Differential scanning calorimetry studies of the inverse temperature transition of the polypentapeptide of elastin and its analogues

Real-time measure of solvation free energy changes upon liquid-liquid phase separation of α-elastin

Biological condensates are known to retain a large fraction of water to remain in a liquid and reversible state. Local solvation contributions from water hydrating hydrophilic and hydrophobic protein surfaces were proposed to play a prominent role for the formation of condensates through liquid-liquid phase separation (LLPS). However, although the total free energy is accessible by calorimetry, the partial solvent contributions to the free energy changes upon LLPS remained experimentally inaccessible so far. Here, we show that the recently developed THz calorimetry approach allows to quantify local hydration enthalpy and entropy changes upon LLPS of α-elastin in real time, directly from experimental THz spectroscopy data. We find that hydrophobic solvation dominates the entropic solvation term, whereas hydrophilic solvation mainly contributes to the enthalpy. Both terms are in the order of hundreds of kJ/mol, which is more than one order of magnitude larger than the total free energy changes at play during LLPS. However, since we show that entropy/enthalpy mostly compensates, a small entropy/enthalpy imbalance is sufficient to tune LLPS. Theoretically, a balance was proposed before. Here we present experimental evidence based on our spectroscopic approach. We finally show that LLPS can be steered by inducing small changes of solvation entropy/enthalpy compensation via concentration or temperature in α-elastin.

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

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

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.

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.

Coupling of RAFT polymerization and chemoselective post-modifications of elastin-like polypeptides for the synthesis of gene delivery hybrid vectors

Hybrid cationic ELPs for nucleic acids transport and delivery were synthetized through the coupling of RAFT polymerization and biorthogonal chemistry of ELPs, introducing a specific number of positive charges to the ELP backbone.

Design of Thermoresponsive Elastin-Like Glycopolypeptides for Selective Lectin Binding and Sorting

Selective lectin binding and sorting was achieved using thermosensitive glycoconjugates derived from recombinant elastin-like polypeptides (ELPs) in simple centrifugation-precipitation assays. A recombinant ELP, (VPGXG)₄₀, containing periodically spaced methionine residues was used to enable chemoselective postsynthetic modification via thioether alkylation using alkyne functional epoxide derivatives. The resulting sulfonium groups were selectively demethylated to give alkyne functionalized homocysteine residues, which were then reacted with azido-functionalized monosaccharides to obtain ELP glycoconjugates with periodic saccharide functionality. These modifications were also found to allow modulation of ELP temperature dependent water solubility. The multivalent ELP glycoconjugates were evaluated for specific recognition, binding and separation of the lectin Ricinus communis agglutinin (RCA₁₂₀) from a complex protein mixture. RCA₁₂₀ and ELP glycoconjugate interactions were evaluated using laser scanning confocal microscopy and dynamic light scattering. Due to the thermoresponsive nature of the ELP glycoconjugates, it was found that heating a mixture of galactose-functionalized ELP and RCA₁₂₀ in complex media selectively yielded a phase separated pellet of ELP-RCA₁₂₀ complexes. Based on these results, ELP glycoconjugates show promise as designer biopolymers for selective protein binding and sorting.

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.

Biotechnological applications of elastin-like polypeptides and the inverse transition cycle in the pharmaceutical industry

Proteins are essential throughout the biological and biomedical sciences and the purification strategies of proteins of interest have advanced over centuries. Elastin-like polypeptides (ELPs) are compound polymers that have recently been highlighted for their sharp and reversible phase transition property when heated above their lower critical solution temperature (LCST). ELPs preserve this behavior when fused to a protein, and as a result providing a simple method to isolate a recombinant ELP fusion protein from cell contaminants by taking the solution through the soluble and insoluble phase of the ELP fusion protein, a technique designated as the inverse transition cycle (ITC). ITC is considered an inexpensive and efficient way of purifying recombinant ELP fusion proteins. In addition, ELPs render recombinant fusion protein more stability and a longer clear time in blood stream, which give ELPs a lot of valuable applications in the biotechnological and pharmaceutical industry. This article reviews the modernizations of ELPs and briefly highlights on the possible use of technologies such as the automatic piston discharge (APD) centrifuges to improve the efficiency of the ITC in the pharmaceutical industry to obtain benefits.

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.

Pi-Pi contacts are an overlooked protein feature relevant to phase separation

Protein phase separation is implicated in formation of membraneless organelles, signaling puncta and the nuclear pore. Multivalent interactions of modular binding domains and their target motifs can drive phase separation. However, forces promoting the more common phase separation of intrinsically disordered regions are less understood, with suggested roles for multivalent cation-pi, pi-pi, and charge interactions and the hydrophobic effect. Known phase-separating proteins are enriched in pi-orbital containing residues and thus we analyzed pi-interactions in folded proteins. We found that pi-pi interactions involving non-aromatic groups are widespread, underestimated by force-fields used in structure calculations and correlated with solvation and lack of regular secondary structure, properties associated with disordered regions. We present a phase separation predictive algorithm based on pi interaction frequency, highlighting proteins involved in biomaterials and RNA processing.

