described assembly protocols efficiently yield protein membrane-based compartments (PMBC) with adjustable size, stability, and net surface charge. PMBCs demonstrate membrane fusion and phase separation behavior and are able to encapsulate structurally and chemically diverse cargo molecules ranging from small molecules to naturally folded proteins. The ability to engineer tailored supramolecular architectures with defined fusion behavior, tunable properties, and encapsulated cargo paves the road for novel drug delivery systems, the design of artificial cells, and confined catalytic nanofactories.

Temperature induced conformational changes in the elastin-like peptide GVG(VPGVG)3

Elastin-like peptides are biopolymers that display LCST behaviour in solution quite similar to other synthetic polymers like polyethylene oxide. Here we study the structure of the peptide GVG(VPGVG)3 in a temperature range of 25 °C to 70 °C with small angle neutron scattering. The LCST for this peptide is outside the experimental range of temperatures. Molecular conformation is well described within the model of a random coil but increasing temperature leads to significant changes. The peptide displays a combination of conformational change and aggregation that show up in the scattering at low and intermediate scattering vector q. The aggregate size is determined from an integral measure of the scattered intensity. It increases with temperature and concentration. For low concentration we find a size variation with temperature that may be related to the collapse of conformation at the inverse temperature transition (ITT).

Harvesting Mechanical Work From Folding-Based Protein Engines: From Single-Molecule Mechanochemical Cycles to Macroscopic Devices

Mechanochemical coupling cycles underlie the work-generation mechanisms of biological systems and are realized by highly regulated conformational changes of the protein machineries. However, it has been challenging to utilize protein conformational changes to do mechanical work at the macroscopic level in biomaterials, and it remains elusive to construct macroscopic mechanochemical devices based on molecular-level mechanochemical coupling systems. Here, the authors demonstrate that protein folding can be utilized to realize protein’s mechanochemical cycles at both single-molecule and macroscopic levels. Our results demonstrate, for the first time, the successful harnessing of mechanical work generated by protein folding in a macroscopic protein hydrogel device, and the work generated by protein folding compares favorably with the energy output of molecular motors. Our work bridges a gap between single-molecule and macroscopic levels, and paves the way to utilizing proteins as building blocks to design protein-based artificial muscles and soft actuators.

Active Targeting of Cancer Cells by Nanobody Decorated Polypeptide Micelle with Bio-orthogonally Conjugated Drug

Polypeptides are promising carriers for chemotherapeutics: they have minimal toxicity, can be recombinantly synthesized with precise control over molecular weight, and enhance drug pharmacokinetics as self-assembled nanoparticles. Polypeptide-based systems also provide the ability to achieve active targeting with genetically encoded targeting ligands. While passive targeting promotes accumulation of nanocarriers in solid tumors, active targeting provides an additional layer of tunable control and widens the therapeutic window. However, fusion of most targeting proteins to polypeptide carriers exposes the limitations of this approach: the residues that are used for drug attachment are also promiscuously distributed on protein surfaces. We present here a universal methodology to solve this problem by the site-specific attachment of extrinsic moieties to polypeptide drug delivery systems without cross-reactivity to fused targeting domains. We incorporate an unnatural amino acid, p-acetylphenylalanine, to provide a biorthogonal ketone for attachment of doxorubicin in the presence of reactive amino acids in a nanobody-targeted, elastin-like polypeptide nanoparticle. These nanoparticles exhibit significantly greater cytotoxicity than nontargeted controls in multiple cancer cell lines.

An Intrinsically Disordered Peptide-Peptide Stapler for Highly Efficient Protein Ligation Both in Vivo and in Vitro

Herein, we report an intrinsically disordered protein SpyStapler that can catalyze the isopeptide bond formation between two peptide tags, that is, SpyTag and BDTag, both in vitro and in vivo. SpyStapler and BDTag are developed by splitting SpyCatcher-the cognate protein partner of SpyTag-at the more solvent exposed second loop region. Regardless of their locations in protein constructs, SpyStapler enables efficient covalent coupling of SpyTag and BDTag under a variety of mild conditions in vitro (yield ∼80%). Co-expression of SpyStapler with telechelic dihydrofolate reductase (DHFR) bearing a SpyTag at N-terminus and a BDTag at C-terminus leads to direct cellular synthesis of a circular DHFR. Mechanistic studies involving circular dichroism and nuclear magnetic resonance spectrometry reveal that SpyStapler alone is disordered in solution and forms a stable folded structure ( T_m ∼ 55 °C) in the presence of both SpyTag and BDTag upon isopeptide bonding. No ordered structure can be formed in the absence of either tag. The catalytically inactive SpyStapler-EQ mutant cannot form a stable physical complex with SpyTag and BDTag, but it can fold into ordered structure in the presence of the ligated product (SpyTag-BDTag). It suggests that the isopeptide bond is important in stabilizing the complex. Given its efficiency, resilience, and robustness, SpyStapler provides new opportunities for bioconjugation and creation of complex protein architectures.

