Conformational Conversion during Controlled Oligomerization into Nonamylogenic Protein Nanoparticles

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.

An Up-to-date Review on Protein-based Nanocarriers in the Management of Cancer

BACKGROUND

A big health issue facing the world's population is cancer. An alarming increase in cancer patients was anticipated by worldwide demographic statistics, which showed that the number of patients with different malignancies was rapidly increasing. By 2025, probably 420 million cases were projected to be achieved. The most common cancers diagnosed are breast, colorectal, prostate, and lung. Conventional treatments, such as surgery, chemotherapy, and radiation therapy, have been practiced.

OBJECTIVE

In recent years, the area of cancer therapy has changed dramatically with expanded studies on the molecular-level detection and treatment of cancer. Recent advances in cancer research have seen significant advances in therapies such as chemotherapy and immunotherapy, although both have limitations in effectiveness and toxicity.

METHODS

The development of nanotechnology for anticancer drug delivery has developed several potentials as nanocarriers, which may boost the pharmacokinetic and pharmacodynamic effects of the drug product and substantially reduce the side effects.

RESULTS

The advancement in non-viral to viral-based protein-based nanocarriers for treating cancer has earned further recognition in this respect. Many scientific breakthroughs have relied on protein-based nanocarriers, and proteins are essential organic macromolecules for life. It allows targeted delivery of passive or active tumors using non-viral-based protein-based nanocarriers to viral-based protein nanocarriers. When targeting cancer cells, both animal and plant proteins may be used in a formulation process to create self-assembled viruses and platforms that can successfully eradicate metastatic cancer cells.

CONCLUSION

This review, therefore, explores in depth the applications of non-viral to viral proteinbased noncarriers with a specific focus on intracellular drug delivery and anti-cancer drug targeting ability.

Biofabrication of functional protein nanoparticles through simple His-tag engineering

We have developed a simple, robust, and fully transversal approach for the a-la-carte fabrication of functional multimeric nanoparticles with potential biomedical applications, validated here by a set of diverse and unrelated polypeptides. The proposed concept is based on the controlled coordination between Zn2+ ions and His residues in His-tagged proteins. This approach results in a spontaneous and reproducible protein assembly as nanoscale oligomers that keep the original functionalities of the protein building blocks. The assembly of these materials is not linked to particular polypeptide features, and it is based on an environmentally friendly and sustainable approach. The resulting nanoparticles, with dimensions ranging between 10 and 15 nm, are regular in size, are architecturally stable, are fully functional, and serve as intermediates in a more complex assembly process, resulting in the formation of microscale protein materials. Since most of the recombinant proteins produced by biochemical and biotechnological industries and intended for biomedical research are His-tagged, the green biofabrication procedure proposed here can be straightforwardly applied to a huge spectrum of protein species for their conversion into their respective nanostructured formats.

Selecting Subpopulations of High-Quality Protein Conformers among Conformational Mixtures of Recombinant Bovine MMP-9 Solubilized from Inclusion Bodies

A detailed workflow to analyze the physicochemical characteristics of mammalian matrix metalloproteinase (MMP-9) protein species obtained from protein aggregates (inclusion bodies-IBs) was followed. MMP-9 was recombinantly produced in the prokaryotic microbial cell factories Clearcoli (an engineered form of Escherichia coli) and Lactococcus lactis, mainly forming part of IBs and partially recovered under non-denaturing conditions. After the purification by affinity chromatography of solubilized MMP-9, four protein peaks were obtained. However, so far, the different conformational protein species forming part of IBs have not been isolated and characterized. Therefore, with the aim to link the physicochemical characteristics of the isolated peaks with their biological activity, we set up a methodological approach that included dynamic light scattering (DLS), circular dichroism (CD), and spectrofluorometric analysis confirming the separation of subpopulations of conformers with specific characteristics. In protein purification procedures, the detailed analysis of the individual physicochemical properties and the biological activity of protein peaks separated by chromatographic techniques is a reliable source of information to select the best-fitted protein populations.

Design and engineering of tumor-targeted, dual-acting cytotoxic nanoparticles

The possibility to conjugate tumor-targeted cytotoxic nanoparticles and conventional antitumoral drugs in single pharmacological entities would open a wide spectrum of opportunities in nanomedical oncology. This principle has been explored here by using CXCR4-targeted self-assembling protein nanoparticles based on two potent microbial toxins, the exotoxin A from Pseudomonas aeruginosa and the diphtheria toxin from Corynebacterium diphtheriae, to which oligo-floxuridine and monomethyl auristatin E respectively have been chemically coupled. The resulting multifunctional hybrid nanoconjugates, with a hydrodynamic size of around 50 nm, are stable and internalize target cells with a biological impact. Although the chemical conjugation minimizes the cytotoxic activity of the protein partner in the complexes, the concept of drug combination proposed here is fully feasible and highly promising when considering multiple drug treatments aimed to higher effectiveness or when facing the therapy of cancers with acquired resistance to classical drugs.