Rational design of new materials using recombinant structural proteins: Current state and future challenges

Sequence-definable polymers are seen as a prerequisite for design of future materials, with many polymer scientists regarding such polymers as the holy grail of polymer science. Recombinant proteins are sequence-defined polymers. Proteins are dictated by DNA templates and therefore the sequence of amino acids in a protein is defined, and molecular biology provides tools that allow redesign of the DNA as required. Despite this advantage, proteins are underrepresented in materials science. In this publication we investigate the advantages and limitations of using proteins as templates for rational design of new materials.

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.

Surface functionalization superparamagnetic nanoparticles conjugated with thermoresponsive poly(epsilon-lysine) dendrons tethered with carboxybetaine for the mild hyperthermia-controlled delivery of VEGF

Vascular endothelial growth factor (VEGF) is the growth factor responsible for the triggering of angiogenesis, the process of blood vessel formation supporting the long-term viability of any repaired or regenerated tissue. As the growth factor is effective only when concentration gradients are generated, new shuttles need to be developed that ensure both the control of gradients at the site of tissue repair and the release of VEGF at physiological levels. Magnetic hyperthermia is the production of heat induced by magnetic materials through their exposure to an external oscillating magnetic field. In this paper, magnetic nanoparticles capable of generating controllable hyperthermia were functionalised with hyperbranched poly(epsilon-lysine) peptides integrating in their core parallel thermoresponsive elastin-like peptide sequences and presenting an uppermost branching generation tethered by the zwitterionic amino acid carboxybetaine. The results show that these functionalised magnetic nanoparticles avidly bind VEGF and release it only upon generation of mild-hyperthermic pulses generated by oscillating magnetic filed. The VEGF release occurred in a temperature range at which the elastin-like peptides collapse. It is proposed that, through the application of an external magnetic field, these magnetic carriers could generated gradients of VEGF in vivo and allow its tuned delivery in a number of clinical applications.

STATEMENT OF SIGNIFICANCE

The present paper for the first time reveals the possibility to control the delivery of VEGF through mild hyperthermia stimuli generated by a oscillating magnetic field. To this purpose, magnetic nanoparticles of high size homogeneity and coated with a thin coating of poly(acrylic acid) were functionalised with a novel class of poly(epsilon lysine) dendrimers integrating in their structure a thermoresponsive amino acid sequence mimicking elastin and exposing at high density a zwitterionic modified amino acid, the carboxybetaine, known to be able to bind macromolecules. Physicochemical and biochemical characterisation elegantly show the link between the thermal properties of the nanoparticles and of the dendrimer change of conformation and how this enable the release of VEGF at temperature values compatible with the growth factor stability.

Biocompatibility of the Bioelastic Materials, Poly(GVGVP) and Its γ-Irradiation Cross-Linked Matrix: Summary of Generic Biological Test Results

The complete series of the recommended generic biological tests for materials and devices in contact with tissues and tissue fluids and blood have been carried out by an independent testing laboratory on the elastic protein-based (bioelastic) polymer, Poly(L-Val1-L-Pro2-Gly3-L-Val 4-Gly5) with a degree of polymerization greater than 120, and its 20 Mrad γ-irradiation cross linked elastic matrix, X20-poly(VPGVG). The specific tests and the summarized results given in parentheses are: (1) the Ames mutagenicity test (non- mutagenic), (2) cytotoxicity-agarose overlay (non-toxic), (3) acute systemic tox icity (non-toxic), (4) intracutaneous toxicity (non-toxic), (5) muscle implantation (favorable), (6) acute intraperitoneal toxicity (non-toxic), (7) systemic antigenic ity (non-antigenic), (8) dermal sensitization—the Magnusson and Kligman maximization method (non-sensitizing), (9) pyrogenicity (non-pyrogenic), (10) Lee White clotting study (normal clotting time), and (11) in vitro hemolysis test (non-hemolytic). Thus, this new elastomeric polypeptide biomaterial which is based on the most striking repeating sequence in the mammalian elastic fiber exhibits an extraordinary biocompatibility. This parent bioelastic material and a wide range of component peptide variations are under development for an equally wide range of potential medical applications such as prevention of adhesions, drug delivery, and synthetic arteries.