Towards synthetic cells using peptide-based reaction compartments

Membrane compartmentalization and growth are central aspects of living cells, and are thus encoded in every cell's genome. For the creation of artificial cellular systems, genetic information and production of membrane building blocks will need to be coupled in a similar manner. However, natural biochemical reaction networks and membrane building blocks are notoriously difficult to implement in vitro. Here, we utilized amphiphilic elastin-like peptides (ELP) to create self-assembled vesicular structures of about 200 nm diameter. In order to genetically encode the growth of these vesicles, we encapsulate a cell-free transcription-translation system together with the DNA template inside the peptide vesicles. We show in vesiculo production of a functioning fluorescent RNA aptamer and a fluorescent protein. Furthermore, we implement in situ expression of the membrane peptide itself and finally demonstrate autonomous vesicle growth due to the incorporation of this ELP into the membrane.

Decellularized matrices in regenerative medicine

Of all biologic matrices, decellularized extracellular matrix (dECM) has emerged as a promising tool used either alone or when combined with other biologics in the fields of tissue engineering or regenerative medicine - both preclinically and clinically. dECM provides a native cellular environment that combines its unique composition and architecture. It can be widely obtained from native organs of different species after being decellularized and is entitled to provide necessary cues to cells homing. In this review, the superiority of the macro- and micro-architecture of dECM is described as are methods by which these unique characteristics are being harnessed to aid in the repair and regeneration of organs and tissues. Finally, an overview of the state of research regarding the clinical use of different matrices and the common challenges faced in using dECM are provided, with possible solutions to help translate naturally derived dECM matrices into more robust clinical use.

STATEMENT OF SIGNIFICANCE

Ideal scaffolds mimic nature and provide an environment recognized by cells as proper. Biologically derived matrices can provide biological cues, such as sites for cell adhesion, in addition to the mechanical support provided by synthetic matrices. Decellularized extracellular matrix is the closest scaffold to nature, combining unique micro- and macro-architectural characteristics with an equally unique complex composition. The decellularization process preserves structural integrity, ensuring an intact vasculature. As this multifunctional structure can also induce cell differentiation and maturation, it could become the gold standard for scaffolds.

Elastin-like Peptide in Confinement: FT-IR and NMR T 1 Relaxation Data

We employed FT-IR and NMR experiments to investigate the influence of a cell-mimicking crowding environment on the structure and dynamics of an elastin-like peptide (ELP) with the sequence GVG(VPGVG)3, which – due to a high number of hydrophobic amino acid side chains – exhibits an inverse temperature transition (ITT). As simplified crowding agent, we used 30 wt% Ficoll. The FT-IR data revealed the well-known broad ITT above ~25°C, as observed by the decrease of the relative population of random coil structures and the concomitant increase of type II β-turns. Interestingly, the addition of Ficoll leads to a destabilizing effect of type II β-turn structures. This is in contrast to the expected excluded-volume effect of the macromolecular crowder, but can be explained by weak interactions of the peptide with the polysaccharide chains of the crowding agent. Further, the crowding agent leads to the onset of a reversal of the folding transition at high temperatures. The full assignment of the ELP allowed for a residue-specific investigation of the dynamic behavior of ELP by NMR. Due to a strong change of microscopic viscosity between native/buffered conditions and crowded conditions, relaxation data remain inconclusive with respect to the observation of an ITT. Hence, no quantitative details in terms of internal conformational changes can be obtained. However, temperature dependent differences in the 13C relaxation behavior between core and terminal parts of the peptide indicate temperature induced changes in the internal dynamics with generally higher internal mobility at chain ends: This is in full agreement with FT-IR data. In harmony with the FT-IR analysis, macromolecular crowding does not lead to significant changes in the relaxation behavior.