Engineering Protein Nanoparticles Out from Components of the Human Microbiome

Nanoscale protein materials are highly convenient as vehicles for targeted drug delivery because of their structural and functional versatility. Selective binding to specific cell surface receptors and penetration into target cells require the use of targeting peptides. Such homing stretches should be incorporated to larger proteins that do not interact with body components, to prevent undesired drug release into nontarget organs. Because of their low interactivity with human body components and their tolerated immunogenicity, proteins derived from the human microbiome are appealing and fully biocompatible building blocks for the biofabrication of nonreactive, inert protein materials within the nanoscale. Several phage and phage-like bacterial proteins with natural structural roles are produced in Escherichia coli as polyhistidine-tagged recombinant proteins, looking for their organization as discrete, nanoscale particulate materials. While all of them self-assemble in a variety of sizes, the stability of the resulting constructs at 37 °C is found to be severely compromised. However, the fine adjustment of temperature and Zn²⁺ concentration allows the formation of robust nanomaterials, fully stable in complex media and under physiological conditions. Then, microbiome-derived proteins show promise for the regulatable construction of scaffold protein nanomaterials, which can be tailored and strengthened by simple physicochemical approaches.

Self-assembling as regular nanoparticles dramatically minimizes photobleaching of tumour-targeted GFP

Fluorescent proteins are useful imaging and theranostic agents, but their potential superiority over alternative dyes is weakened by substantial photobleaching under irradiation. Enhancing protein photostability has been attempted through diverse strategies, with irregular results and limited applicability. In this context, we wondered if the controlled oligomerization of Green Fluorescent Protein (GFP) as nanoscale supramolecular complexes could stabilize the fluorophore through the newly formed protein-protein contacts, and thus, enhance its global photostability. For that, we have here analyzed the photobleaching profile of several GFP versions, engineered to self-assemble as tumour-homing nanoparticles with different targeting, size and structural stability. This has been done under prolonged irradiation in confocal laser scanning microscopy and by small-angle X-ray scattering. The results show that the oligomerization of GFP at the nanoscale enhances, by more than seven-fold, the stability of fluorescence emission. Interestingly, GFP nanoparticles are much more resistant to X-ray damage than the building block counterparts, indicating that the gained photostability is linked to enhanced structural resistance to radiation. Therefore, the controlled oligomerization of self-assembling fluorescent proteins as protein nanoparticles is a simple, versatile and powerful method to enhance their photostability for uses in precision imaging and therapy. STATEMENT OF SIGNIFICANCE: Fluorescent protein assembly into regular and highly symmetric nanoscale structures has been identified to confer enhanced structural stability against radiation stresses dramatically reducing their photobleaching. Being this the main bottleneck in the use of fluorescent proteins for imaging and theranostics, this protein architecture engineering principle appears as a powerful method to enhance their photostability for a broad applicability in precision imaging, drug delivery and theranostics.

Engineering Secretory Amyloids for Remote and Highly Selective Destruction of Metastatic Foci

Functional amyloids produced in bacteria as nanoscale inclusion bodies are intriguing but poorly explored protein materials with wide therapeutic potential. Since they release functional polypeptides under physiological conditions, these materials can be potentially tailored as mimetic of secretory granules for slow systemic delivery of smart protein drugs. To explore this possibility, bacterial inclusion bodies formed by a self-assembled, tumor-targeted Pseudomonas exotoxin (PE24) are administered subcutaneously in mouse models of human metastatic colorectal cancer, for sustained secretion of tumor-targeted therapeutic nanoparticles. These proteins are functionalized with a peptidic ligand of CXCR4, a chemokine receptor overexpressed in metastatic cancer stem cells that confers high selective cytotoxicity in vitro and in vivo. In the mouse models of human colorectal cancer, time-deferred anticancer activity is detected after the subcutaneous deposition of 500 µg of PE24-based amyloids, which promotes a dramatic arrest of tumor growth in the absence of side toxicity. In addition, long-term prevention of lymphatic, hematogenous, and peritoneal metastases is achieved. These results reveal the biomedical potential and versatility of bacterial inclusion bodies as novel tunable secretory materials usable in delivery, and they also instruct how therapeutic proteins, even with high functional and structural complexity, can be packaged in this convenient format.