Elastin-like polypeptide based nanoparticles: design rationale toward nanomedicine

Elastin-like polypeptides (ELPs) are characterized by a high sequence control, temperature responsiveness and biocompatibility, which make them highly interesting as smart materials for application in nanomedicine. In particular the construction of ELP-based nanoparticles has recently become a focal point of attention in materials research. This review will give an overview of the ELP-based nanoparticles that have been developed until now and their underlying design principles. First a short introduction on ELPs and their stimulus-responsive behavior will be given. This characteristic has been applied for the development of ELP-based block copolymers that can self-assemble into nanoparticles. Both the fully ELP-based as well as several ELP hybrid materials that have been reported to form nanoparticles will be discussed, which is followed by a concise description of the promising biomedical applications reported for this class of materials.

The fusions of elastin-like polypeptides and xylanase self-assembled into insoluble active xylanase particles

We fused the genes of elastin-like polypeptides (ELPs) and xylanase and then expressed them in Escherichia coli. Unexpectedly, the fusion proteins self-assembled into insoluble active particles as the ELPs underwent a hardly reversible phase transition. The specific activity of the particles was 92% of the native counterparts, which means it can act as a pull-down handler for converting soluble proteins into active aggregates. We evaluated the characterizations of the insoluble active xylanase particles in detail and the results were encouraging. The pH optimum (6.0) of the particles was the same as the free one, but the optimum pH range was 5-7, while the free xylanase was 6-7. The free xylanase had an optimum temperature of 50°C, whereas the insoluble active xylanase particles shifted to 70°C. The pH stability, thermostability and storage stability of the xylanase particles increased significantly when compared with the free xylanase. We also observed an increase of the Km values of the free xylanase from 0.374gL(-1) to 0.980gL(-1) at the insoluble state. The considerable higher activity and stability of the xylanase particles were much like immobilized xylanases and could be valuable for its industrial application.

Conformational transitions of the cross-linking domains of elastin during self-assembly

Elastin is the intrinsically disordered polymeric protein imparting the exceptional properties of extension and elastic recoil to the extracellular matrix of most vertebrates. The monomeric precursor of elastin, tropoelastin, as well as polypeptides containing smaller subsets of the tropoelastin sequence, can self-assemble through a colloidal phase separation process called coacervation. Present understanding suggests that self-assembly is promoted by association of hydrophobic domains contained within the tropoelastin sequence, whereas polymerization is achieved by covalent joining of lysine side chains within distinct alanine-rich, α-helical cross-linking domains. In this study, model elastin polypeptides were used to determine the structure of cross-linking domains during the assembly process and the effect of sequence alterations in these domains on assembly and structure. CD temperature melts indicated that partial α-helical structure in cross-linking domains at lower temperatures was absent at physiological temperature. Solid-state NMR demonstrated that β-strand structure of the cross-linking domains dominated in the coacervate state, although α-helix was predominant after subsequent cross-linking of lysine side chains with genipin. Mutation of lysine residues to hydrophobic amino acids, tyrosine or alanine, leads to increased propensity for β-structure and the formation of amyloid-like fibrils, characterized by thioflavin-T binding and transmission electron microscopy. These findings indicate that cross-linking domains are structurally labile during assembly, adapting to changes in their environment and aggregated state. Furthermore, the sequence of cross-linking domains has a dramatic effect on self-assembly properties of elastin-like polypeptides, and the presence of lysine residues in these domains may serve to prevent inappropriate ordered aggregation.

Spheroid organization kinetics of H35 rat hepatoma model cell system on elastin-like polypeptide-polyethyleneimine copolymer substrates

Though two-dimensional systems have yielded some success in deriving morphological and functional markers of hepatocyte culture, they largely fail to capture the three-dimensional organization, long-term viability, and functionality of the hepatic tissue. We have engineered a system for inducing self-assembly of model H35 rat hepatoma spheroids using a copolymer comprised of biocompatible elastin-like polypeptide (ELP) chemically conjugated to positively charged polyethyleneimine (PEI). We have achieved a conjugation ratio of 30 mol %, though our studies analyzing spheroid organization kinetics indicate conjugate ratios of 5 mol % and greater to be optimal for cell culture based on least variability in spheroid sizes and minimum incidence of overgrown aggregates. Furthermore, our ELP-PEI system indicated the potential for influencing ultimate spheroid dimensions, with spheroid size inversely related to polyelectrolyte conjugation. Overall, this study provides a good starting point to investigate functional correlations between spheroid size and functional markers and their future use as an in vitro diagnostic or tissue engineering tool.

