Concentration-Independent Multivalent Targeting of Cancer Cells by Genetically Encoded Core-Crosslinked Elastin/Resilin-like Polypeptide Micelles

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.

Encoding Structure in Intrinsically Disordered Protein Biomaterials

In nature, proteins range from those with highly ordered secondary and tertiary structures to those that completely lack a well-defined three-dimensional structure, termed intrinsically disordered proteins (IDPs). IDPs are generally characterized by one or more segments that have a compositional bias toward small hydrophilic amino acids and proline residues that promote structural disorder and are called intrinsically disordered regions (IDRs). The combination of IDRs with ordered regions and the interactions between the two determine the phase behavior, structure, and function of IDPs. Nature also diversifies the structure of proteins and thereby their functions by hybridization of the proteins with other moieties such as glycans and lipids; for instance, post-translationally glycosylated and lipidated proteins are important cell membrane components. Additionally, diversity in protein structure and function is achieved in nature through cross-linking proteins within themselves or with other domains to create various topologies. For example, an essential characteristic of the extracellular matrix (ECM) is the cross-linking of its network components, including proteins such as collagen and elastin, as well as polysaccharides such as hyaluronic acid (HA). Inspired by nature, synthetic IDP (SynIDP)-based biomaterials can be designed by employing similar strategies with the goal of introducing structural diversity and hence unique physiochemical properties. This Account describes such materials produced over the past decade and following one or more of the following approaches: (1) incorporating highly ordered domains into SynIDPs, (2) conjugating SynIDPs to other moieties through either genetically encoded post-translational modification or chemical conjugation, and (3) engineering the topology of SynIDPs via chemical modification. These approaches introduce modifications to the primary structure of SynIDPs, which are then translated to unique three-dimensional secondary and tertiary structures. Beginning with completely disordered SynIDPs as the point of origin, structure may be introduced into SynIDPs by each of these three unique approaches individually along orthogonal axes or by combinations of the three, enabling bioinspired designs to theoretically span the entire range of three-dimensional structural possibilities. Furthermore, the resultant structures span a wide range of length scales, from nano- to meso- to micro- and even macrostructures. In this Account, emphasis is placed on the physiochemical properties and structural features of the described materials. Conjugates of SynIDPs to synthetic polymers and materials achieved by simple mixing of components are outside the scope of this Account. Related biomedical applications are described briefly. Finally, we note future directions for the design of functional SynIDP-based biomaterials.

Peptide-Conjugated Micelles Make Effective Mimics of the TRAIL Protein for Driving Apoptosis in Colon Cancer

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) drives apoptosis selectively in cancer cells by clustering death receptors (DR4 and DR5). While it has excellent in vitro selectivity and toxicity, the TRAIL protein has a very low circulation half-life in vivo, which has hampered clinical development. Here, we developed core-cross-linked micelles that present multiple copies of a TRAIL-mimicking peptide at its surface. These micelles successfully induce apoptosis in a colon cancer cell line (COLO205) via DR4/5 clustering. Micelles with a peptide density of 15% (roughly 1 peptide/45 nm2) displayed the strongest activity with an IC50 value of 0.8 μM (relative to peptide), demonstrating that the precise spatial arrangement of ligands imparted by a protein such as a TRAIL may not be necessary for DR4/5/signaling and that a statistical network of monomeric ligands may suffice. As micelles have long circulation half-lives, we propose that this could provide a potential alternative drug to TRAIL and stimulate the use of micelles in other membrane receptor clustering networks.

