Self-Assembly of Stimuli-Responsive Biohybrid Synthetic-b-Recombinant Block Copolypeptides

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.

Protease-Driven Phase Separation of Elastin-Like Polypeptides

Elastin-like polypeptides (ELPs) are a promising material platform for engineering stimuli-responsive biomaterials, as ELPs undergo phase separation above a tunable transition temperature. ELPs with phase behavior that is isothermally regulated by biological stimuli remain attractive for applications in biological systems. Herein, we report protease-driven phase separation of ELPs. Protease-responsive "cleavable" ELPs comprise a hydrophobic ELP block connected to a hydrophilic ELP block by a protease cleavage site linker. The hydrophilic ELP block acts as a solubility tag for the hydrophobic ELP block, creating a temperature window in which the cleavable ELP reactant is soluble and the proteolytically generated hydrophobic ELP block is insoluble. Within this temperature window, isothermal, protease-driven phase separation occurs when a critical concentration of hydrophobic cleavage product accumulates. Furthermore, protease-driven phase separation is generalizable to four compatible protease-cleavable ELP pairings. This work presents exciting opportunities to regulate ELP phase behavior in biological systems using proteases.

Supramolecular Assembly and Thermogelation Strategies Using Peptide-Polymer Conjugates

Peptide-polymer conjugates (PPCs) are of particular interest in the development of responsive, adaptive, and interactive materials due to the benefits offered by combining both building blocks and components. This review presents pioneering work as well as recent advances in the design of peptide-polymer conjugates, with a specific focus on their thermoresponsive behavior. This unique class of materials has shown great promise in the development of supramolecular structures with physicochemical properties that are modulated using soft and biorthogonal external stimuli. The temperature-induced self-assembly of PPCs into various supramolecular architectures, gelation processes, and tuning of accessible processing parameters to biologically relevant temperature windows are described. The discussion covers the chemical design of the conjugates, the supramolecular driving forces involved, and the mutual influence of the polymer and peptide segments. Additionally, some selected examples for potential biomedical applications of thermoresponsive PPCs in tissue engineering, delivery systems, tumor therapy, and biosensing are highlighted, as well as perspectives on future challenges.

Thermoresponsive Core-cross-linked Nanoparticles from HA-b-ELP Diblock Copolymers

Stabilization against the dilution-dependent disassembly of self-assembled nanoparticles is a requirement for in vivo application. Herein, we propose a simple and biocompatible cross-linking reaction for the stabilization of a series of nanoparticles formed by the self-assembly of amphiphilic HA-b-ELP block copolymers, through the alkylation of methionine residues from the ELP block with diglycidyl ether compounds. The core-cross-linked nanoparticles retain their colloidal properties, with a spherical core-shell morphology, while maintaining thermoresponsive behavior. As such, instead of a reversible disassembly when non-cross-linked, a reversible swelling of nanoparticles' core and increase of hydrodynamic diameter are observed with lowering of the temperature.

Emerging opportunities in bioconjugates of Elastin-like polypeptides with synthetic or natural polymers

Nature is an everlasting source of inspiration for chemical and polymer scientists seeking to develop ever more innovative materials with greater performances. Natural structural proteins are particularly scrutinized to design biomimetic materials. Often characterized by repeat peptide sequences, that together interact by inter- and intramolecular interactions and form a 3D skeleton, they contribute to the mechanical properties of individual cells, tissues, organs, and whole organisms. (Numata, K. Polymer Journal 2020, 52, 1043-1056) Among them elastin, and its main repeat sequences, have been a source of intense studies for more than 50 years resulting in the specific research field dedicated to elastin-like polypeptides (ELPs). These are currently widely investigated in different applications, namely protein purification, tissue engineering, and drug delivery, and some technologies based on ELPs are currently explored by several start-up companies. In the present review, we have summarized pioneering contributions on ELPs, progress made in their genetic engineering, and understanding of their thermal behavior and self-assembly properties. Considered as intrinsically disordered protein polymers, we have finally focused on the works where ELPs have been conjugated to other synthetic macromolecules as covalent hybrid, statistical, graft, or block copolymers, highlighting the huge opportunities that have still not been explored so far.

Adaptive Recombinant Nanoworms from Genetically Encodable Star Amphiphiles

Recombinant nanoworms are promising candidates for materials and biomedical applications ranging from the templated synthesis of nanomaterials to multivalent display of bioactive peptides and targeted delivery of theranostic agents. However, molecular design principles to synthesize these assemblies (which are thermodynamically favorable only in a narrow region of the phase diagram) remain unclear. To advance the identification of design principles for the programmable assembly of proteins into well-defined nanoworms and to broaden their stability regimes, we were inspired by the ability of topologically engineered synthetic macromolecules to acess rare mesophases. To test this design principle in biomacromolecular assemblies, we used post-translational modifications (PTMs) to generate lipidated proteins with precise topological and compositional asymmetry. Using an integrated experimental and computational approach, we show that the material properties (thermoresponse and nanoscale assembly) of these hybrid amphiphiles are modulated by their amphiphilic architecture. Importantly, we demonstrate that the judicious choice of amphiphilic architecture can be used to program the assembly of proteins into adaptive nanoworms, which undergo a morphological transition (sphere-to-nanoworms) in response to temperature stimuli.