Enzymatic cross-linking of human recombinant elastin (HELP) as biomimetic approach in vascular tissue engineering

The use of polymers naturally occurring in the extracellular matrix (ECM) is a promising strategy in regenerative medicine. If compared to natural ECM proteins, proteins obtained by recombinant DNA technology have intrinsic advantages including reproducible macromolecular composition, sequence and molecular mass, and overcoming the potential pathogens transmission related to polymers of animal origin. Among ECM-mimicking materials, the family of recombinant elastin-like polymers is proposed for drug delivery applications and for the repair of damaged elastic tissues. This work aims to evaluate the potentiality of a recombinant human elastin-like polypeptide (HELP) as a base material of cross-linked matrices for regenerative medicine. The cross-linking of HELP was accomplished by the insertion of cross-linking sites, glutamine and lysine, in the recombinant polymer and generating ε-(γ-glutamyl) lysine links through the enzyme transglutaminase. The cross-linking efficacy was estimated by infrared spectroscopy. Freeze-dried cross-linked matrices showed swelling ratios in deionized water (≈2500%) with good structural stability up to 24 h. Mechanical compression tests, performed at 37°C in wet conditions, in a frequency sweep mode, indicated a storage modulus of 2/3 kPa, with no significant changes when increasing number of cycles or frequency. These results demonstrate the possibility to obtain mechanically resistant hydrogels via enzymatic crosslinking of HELP. Cytotoxicity tests of cross-linked HELP were performed with human umbilical vein endothelial cells, by use of transwell filter chambers for 1-7 days, or with its extracts in the opportune culture medium for 24 h. In both cases no cytotoxic effects were observed in comparison with the control cultures. On the whole, the results suggest the potentiality of this genetically engineered HELP for regenerative medicine applications, particularly for vascular tissue regeneration.

Coacervation of tropoelastin

The coacervation of tropoelastin represents the first major stage of elastic fiber assembly. The process has been modeled in vitro by numerous studies, initially with mixtures of solubilized elastin, and subsequently with synthetic elastin peptides that represent hydrophobic repeat units, isolated hydrophobic domains, segments of alternating hydrophobic and cross-linking domains, or the full-length monomer. Tropoelastin coacervation in vitro is characterized by two stages: an initial phase separation, which involves a reversible inverse temperature transition of monomer to n-mer; and maturation, which is defined by the irreversible coalescence of coacervates into large species with fibrillar structures. Coacervation is an intrinsic ability of tropoelastin. It is primarily influenced by the number, sequence, and contextual arrangement of hydrophobic domains, although hydrophilic sequences can also affect the behavior of the hydrophobic domains and thus affect coacervation. External conditions including ionic strength, pH, and temperature also directly influence the propensity of tropoelastin to self-associate. Coacervation is an endothermic, entropically-driven process driven by the cooperative interactions of hydrophobic domains following destabilization of the clathrate-like water shielding these regions. The formation of such assemblies is believed to follow a helical nucleation model of polymerization. Coacervation is closely associated with conformational transitions of the monomer, such as increased β-structures in hydrophobic domains and α-helices in cross-linking domains. Tropoelastin coacervation in vivo is thought to mainly involve the central hydrophobic domains. In addition, cell-surface glycosaminoglycans and microfibrillar proteins may regulate the process. Coacervation is essential for progression to downstream elastogenic stages, and impairment of the process can result in elastin haploinsufficiency disorders such as supravalvular aortic stenosis.

Hierarchical and Modulable Hydrophobic Folding and Self-assembly in Elastic Protein-based Polymers: Implications for Signal Transduction

When the hydrophobic (apolar) and polar moieties of elastomeric polypeptides are properly balanced, the polypeptides are soluble in water at lower temperatures but undergo folding and assembly transitions to increased order on raising the temperature. The temperatures, Tt, and heats, ΔHt, of these inverse temperature transitions are determined by differential scanning calorimetry for a series of elastomeric polypentapeptides: poly(VPAVG), poly(IPAVG), poly(VPGVG), poly(IPGVG), poly[0.5(VPGVG),0.5(IPGVG)] and poly[0.82(IPGVG),0.18(IPGEG)] where V = Val, P = Pro, A = Ala, G = Gly, I = lle and E = Glu.

On increasing the hydrophobicity as when replacing V(Val) by I(lle) which is the addition of one CH2 moiety per pentamer, the temperature of the transition is lowered by 15 to 20°C and the heat of the transition is increased by more than one kcal/mole, for the above examples, by more than a factor of two.

When differential scanning calorimetry thermograms are obtained on mixtures of poly(VPAVG) plus poly(IPAVG) or of poly(VPGVG) plus poly(IPGVG), it is found that the polypentapeptides self-separate, i.e., they de-mix, even though in the latter case the conformations have been shown to be essentially identical before and after their respective transitions.

When the polymer, poly[0.82(IPGVG),0.18(IPGEG)], is studied as a function of pH, increasing the degree of ionization is found to increase the temperature and to decrease the heat of the transition such that, with the correct balance of I with the variable E(GluCOO−), the values of Tt and ΔHt can be made to approach those of poly(VPGVG). Acid-base titration studies indicate that less than one Glu(COO−) in 200 residues can raise the value of Tt by 25°C and decrease ΔHt by 90%.