Molecular Dynamics Simulations of Water, Silica, and Aqueous Mixtures in Bulk and Confinement

Aqueous systems are omnipresent in nature and technology. They show complex behaviors, which often originate in the existence of hydrogen-bond networks. Prominent examples are the anomalies of water and the non-ideal behaviors of aqueous solutions. The phenomenology becomes even richer when aqueous liquids are subject to confinement. To this day, many properties of water and its mixtures, in particular, under confinement, are not understood. In recent years, molecular dynamics simulations developed into a powerful tool to improve our knowledge in this field. Here, our simulation results for water and aqueous mixtures in the bulk and in various confinements are reviewed and some new simulation data are added to improve our knowledge about the role of interfaces. Moreover, findings for water are compared with results for silica, exploiting that both systems form tetrahedral networks.

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.

Photo-Crosslinkable Unnatural Amino Acids Enable Facile Synthesis of Thermoresponsive Nano- to Microgels of Intrinsically Disordered Polypeptides

Hydrogel particles are versatile materials that provide exquisite, tunable control over the sequestration and delivery of materials in pharmaceutics, tissue engineering, and photonics. The favorable properties of hydrogel particles depend largely on their size, and particles ranging from nanometers to micrometers are used in different applications. Previous studies have only successfully fabricated these particles in one specific size regime and required a variety of materials and fabrication methods. A simple yet powerful system is developed to easily tune the size of polypeptide-based, thermoresponsive hydrogel particles, from the nano- to microscale, using a single starting material. Particle size is controlled by the self-assembly and unique phase transition behavior of elastin-like polypeptides in bulk and within microfluidic-generated droplets. These particles are then stabilized through ultraviolet irradiation of a photo-crosslinkable unnatural amino acid (UAA) cotranslationally incorporated into the parent polypeptide. The thermoresponsive property of these particles provides an active mechanism for actuation and a dynamic responsive to the environment. This work represents a fundamental advance in the generation of crosslinked biomaterials, especially in the form of soft matter colloids, and is one of the first demonstrations of successful use of UAAs in generating a novel material.

Elastin-like Polypeptide Linkers for Single-Molecule Force Spectroscopy

Single-molecule force spectroscopy (SMFS) is by now well established as a standard technique in biophysics and mechanobiology. In recent years, the technique has benefitted greatly from new approaches to bioconjugation of proteins to surfaces. Indeed, optimized immobilization strategies for biomolecules and refined purification schemes are being steadily adapted and improved, which in turn has enhanced data quality. In many previously reported SMFS studies, poly(ethylene glycol) (PEG) was used to anchor molecules of interest to surfaces and/or cantilever tips. The limitation, however, is that PEG exhibits a well-known trans-trans-gauche to all-trans transition, which results in marked deviation from standard polymer elasticity models such as the worm-like chain, particularly at elevated forces. As a result, the assignment of unfolding events to protein domains based on their corresponding amino acid chain lengths is significantly obscured. Here, we provide a solution to this problem by implementing unstructured elastin-like polypeptides as linkers to replace PEG. We investigate the suitability of tailored elastin-like polypeptides linkers and perform direct comparisons to PEG, focusing on attributes that are critical for single-molecule force experiments such as linker length, monodispersity, and bioorthogonal conjugation tags. Our results demonstrate that by avoiding the ambiguous elastic response of mixed PEG/peptide systems and instead building the molecular mechanical systems with only a single bond type with uniform elastic properties, we improve data quality and facilitate data analysis and interpretation in force spectroscopy experiments. The use of all-peptide linkers allows alternative approaches for precisely defining elastic properties of proteins linked to surfaces.

Topology Engineering of Proteins in Vivo Using Genetically Encoded, Mechanically Interlocking SpyX Modules for Enhanced Stability

Recombinant proteins are traditionally limited to linear configuration. Herein, we report in vivo protein topology engineering using highly efficient, mechanically interlocking SpyX modules named AXB and BXA. SpyX modules are protein domains composed of p53dim (X), SpyTag (A), and SpyCatcher (B). The p53dim guides the intertwining of the two nascent protein chains followed by autocatalytic isopeptide bond formation between SpyTag and SpyCatcher to fulfill the interlocking, leading to a variety of backbone topologies. Direct expression of AXB or BXA produces protein catenanes with distinct ring sizes. Recombinant proteins containing SpyX modules are obtained either as mechanically interlocked obligate dimers if the protein of interest is fused to the N- or C-terminus of SpyX modules, or as star proteins if the protein is fused to both N- and C-termini. As examples, cellular syntheses of dimers of (GB1)₂ (where GB1 stands for immunoglobulin-binding domain B1 of streptococcal protein G) and of four-arm elastin-like star proteins were demonstrated. Comparison of the catenation efficiencies in different constructs reveals that BXA is generally much more effective than AXB, which is rationalized by the arrangement of three domains in space. Mechanical interlocking induces considerable stability enhancement. Both AXB and BXA have a melting point ∼20 °C higher than the linear controls and the BXA catenane has a melting point ~2 °C higher than the cyclic control BX'A. Notably, four-arm elastin-like star proteins demonstrate remarkable tolerance against trypsin digestion. The SpyX modules provide a convenient and versatile approach to construct unconventional protein topologies via the "assembly-reaction" synergy, which opens a new horizon in protein science for stability enhancement and function reinforcement via topology engineering.