Fibril Assembly and Gelation of Macromolecules with Amphiphilic Repeating Units

This paper reports the self-assembly of the fibrillar network in a concentrated solution of macromolecules with an amphiphilic structure of repeating units. The investigation of amphiphilic homopolymers and alternating copolymers with the linear and cyclic topologies, the solution with different polymer concentrations and solvent qualities, allows us to conclude that the ability to form a fibrillar gel with branched fibrils and regular subchain thickness is inherent for macromolecules with the solvophobic backbone and solvophilic pendants. The elements of the gel structure, such as the mesh size and fibrillar thickness, the number of cross-links, and their functionality, can be tuned and customized according to the requirements of their application. The results could be helpful for the directed design of the synthetic analogue of the relevant extracellular matrix, in tissue engineering, for fibrotic disease treatment and cell encapsulation.

Self-Assembly Behavior of Elastin-like Polypeptide Diblock Copolymers Containing a Charged Moiety

Elastin-like polypeptides (ELPs) are stimulus-responsive protein-based biopolymers, and some ELP block copolymers can assemble into spherical nanoparticles with thermosensitivity. In this study, two different ELP diblock copolymers, each composed of a hydrophobic and a charged moiety, were synthesized, and the dependence of their physical properties on pH, temperature, and salt concentration was investigated. A series of analyses revealed that hydrophobic core micelles could be generated in response to a change in their surroundings and that micelles did not self-aggregate, a phenomenon due to the repulsive forces between like-charged molecules on the surface. We also demonstrated that self-assembly behavior was closely dependent on the character of the charged amino acid and the specific anion in solution.

Secondary Structure-Driven Self-Assembly of Thiol-Reactive Polypept(o)ides

Secondary structure formation differentiates polypeptides from most of the other synthetic polymers, and the transitions from random coils to rod-like α-helices or β-sheets represent an additional parameter to direct self-assembly and the morphology of nanostructures. We investigated the influence of distinct secondary structures on the self-assembly of reactive amphiphilic polypept(o)ides. The individual morphologies can be preserved by core cross-linking via chemoselective disulfide bond formation. A series of thiol-responsive copolymers of racemic polysarcosine-block-poly(S-ethylsulfonyl-dl-cysteine) (pSar-b-p(dl)Cys), enantiopure polysarcosine-block-poly(S-ethylsulfonyl-l-cysteine) (pSar-b-p(l)Cys), and polysarcosine-block-poly(S-ethylsulfonyl-l-homocysteine) (pSar-b-p(l)Hcy) was prepared by N-carboxyanhydride polymerization. The secondary structure of the peptide segment varies from α-helices (pSar-b-p(l)Hcy) to antiparallel β-sheets (pSar-b-p(l)Cys) and disrupted β-sheets (pSar-b-p(dl)Cys). When subjected to nanoprecipitation, copolymers with antiparallel β-sheets display the strongest tendency to self-assemble, whereas disrupted β-sheets hardly induce aggregation. This translates to worm-like micelles, solely spherical micelles, or ellipsoidal structures, as analyzed by atomic force microscopy and cryogenic transmission electron microscopy, which underlines the potential of secondary structure-driven self-assembly of synthetic polypeptides.

Designing peptide nanoparticles for efficient brain delivery

The targeted delivery of therapeutic compounds to the brain is arguably the most significant open problem in drug delivery today. Nanoparticles (NPs) based on peptides and designed using the emerging principles of molecular engineering show enormous promise in overcoming many of the barriers to brain delivery faced by NPs made of more traditional materials. However, shortcomings in our understanding of peptide self-assembly and blood-brain barrier (BBB) transport mechanisms pose significant obstacles to progress in this area. In this review, we discuss recent work in engineering peptide nanocarriers for the delivery of therapeutic compounds to the brain: from synthesis, to self-assembly, to in vivo studies, as well as discussing in detail the biological hurdles that a nanoparticle must overcome to reach the brain.

Supramolecular assembly of functional peptide–polymer conjugates

The following review gives an overview about synthetic peptide–polymer conjugates as macromolecular building blocks and their self-assembly into a variety of supramolecular architectures, from supramolecular polymer chains, to anisotropic 1D arrays, 2D layers, and more complex 3D networks.