These and additional data are interpreted to mean that there exists an hierarchical hydrophobic folding, that the hierarchical hydrophobic folding can be modulated by changing the degree of ionization or by changes in a number of intensive variables, that changes in these intensive variables can be used to drive folding/unfolding-assembly/disassembly transitions under isothermal conditions, and that these unfolding/folding and disassembly/assembly transitions can be used to achieve signal transduction. This is called the ΔTt mechanism of free energy (signal) transduction.

Elastin-like polypeptides as a purification tag for recombinant proteins

This unit presents a recombinant protein purification method that employs an elastin-like polypeptide (ELP) as a purification tag. ELPs undergo a sharp and reversible phase transition when heated above their lower critical solution temperature. ELPs retain this behavior when they are fused to a protein, and thereby provide a simple method to isolate a recombinant ELP fusion protein from cell contaminants by cycling the solution through the insoluble and soluble phase of the ELP fusion protein using a procedure that is termed Inverse Transition Cycling. This method does not require the use of chromatography, so it is cost-effective, easy to scale up, and easy to multiplex.

Cellular response to nanoscale elastin-like polypeptide polyelectrolyte multilayers

Ionic elastin-like polypeptide (ELP) conjugates are a new class of biocompatible, self-assembling biomaterials. ELPs composed of the repeat unit (GVGVP)(n) are derived from the primary sequence of mammalian elastin and produced in Escherichia coli. These biopolymers exhibit an inverse transition temperature that renders them extremely useful for applications in cell-sheet engineering. Cationic and anionic conjugates were synthesized by the chemical coupling of ELP to polyethyleneimine (PEI) and polyacrylic acid (PAA). The self-assembly of ELP-PEI and ELP-PAA using the layer-by-layer deposition of alternately charged polyelectrolytes is a simple, versatile technique to generate bioactive and biomimetic surfaces with the ability to modulate cell-substratum interactions. Our studies are focused on cellular response to self-assembled multilayers of ionic (GVGVP)(40) incorporated within the polymeric sequence H(2)N-MVSACRGPG-(GVGVP)(40)-WP-COOH. Angle-dependent XPS studies indicated a difference in the chemical composition at the surface ( approximately 10A below the surface) and subsurface regions. These studies provided additional insight into the growth of the nanoscale multilayer assembly as well as the chemical environment that the cells can sense. Overall, cellular response was enhanced on glass substrata coated with ELP conjugates compared with uncoated surfaces. We report significant differences in cell proliferation, focal adhesions and cytoskeletal organization as a function of the number of bilayers in each assembly. These multilayer assemblies have the potential to be successfully utilized in the rational design of coatings on biomaterials to elicit a desired cellular response.

Recombinant protein purification by self-cleaving aggregation tag

A simple technique is presented for non-chromatographic purification of recombinant proteins expressed in Escherichia coli. This method is based on a reversibly precipitating, self-cleaving purification tag. The tag is made up of two components: an elastin-like polypeptide (ELP), which reversibly self-associates in high-salt buffers at temperatures above 30 degrees C; and an intein, which causes the ELP tag to self-cleave in response to a mild pH shift. Thus, a tripartite ELP-intein-target protein precursor can be purified by cycles of salt addition, heating and centrifugation. Once purified, intein-mediated self-cleavage, followed by precipitation of the cleaved ELP tag, allows easy and effective isolation of the pure, native target protein without the need for chromatographic separations. Recoveries of 50-100 mg of cleaved, native target protein per liter of shake-flask culture have been achieved for over a dozen proteins, typically in 8-24 h depending on specific process parameters.

Thermal denaturation of a recombinant mouse amelogenin: circular dichroism and differential scanning calorimetric studies

Conformational analyses of a recombinant mouse tooth enamel amelogenin (rM179) were performed using circular dichroism (CD), fluorescence, differential scanning calorimetry, and sedimentation equilibrium studies. The results show that the far-UV CD spectra of rM179 at acidic pH and 10 degrees C are different from the spectra of random coil in 6 M GdnHCl. A near-UV CD spectrum of rM179 at 10 degrees C is similar to that of rM179 in 6 M GdnHCl, which indicates that aromatic residues of native structure are exposed to solvent and rotate freely. Far-UV CD values of rM179 at 80 degrees C are different from that of random-coil structure in 6 M GdnHCl, which suggests that rM179 at 80 degrees C has specific secondary structures. A gradual thermal transition was observed by far-UV CD, which is interpreted as a weak cooperative transition from specific secondary structures to other specific secondary structures. The fluorescence emission maximum for the spectrum due to Trp residues in rM179 at 10 degrees C shows the same fluorescence emission maximum as rM179 in 6 M GdnHCl and amino acid Trp, which indicates that the three Trp in rM179 are exposed to solvent. Deconvolution of differential scanning calorimetry curve gives the population of three states (A, I, and C states). These results indicate that three states (A, I, and C) have specific secondary structures, in which hydrophobic and Trp residues are exposed to the solvent. The thermodynamic characteristics of rM179 are unique and different from a typical globular protein, proline-rich peptides, and a molten globule state.