Synergistic Integration of Experimental and Simulation Approaches for the de Novo Design of Silk-Based Materials

Tailored biomaterials with tunable functional properties are crucial for a variety of task-specific applications ranging from healthcare to sustainable, novel bio-nanodevices. To generate polymeric materials with predictive functional outcomes, exploiting designs from nature while morphing them toward non-natural systems offers an important strategy. Silks are Nature's building blocks and are produced by arthropods for a variety of uses that are essential for their survival. Due to the genetic control of encoded protein sequence, mechanical properties, biocompatibility, and biodegradability, silk proteins have been selected as prototype models to emulate for the tunable designs of biomaterial systems. The bottom up strategy of material design opens important opportunities to create predictive functional outcomes, following the exquisite polymeric templates inspired by silks. Recombinant DNA technology provides a systematic approach to recapitulate, vary, and evaluate the core structure peptide motifs in silks and then biosynthesize silk-based polymers by design. Post-biosynthesis processing allows for another dimension of material design by controlled or assisted assembly. Multiscale modeling, from the theoretical prospective, provides strategies to explore interactions at different length scales, leading to selective material properties. Synergy among experimental and modeling approaches can provide new and more rapid insights into the most appropriate structure-function relationships to pursue while also furthering our understanding in terms of the range of silk-based systems that can be generated. This approach utilizes nature as a blueprint for initial polymer designs with useful functions (e.g., silk fibers) but also employs modeling-guided experiments to expand the initial polymer designs into new domains of functional materials that do not exist in nature. The overall path to these new functional outcomes is greatly accelerated via the integration of modeling with experiment. In this Account, we summarize recent advances in understanding and functionalization of silk-based protein systems, with a focus on the integration of simulation and experiment for biopolymer design. Spider silk was selected as an exemplary protein to address the fundamental challenges in polymer designs, including specific insights into the role of molecular weight, hydrophobic/hydrophilic partitioning, and shear stress for silk fiber formation. To expand current silk designs toward biointerfaces and stimuli responsive materials, peptide modules from other natural proteins were added to silk designs to introduce new functions, exploiting the modular nature of silk proteins and fibrous proteins in general. The integrated approaches explored suggest that protein folding, silk volume fraction, and protein amino acid sequence changes (e.g., mutations) are critical factors for functional biomaterial designs. In summary, the integrated modeling-experimental approach described in this Account suggests a more rationally directed and more rapid method for the design of polymeric materials. It is expected that this combined use of experimental and computational approaches has a broad applicability not only for silk-based systems, but also for other polymer and composite materials.

The Coupled Bio-Chemo-Electro-Mechanical Behavior of Glucose Exposed Arterial Elastin

Elastin, the principle protein component of the elastic fiber, is a critical extracellular matrix (ECM) component of the arterial wall providing structural resilience and biological signaling essential in vascular morphogenesis and maintenance of mechanical homeostasis. Pathogenesis of many cardiovascular diseases have been associated with alterations of elastin. As a long-lived ECM protein that is deposited and organized before adulthood, elastic fibers can suffer from cumulative effects of biochemical exposure encountered during aging and/or disease, which greatly compromise their mechanical function. This review article covers findings from recent studies of the mechanical and structural contribution of elastin to vascular function, and the effects of biochemical degradation. Results from diverse experimental methods including tissue-level mechanical characterization, fiber-level nonlinear optical imaging, piezoelectric force microscopy, and nuclear magnetic resonance are reviewed. The intriguing coupled bio-chemo-electro-mechanical behavior of elastin calls for a multi-scale and multi-physical understanding of ECM mechanics and mechanobiology in vascular remodeling.