Genetic Fusion of Thermoresponsive Polypeptides with UCST-type Behavior Mediates 1D Assembly of Coiled-Coil Bundlemers

Thermoresponsive resilin-like polypeptides (RLPs) of various lengths were genetically fused to two different computationally designed coiled coil-forming peptides with distinct thermal stability, to develop new strategies to assemble coiled coil peptides via temperature-triggered phase separation of the RLP units. Their successful production in bacterial expression hosts was verified via gel electrophoresis, mass spectrometry, and amino acid analysis. Circular dichroism (CD) spectroscopy, ultraviolet-visible (UV/Vis) turbidimetry, and dynamic light scattering (DLS) measurements confirmed the stability of the coiled coils and showed that the thermosensitive phase behavior of the RLPs was preserved in the genetically fused hybrid polypeptides. Cryogenic-transmission electron microscopy and coarse-grained modeling revealed that functionalizing the coiled coils with thermoresponsive RLPs leads to their thermally triggered noncovalent assembly into nanofibrillar assemblies.

Protein-Based Biological Materials: Molecular Design and Artificial Production

Polymeric materials produced from fossil fuels have been intimately linked to the development of industrial activities in the 20th century and, consequently, to the transformation of our way of living. While this has brought many benefits, the fabrication and disposal of these materials is bringing enormous sustainable challenges. Thus, materials that are produced in a more sustainable fashion and whose degradation products are harmless to the environment are urgently needed. Natural biopolymers─which can compete with and sometimes surpass the performance of synthetic polymers─provide a great source of inspiration. They are made of natural chemicals, under benign environmental conditions, and their degradation products are harmless. Before these materials can be synthetically replicated, it is essential to elucidate their chemical design and biofabrication. For protein-based materials, this means obtaining the complete sequences of the proteinaceous building blocks, a task that historically took decades of research. Thus, we start this review with a historical perspective on early efforts to obtain the primary sequences of load-bearing proteins, followed by the latest developments in sequencing and proteomic technologies that have greatly accelerated sequencing of extracellular proteins. Next, four main classes of protein materials are presented, namely fibrous materials, bioelastomers exhibiting high reversible deformability, hard bulk materials, and biological adhesives. In each class, we focus on the design at the primary and secondary structure levels and discuss their interplays with the mechanical response. We finally discuss earlier and the latest research to artificially produce protein-based materials using biotechnology and synthetic biology, including current developments by start-up companies to scale-up the production of proteinaceous materials in an economically viable manner.

Recombinant protein-based injectable materials for biomedical applications

Injectable nanocarriers and hydrogels have found widespread use in a variety of biomedical applications such as local and sustained biotherapeutic cargo delivery, and as cell-instructive matrices for tissue engineering. Recent advances in the development and application of recombinant protein-based materials as injectable platforms under physiological conditions have made them useful platforms for the development of nanoparticles and tissue engineering matrices, which are reviewed in this work. Protein-engineered biomaterials are highly customizable, and they provide distinctly tunable rheological properties, encapsulation efficiencies, and delivery profiles. In particular, the key advantages of emerging technologies which harness the stimuli-responsive properties of recombinant polypeptide-based materials are highlighted in this review.

Elastin-like polypeptide-based micelles as a promising platform in nanomedicine

New and improved nanomaterials are constantly being developed for biomedical purposes. Nanomaterials based on elastin-like polypeptides (ELPs) have increasingly shown potential over the past two decades. These polymers are artificial proteins of which the design is based on human tropoelastin. Due to this similarity, ELP-based nanomaterials are biodegradable and therefore well suited to drug delivery. The assembly of ELP molecules into nanoparticles spontaneously occurs at temperatures above a transition temperature (T_t). The ELP sequence influences both the T_t and the physicochemical properties of the assembled nanomaterial. Nanoparticles with desired properties can hence be designed by choosing the appropriate sequence. A promising class of ELP nanoparticles are micelles assembled from amphiphilic ELP diblock copolymers. Such micelles are generally uniform and well defined. Furthermore, site-specific attachment of cargo to the hydrophobic block results in micelles with the cargo shielded inside their core, while conjugation to the hydrophilic block causes the cargo to reside in the corona where it is available for interactions. Such control over particle design is one of the main contributing factors for the potential of ELP-based micelles as a drug delivery system. Additionally, the micelles are easily loaded with protein or peptide-based cargo by expressing it as a fusion protein. Small molecule drugs and other cargo types can be either covalently conjugated to ELP domains or physically entrapped inside the micelle core. This review aims to give an overview of ELP-based micelles and their applications in nanomedicine.