Temperature-responsive stationary phase utilizing a polymer of proline derivative for hydrophobic interaction chromatography using an aqueous mobile phase

A new method of chromatography is proposed, utilizing a thermo-responsive polymer carrying an amino acid ester residue for the stationary phase of high-performance liquid chromatography (HPLC). We have been investigating the new concept of chromatography, a temperature-responsive chromatography, using temperature-responsive poly(N-isopropylacrylamide) (PNIPAAm)-modified surface for HPLC with a constant aqueous media as the mobile phase. In this study, we designed and synthesized thermo-responsive poly(acryloyl-L-proline methyl ester) and its copolymer with N-isopropylacrylamide (NIPAAm). Homopolymers of acryloyl-L-proline methyl ester and copolymer were prepared by the reaction of radical telomerization. These polymers underwent a reversible phase transition from water-soluble forms into aggregates by changing the temperature, similar to PNIPAAm. The surface properties and functions of stationary phases modified with poly(acryloyl-L-proline methyl ester) were controlled by the external temperature. In the chromatographic system, we separated steroids and amino acids with a variety of hydrophobicities using a sole aqueous mobile phase. In contrast to a PNIPAAm-modified surface, a poly(acryloyl-L-proline methyl ester)-modified surface showed a greater affinity for hydrophobic amino acids.

Mechanism for the Phase Transition of a Genetically Engineered Elastin Model Peptide (VPGIG)40in Aqueous Solution

The concentration dependence of the pressure- and temperature-induced cloud point transition (Pc and Tc, respectively) of aqueous solutions of an elastin-like polypeptide with a repeating pentapeptide Val-Pro-Gly-Ile-Gly sequence (MGLDGSMG(VPGIG)40VPLE) was investigated by using apparent light scattering, differential scanning calorimetry, and circular dichroism methods. In addition, the effects of salts and surfactants on these properties were investigated. The Pc and Tc of the present peptide in aqueous solution were strongly concentration dependent. The calorimetric measurements showed that the enthalpy of transitions was 300-400 kJ/mol, i.e., 7-10 kJ/mol per VPGIG pentamer. The Tc of the (VPGIG)40 solution was highly affected by the addition of inert salts or SDS. The effects of salts were consistent with those observed in the lyotropic series or Hoffmeister series. The CD spectrum at low peptide concentrations indicated that the present peptide forms type II beta-turn-like structure(s) at higher temperatures, but the temperature dependence of random coil diminishment (195 nm) and beta-turn formation (210 nm) were not exactly coincident. A hypothetical mechanism of the (VPGIG)40 phase transition that could account for these observations was postulated. Observations suggest that the temperature-responsive properties of the elastin model peptides occur via a mechanism involving conformational change-association-aggregation and that the first two are strongly interactive.

Genetic synthesis and characterization of pH- and temperature-sensitive silk-elastinlike protein block copolymers

The purpose of this work was to synthesize and characterize a pH- and temperature-sensitive block copolymer containing repeating sequences from silk (Gly-Ala-Gly-Ala-Gly-Ser) and elastin (Gly-Val-Gly-Val-Pro) protein. The monomer contained one repeat of silk and eight repeat units of elastin, with the first valine in one of the elastin repeats being replaced by glutamic acid. The copolymer was synthesized using genetic engineering techniques. The sensitivity of the copolymer to pH and temperature was examined at various polymer concentrations and ionic strengths. Turbidity measurements were carried out over a temperature range of 20 to 100 degrees C at various pH, concentration, and ionic strength values. The introduction of an ionizable residue (glutamic acid) rendered the copolymer sensitive to changes in pH. The transition termperature (T(t)), the temperature at which the polymer became insoluble upon increase in temperature, was modulated by changing the pH. In general, the T(t) value, was found: (1) to increase with an increase in pH, (2) to decrease with increasing ionic strength, and (3) to decrease with increasing concentration. Results of these studies suggest that by strategic placement of charged amino acids in genetically engineered silk-elastinlike protein block copolymers it is possible to precisely control sensitivity to stimuli such as pH and temperature.

Effects of temperature and pressure on the aggregation properties of an engineered elastin model polypeptide in aqueous solution

The pressure and temperature dependence of the cloud point transition of an aqueous solution of an elastin-like polypeptide (MGLDGSMG(VPGIG)40VPLE), prepared by bacterial expression of the corresponding artificial gene, was measured. A temperature-pressure diagram was constructed over a wide range of conditions. The (VPGIG)40 solution exhibited a well-defined pressure-induced cloudpoint (Pc), as well as a temperature-induced transition (Tc). From near atmospheric pressure up to 100 MPa, Tc increased with increasing pressure, but decreased with further increases in pressure above 200 MPa. The maximum Tc was reached at 100-200 MPa. Between 10 and 25 degrees C, the Pc decreased with increasing temperature, and a broad maximum in Pc was observed in the range -10 to 0 degree C. These results are compared with our previous results on synthetic thermoresponsive vinyl polymers.

