The Poly-Histidine Tag H6 Mediates Structural and Functional Properties of Disintegrating, Protein-Releasing Inclusion Bodies

Hybrid Micro-/Nanoprotein Platform Provides Endocrine-like and Extracellular Matrix-like Cell Delivery of Growth Factors

Protein materials are versatile tools in diverse biomedical fields. Among them, artificial secretory granules (SGs), mimicking those from the endocrine system, act as mechanically stable reservoirs for the sustained release of proteins as oligomeric functional nanoparticles. Only validated in oncology, the physicochemical properties of SGs, along with their combined drug-releasing and scaffolding abilities, make them suitable as smart topographies in regenerative medicine for the prolonged delivery of growth factors (GFs). Thus, considering the need for novel, safe, and cost-effective materials to present GFs, in this study, we aimed to biofabricate a protein platform combining both endocrine-like and extracellular matrix fibronectin-derived (ECM-FN) systems. This approach is based on the sustained delivery of a nanostructured histidine-tagged version of human fibroblast growth factor 2. The GF is presented onto polymeric surfaces, interacting with FN to spontaneously generate nanonetworks that absorb and present the GF in the solid state, to modulate mesenchymal stromal cell (MSC) behavior. The results show that SGs-based topographies trigger high rates of MSCs proliferation while preventing differentiation. While this could be useful in cell therapy manufacture demanding large numbers of unspecialized MSCs, it fully validates the hybrid platform as a convenient setup for the design of biologically active hybrid surfaces and in tissue engineering for the controlled manipulation of mammalian cell growth.

Atomic force microscopy of spherical intermediates on the pathway to fibril formation of influenza A virus nuclear export protein

The nuclear export protein of the influenza A virus (NEP) is involved in many important processes of the virus life cycle. This makes it an attractive target for the treatment of a disease caused by a virus. Previously it has been shown, that recombinant variants of NEP are highly prone to aggregation in solution under various conditions with the formation of amyloid-like aggregates. In the present work, the amyloid nature of NEP aggregates was evidenced by Congo red binding assays. Atomic force microscopy has shown that NEP can form two types of spherical nanoparticles, which provide an alternative pathway for the formation of amyloid-like fibrils. Type I of these "fibrillogenic" spheres, formed under physiological conditions, represents the micelle-like particles with height 10-60 nm, which can generate worm-like flexible fibrils with the diameter 2.5-4.0 nm, length 20-500 nm and the Young's modulus ~73 MPa. Type II spherical aggregates with size of about 400-1000 nm, formed at elevated temperatures, includes fractions of drop-like and vesicle-like particles, generating more rigid amyloid-like fibrils with height of ~8 nm, and length of up to 2 μm. The hypothetical mechanism of fibril formation via nanospherical structures was suggested. RESEARCH HIGHLIGHTS: AFM has revealed two types of the influenza A virus nuclear export protein spherical aggregates. They provide an alternative pathway for the formation of amyloid-like fibrils. The mechanism of fibril formation via spherical structures is suggested.

Structural Stabilization of Clinically Oriented Oligomeric Proteins During their Transit through Synthetic Secretory Amyloids

Developing time-sustained drug delivery systems is a main goal in innovative medicines. Inspired by the architecture of secretory granules from the mammalian endocrine system it has generated non-toxic microscale amyloid materials through the coordination between divalent metals and poly-histidine stretches. Like their natural counterparts that keep the functionalities of the assembled protein, those synthetic structures release biologically active proteins during a slow self-disintegration process occurring in vitro and upon in vivo administration. Being these granules formed by a single pure protein species and therefore, chemically homogenous, they act as highly promising time-sustained drug delivery systems. Despite their enormous clinical potential, the nature of the clustering process and the quality of the released protein have been so far neglected issues. By using diverse polypeptide species and their protein-only oligomeric nanoscale versions as convenient models, a conformational rearrangement and a stabilization of the building blocks during their transit through the secretory granules, being the released material structurally distinguishable from the original source is proved here. This fact indicates a dynamic nature of secretory amyloids that act as conformational arrangers rather than as plain, inert protein-recruiting/protein-releasing granular depots.

Efficient Delivery of Antimicrobial Peptides in an Innovative, Slow-Release Pharmacological Formulation

Both nanostructure and multivalency enhance the biological activities of antimicrobial peptides (AMPs), whose mechanism of action is cooperative. In addition, the efficacy of a particular AMP should benefit from a steady concentration at the local place of action and, therefore, from a slow release after a dynamic repository. In the context of emerging multi-resistant bacterial infections and the urgent need for novel and effective antimicrobial drugs, we tested these concepts through the engineering of four AMPs into supramolecular complexes as pharmacological entities. For that purpose, GWH1, T22, Pt5, and PaD, produced as GFP or human nidogen-based His-tagged fusion proteins, were engineered as self-assembling oligomeric nanoparticles ranging from 10 to 70 nm and further packaged into nanoparticle-leaking submicron granules. Since these materials slowly release functional nanoparticles during their time-sustained unpacking, they are suitable for use as drug depots in vivo. In this context, a particular AMP version (GWH1-NIDO-H6) was selected for in vivo validation in a zebrafish model of a complex bacterial infection. The GWH1-NIDO-H6-secreting protein granules are protective in zebrafish against infection by the multi-resistant bacterium Stenotrophomonas maltophilia, proving the potential of innovative formulations based on nanostructured and slowly released recombinant AMPs in the fight against bacterial infections.