Sequence Context and Complex Hofmeister Salt Interactions Dictate Phase Separation Propensity of Resilin-like Polypeptides

Resilin is an elastic material found in insects with exceptional durability, resilience, and extensibility, making it a promising biomaterial for tissue engineering. The monomeric precursor, pro-resilin, undergoes thermo-responsive self-assembly through liquid-liquid phase separation (LLPS). Understanding the molecular details of this assembly process is critical to developing complex biomaterials. The present study investigates the interplay between the solvent, sequence syntax, structure, and dynamics in promoting LLPS of resilin-like-polypeptides (RLPs) derived from domains 1 and 3 of Drosophila melanogaster pro-resilin. NMR, UV-vis, and microscopy data demonstrate that while kosmotropic salts and low pH promote LLPS, the effects of chaotropic salts with increasing pH are more complex. Subtle variations between the repeating amino acid motifs of resilin domain 1 and domain 3 lead to significantly different salt and pH dependence of LLPS, with domain 3 sequence motifs more strongly favoring phase separation under most conditions. These findings provide new insight into the molecular drivers of RLP phase separation and the complex roles of both RLP sequence and solution composition in fine-tuning assembly conditions.

Expanding the chemical repertoire of protein-based polymers for drug-delivery applications

Expanding the chemical repertoire of natural and artificial protein-based polymers (PBPs) can enable the production of sequence-defined, yet chemically diverse, biopolymers with customized or new properties that cannot be accessed in PBPs composed of only natural amino acids. Various approaches can enable the expansion of the chemical repertoire of PBPs, including chemical and enzymatic treatments or the incorporation of unnatural amino acids. These techniques are employed to install a wide variety of chemical groups-such as bio-orthogonally reactive, cross-linkable, post-translation modifications, and environmentally responsive groups-which, in turn, can facilitate the design of customized PBP-based drug-delivery systems with modified, fine-tuned, or entirely new properties and functions. Here, we detail the existing and emerging technologies for expanding the chemical repertoire of PBPs and review several chemical groups that either demonstrate or are anticipated to show potential in the design of PBP-based drug delivery systems. Finally, we provide our perspective on the remaining challenges and future directions in this field.

The Two Sweet Sides of Janus Lectin Drive Crosslinking of Liposomes to Cancer Cells and Material Uptake

A chimeric, bispecific Janus lectin has recently been engineered with different, rationally oriented recognition sites. It can bind simultaneously to sialylated and fucosylated glycoconjugates. Because of its multivalent architecture, this lectin reaches nanomolar avidities for sialic acid and fucose. The lectin was designed to detect hypersialylation—a dysregulation in physiological glycosylation patterns, which promotes the tumor growth and progression of several cancer types. In this study, the characteristic properties of this bispecific Janus lectin were investigated on human cells by flow cytometry and confocal microscopy in order to understand the fundamentals of its interactions. We evaluated its potential in targeted drug delivery, precisely leading to the cellular uptake of liposomal content in human epithelial cancer cells. We successfully demonstrated that Janus lectin mediates crosslinking of glyco-decorated giant unilamellar vesicles (GUVs) and H1299 lung epithelial cells. Strikingly, the Janus lectin induced the internalization of liposomal lipids and also of complete GUVs. Our findings serve as a solid proof of concept for lectin-mediated targeted drug delivery using glyco-decorated liposomes as possible drug carriers to cells of interest. The use of Janus lectin for tumor recognition certainly broadens the possibilities for engineering diverse tailor-made lectin constructs, specifically targeting extracellular structures of high significance in pathological conditions.