Differential scanning calorimetry study of the hydrophobic hydration of the elastin-based polypentapeptide, poly(VPGVG), from deficiency to excess of water

The polypentapeptide of elastin, poly(VPGVG), has become an interesting model polypeptide in understanding the mechanism of protein folding and assembly. Due to its simple amino acid composition and the predominance of apolar side chains, this polymer shows strong hydrophobic-hydration phenomena. This paper explores, by calorimetric methods, the nature and structure of the clathrate-like arrangements that take place, surrounding the apolar side chains of the polymer. The performance of these methods, especially differential scanning calorimetry, has a well-gained reputation. In this work, the development of the clathrate-like structures around this model polymer has been followed from water deficiency to water-excess states. Two main conclusions have been obtained from the data obtained. First, there is an upper limit of about 170 water molecules per pentamer as the number of water molecules required to form all the possible clathrate-like structures. Second, these structures exist as an inhomogeneous population with energies spreading in a significantly broad range, which is likely related to differences in geometrical parameters (bond lengths and angles) of the clathrate structure.

Characterizations of critical processes in liquid-liquid phase separation of the elastomeric protein-water system: microscopic observations and light scattering measurements

Biological self-assembly process of tropoelastin in an extracellular space, viewed as a key step of the elastogenesis, can be mimicked by the temperature-dependent coacervation of the elastin-related polypeptide-water system. Early and late stages of the phase separation behavior of the bovine neck ligamental alpha-elastin-water system were examined respectively by the laser light scattering photometry and phase contrast microscopy. Changes in the hydrodynamic size of molecular assemblies and visible microcoacervate droplet size were traced as a function of the concentration of alpha-elastin and temperature. Near the critical point, alpha-elastin concentration of 0.11 mg/mL and temperature of 21.5 degrees C, the phase separation was initiated after fast increase of the hydrodynamic size of primary aggregates as scattering particles and followed by the appearance of larger microcoacervate droplets with a broad size distribution. Whereas in the off-critical region, slow decrease of the hydrodynamic size of primary particles induced phase separation with smaller droplets of a narrow size distribution. Observation of the phase separation processes in the alpha-elastin-water system with metal chlorides and hydrophobic synthetic model polypeptide-water system indicated that the fast and slow molecular assembly processes were based on the fundamental hydrophobic interactions and involvements of electrostatic interactions between charged amino acid residues, respectively.

Short elastin-like peptides exhibit the same temperature-induced structural transitions as elastin polymers: implications for protein engineering

Elastin is a major protein component of the vascular wall and is responsible for its unusual elastic properties. Polymers of its repeating VPGVG sequences have been synthesised and shown to exhibit an inverse temperature transition where, as temperature rises, the polymer collapses from an extended chain to a beta-spiral structure with three VPGVG units per turn, each pentamer adopting a type II beta-turn conformation. These studies, however, have not established whether the temperature-driven conformational change is an intrinsic property of the individual pentameric sequences or a global, co-operative effect of many pentamers within the beta-spiral structure. Here, we examine by circular dichroism the behaviour of elastin-like peptides (VPGVG)n, where n varies between 1 and 5. Remarkably, we find that all lengths of peptide undergo an extended left and right arrow beta-turn transition with increasing temperature, suggesting that the induction of the beta-spiral occurs at the level of single pentameric units. The origin of this effect is a positive DeltaS term for the transition. At 35 degreesC, the average transition midpoint temperature, the value of TDeltaS is about 15 kcal mol-1. With larger oligomers (n=3), there is only a modest rise in DeltaS, suggesting that the dominant entropic effect resides within the monomer and that interactions between these units make only a small contribution to the energetics of the transition. Charges at the termini, and residue replacements or additions, regulate the transitions for the short peptides in a manner similar to that observed for the longer polymers. The behaviour of the same peptides in trifluoroethanol and SDS solutions is consistent with formation of the beta-turn being driven by interactions between non-polar groups. The significance of this behaviour for the rational design of temperature-induced responses in proteins is discussed.