Collaborative membrane activity and receptor-dependent tumor cell targeting for precise nanoparticle delivery in CXCR4+ colorectal cancer

By the appropriate selection of functional peptides and proper accommodation sites, we have generated a set of multifunctional proteins that combine selectivity for CXCR4⁺ cell binding and relevant endosomal escape capabilities linked to the viral peptide HA2. In particular, the construct T22-GFP-HA2-H6 forms nanoparticles that upon administration in mouse models of human, CXCR4⁺ colorectal cancer, accumulates in primary tumor at levels significantly higher than the parental T22-GFP-H6 HA2-lacking version. The in vivo application of a CXCR4 antagonist has confirmed the prevalence of the CXCR4⁺ tumor tissue selectivity over unspecific cell penetration, upon systemic administration of the material. Such specificity is combined with improved endosomal escape, what overall results in a precise and highly efficient tumor biodistribution. These data strongly support the functional recruitment as a convenient approach to generate protein materials for clinical applications. More precisely, they also support the unexpected concept that enhancing the unspecific membrane activity of a protein material does not necessarily compromise, but it can even improve, the selective cell targeting offered by an accompanying functional module. STATEMENT OF SIGNIFICANCE: We have shown here that the combination of cell-penetrating and tumor cell-targeting peptides dramatically enhances precise tumor accumulation of protein-only nanoparticles intended for selective drug delivery, in mouse models of human colorectal cancer. This fact is a step forward for the rational design of multifunctional protein nanomaterials for improved cancer therapies.

Recruiting potent membrane penetrability in tumor cell-targeted protein-only nanoparticles

The membrane pore-forming activities of the antimicrobial peptide GWH1 have been evaluated in combination with the CXCR4-binding properties of the peptide T22, in self-assembling protein nanoparticles with high clinical potential. The resulting materials, of 25 nm in size and with regular morphologies, show a dramatically improved cell penetrability into CXCR4⁺ cells (more than 10-fold) and enhanced endosomal escape (the lysosomal degradation dropping from 90% to 50%), when compared with equivalent protein nanoparticles lacking GWH1. These data reveal that GWH1 retains its potent membrane activity in form of nanostructured protein complexes. On the other hand, the specificity of T22 in the CXCR4 receptor binding is subsequently minimized but, unexpectedly, not abolished by the presence of the antimicrobial peptide. The functional combination T22-GWH1 results in 30% of the nanoparticles entering cells via CXCR4 while also exploiting pore-based uptake. Such functional materials are capable to selectively deliver highly potent cytotoxic drugs upon chemical conjugation, promoting CXCR4-dependent cell death. These data support the further development of GWH1-empowered cell-targeted proteins as nanoscale drug carriers for precision medicines. This is a very promising approach to overcome lysosomal degradation of protein nanostructured materials with therapeutic value.

Protein-driven nanomedicines in oncotherapy

Proteins are organic macromolecules essential in life but exploited, mainly in recombinant versions, as drugs or vaccine components, among other uses in industry or biomedicine. In oncology, individual proteins or supramolecular complexes have been tailored as small molecular weight drug carriers for passive or active tumor cell-targeted delivery, through the de novo design of appropriate drug stabilizing vehicles, or by generating constructs with different extents of mimesis of natural cell-targeted entities, such as viruses. In most of these approaches, a convenient nanoscale size is achieved through the oligomeric organization of the protein component in the drug conjugate. Among the different taken strategies, highly cytotoxic proteins such as microbial or plant toxins have been conveniently engineered to self-assemble as self-delivered virus-like, nanometric structures, chemically homogeneous that target metastatic cancer stem cells for the destruction of metastasis in absence of any partner vehicle.

Assembly of histidine-rich protein materials controlled through divalent cations

Nanostructured protein materials show exciting biomedical applications, since both structure and function can be genetically programmed. In particular, self-assembling histidine-rich proteins benefit from functional plasticity that allows the generation of protein-only nanoparticles for cell targeted drug delivery. However, the rational development of constructs with improved functions is limited by a poor control of the oligomerization process. By exploring cross-interactions between histidine-tagged building blocks, we have identified a critical architectonic role of divalent cations. The obtained data instruct about how histidine-rich protein materials can be assembled, disassembled and reassembled within the nanoscale through the stoichiometric manipulation of divalent ions, in a biochemical approach to biomaterials design. STATEMENT OF SIGNIFICANCE: Divalent metal and non-metal cations such as Ni²⁺, Cu²⁺ Ca²⁺ and Zn²⁺ have been identified as unexpected molecular tools to control the assembling, disassembling and reassembling of histidine-rich protein materials at the nanoscale. Their stoichiometric manipulation allows generating defined protein-protein cross-molecular contacts between building blocks, for a powerful nano-biochemical manipulation of the material's architecture.