Protein features instruct the secretion dynamics from metal-supported synthetic amyloids

Hexahistidine-tagged proteins can be clustered by divalent cations into self-containing, dynamic protein depots at the microscale, which under physiological conditions leak functional protein. While such protein granules show promise in clinics as time-sustained drug delivery systems, little is known about how the nature of their components, that is, the protein and the particular cation used as cross-linker, impact on the disintegration of the material and on its secretory performance. By using four model proteins and four different cation formulations to control aggregation, we have here determined a moderate influence of the used cation and a potent impact of some protein properties on the release kinetics and on the final fraction of releasable protein. In particular, the electrostatic charge at the amino terminus and the instability and hydropathicity indexes determine the disintegration profile of the depot. These data offer clues for the fabrication of efficient and fully exploitable secretory granules that being biocompatible and chemically homogenous allow their tailored use as drug delivery platforms in biological systems.

Recombinant Proteins for Assembling as Nano- and Micro-Scale Materials for Drug Delivery: A Host Comparative Overview

By following simple protein engineering steps, recombinant proteins with promising applications in the field of drug delivery can be assembled in the form of functional materials of increasing complexity, either as nanoparticles or nanoparticle-leaking secretory microparticles. Among the suitable strategies for protein assembly, the use of histidine-rich tags in combination with coordinating divalent cations allows the construction of both categories of material out of pure polypeptide samples. Such molecular crosslinking results in chemically homogeneous protein particles with a defined composition, a fact that offers soft regulatory routes towards clinical applications for nanostructured protein-only drugs or for protein-based drug vehicles. Successes in the fabrication and final performance of these materials are expected, irrespective of the protein source. However, this fact has not yet been fully explored and confirmed. By taking the antigenic RBD domain of the SARS-CoV-2 spike glycoprotein as a model building block, we investigated the production of nanoparticles and secretory microparticles out of the versions of recombinant RBD produced by bacteria (Escherichia coli), insect cells (Sf9), and two different mammalian cell lines (namely HEK 293F and Expi293F). Although both functional nanoparticles and secretory microparticles were effectively generated in all cases, the technological and biological idiosyncrasy of each type of cell factory impacted the biophysical properties of the products. Therefore, the selection of a protein biofabrication platform is not irrelevant but instead is a significant factor in the upstream pipeline of protein assembly into supramolecular, complex, and functional materials.

Protein scaffolds in human clinics

Fundamental clinical areas such as drug delivery and regenerative medicine require biocompatible materials as mechanically stable scaffolds or as nanoscale drug carriers. Among the wide set of emerging biomaterials, polypeptides offer enticing properties over alternative polymers, including full biocompatibility, biodegradability, precise interactivity, structural stability and conformational and functional versatility, all of them tunable by conventional protein engineering. However, proteins from non-human sources elicit immunotoxicities that might bottleneck further development and narrow their clinical applicability. In this context, selecting human proteins or developing humanized protein versions as building blocks is a strict demand to design non-immunogenic protein materials. We review here the expanding catalogue of human or humanized proteins tailored to execute different levels of scaffolding functions and how they can be engineered as self-assembling materials in form of oligomers, polymers or complex networks. In particular, we emphasize those that are under clinical development, revising their fields of applicability and how they have been adapted to offer, apart from mere mechanical support, highly refined functions and precise molecular interactions.

Rapid Quantification of Polyhydroxybutyrate Polymer from Single Bacterial Cells with Mass Spectrometry

Polyhydroxyalkanoate (PHA) is one of the biocompatible and biodegradable plastics that can be produced and accumulated as granules inside microorganisms. In this study, a new approach to rapidly quantify a short-chain-length PHA, polyhydroxybutyrate (PHB), produced from genetically engineered Escherichia coli containing phaCAB is presented. The mass of each bacterial cell was measured using a laser-induced radio frequency (rf) plasma charge detection quadrupole ion trap mass spectrometer (LIRFP CD QIT-MS), and then, the PHB contents were determined by calculating the change in cellular mass. The quantitative results showed that the PHB contents measured by LIRFP CD QIT-MS were consistent with those by reference analysis, gas chromatography (GC). The PHB content of each bacterial sample can be obtained within 20 min from sampling using LIRFP CD QIT-MS while GC analysis takes 2 days. In addition, LIRFP CD QIT-MS does not use any hazardous chemicals in cellular mass quantification as compared to GC. This indicates that LIRFP CD QIT-MS has potential in routine monitoring of PHB production.