Penetration of dimyristoylphosphatidylcholine monolayers and bilayers by model beta-blocker agents of varying lipophilicity

Penetration of model beta-blockers, propranolol, oxprenolol, metaprolol, and nadolol, into dimyristoylphosphatidylcholine (DMPC) monolayers cast on a pH 7.4 phosphate buffer (mu = 0.155 adjusted with NaCl) at 25 degreesC was monitored using a film balance equipped with a Wilhelmy plate for measuring changes in surface pressure. Drug solution (pH 7.4) is injected below the surface of the monolayer. The difference in surface pressure, Delta pi, for each drug concentration added to the monolayer was measured at equilibrium. Delta pi increased with increasing drug concentration. Consistent with the relative lipophilicities, the Delta pi vs drug concentration slopes were as follows: propranolol > metaprolol > oxprenolol > nadolol. The intrinsic surface activity of the beta-blockers was also determined in the absence of the lipid. Differential scanning calorimetry (DSC) measurements were also made on DMPC bilayers in the above buffer. DMPC suspended in buffered drug solutions were scanned over a temperature range of 5 degrees to 40 degreesC at a scan rate of 0.091 degreesC/min. The DSC studies indicate that the DMPC thermotropic phase behavior is modulated by these compounds as follows: propranolol >> metaprolol congruent with oxprenolol > nadolol which agrees with reported partition coefficients as well as the above Delta pi observations. However, an accounting of the intrinsic surface activity of these compounds results in a lower than expected affinity for the DMPC monolayer.

Coacervation characteristics of recombinant human tropoelastin

Coacervation of soluble tropoelastin molecules is characterized by thermodynamically reversible association as temperature is increased under appropriately juxtaposed ionic conditions, protein concentration and pH. Coacervation plays a critical role in the assembly of these elastin precursors in elastic fiber formation. To examine the effect of physiological parameters on the ability of tropoelastin molecules to associate, solutions of recombinant human tropoelastin were monitored spectrophotometrically by light scattering over a broad range of temperatures. Coacervation of recombinant human tropoelastin is strongly influenced by the concentration of protein and NaCl and to a lesser extent on pH. Trends towards maximal association are apparent when each of these parameters is varied. Remarkably, optimal coacervation is found at 37 degrees C, 150 mM NaCl and pH 7-8. Using the data generated by time courses, estimates of thermodynamic parameters were made. These estimates confirm that coacervation is endothermic and is marked by a strong entropic contribution. Circular dichroism of recombinant human tropoelastin revealed that, rather than being random, the structure is compatible with being largely that, of an all-beta protein (with secondary structure estimated to be 3% alpha-helix, 41% beta-sheet, 21% beta-turn and 33% other), exhibiting a spectrum as previously seen for tropoelastin populations and soluble elastin from naturally-derived sources.

Synthesis and characterization of the human elastin W4 sequence

Following the nomenclature of Sandberg, the W4 sequence of human elastin, [sequence: see text], has been synthesized by solid-phase methods and characterized by carbon-13 nuclear magnetic resonance, amino-acid analysis, mass spectra and elemental analysis. This sequence was then polymerized to greater than 50 kDa as determined by retention in 50 kDa molecular weight cut-off dialysis tubing. It has been successfully cross-linked by gamma-irradiation (20 Mrad) to form an elastomeric matrix, designated as X20-poly(W4). Physical characterizations such as stress/strain, thermolelasticity, acid-base titration and inverse temperature transition studies have been carried out on this elastomer, which is homologous to the striking, poly(VPGVG), W4 sequence of bovine and porcine elastins. These results are compared with previous results on the polypentapeptide of elastin, (VPGVG)n, and it has been demonstrated that X20-poly(W4) also is a dominantly entropic elastomer. Finally, the working model for the structure of this human elastin sequence was derived computationally using molecular mechanics and dynamics calculations. Thus the human W4 sequence appears to be structurally and functionally equivalent to the bovine and porcine W4 sequences in spite of the less regular repeating pentamer sequence.

Self-assembly of bioelastomeric structures from solutions: mean-field critical behavior and Flory-Huggins free energy of interactions

Elastic and quasi-elastic light scattering studies were performed on aqueous solutions of poly(Val-Pro-Gly-Gly), a representative synthetic bioelastomer that differs from the previously studied poly(Val-Pro-Gly-Val-Gly) by the deletion of the hydrophobic Val in position four. When the spinodal line was approached from the region of thermodynamic stability, the intensity of light scattered by fluctuations, and the related lifetime and correlation length, were observed to diverge with mean-field critical exponents for both systems. Fitting of the experimental data allowed determining the spinodal and binodal (coexistence) lines that characterize the phase diagrams of the two systems, and it also allowed a quantitative sorting out of the enthalpic and entropic contributions to the Flory-Huggins interaction parameters. The contribution of valine is derived by comparison of the two cases. This can be viewed as sorting out the effect of a modulation of the solute. The same approach may allow sorting out the entropic and enthalpic effect of modulations of the solvent by cosolutes (or by cosolvents). This could be of particular interest in the case of small osmolytes, affording important adaptive roles in nature, at the cost of very limited changes in genetic information. Finally, the suggestion is further supported that statistical fluctuations of anomalous amplitude, such as those occurring in proximity of the spinodal line, have a role in promoting the process of self-assembly of extended supramolecular structures. On the practical side, the present approach appears useful in the design of novel synthetic model systems for bioelastomers.