Small Heat Shock Protein as a Multifunctional Scaffold: Integrated Tumor Targeting and Caspase Imaging within a Single Cage

Manufacturing of non-viral protein nanocages for biotechnological and biomedical applications

Protein nanocages are highly ordered nanometer scale architectures, which are typically formed by homo- or hetero-self-assembly of multiple monomers into symmetric structures of different size and shape. The intrinsic characteristics of protein nanocages make them very attractive and promising as a biological nanomaterial. These include, among others, a high surface/volume ratio, multi-functionality, ease to modify or manipulate genetically or chemically, high stability, mono-dispersity, and biocompatibility. Since the beginning of the investigation into protein nanocages, several applications were conceived in a variety of areas such as drug delivery, vaccine development, bioimaging, biomineralization, nanomaterial synthesis and biocatalysis. The ability to generate large amounts of pure and well-folded protein assemblies is one of the keys to transform nanocages into clinically valuable products and move biomedical applications forward. This calls for the development of more efficient biomanufacturing processes and for the setting up of analytical techniques adequate for the quality control and characterization of the biological function and structure of nanocages. This review concisely covers and overviews the progress made since the emergence of protein nanocages as a new, next-generation class of biologics. A brief outline of non-viral protein nanocages is followed by a presentation of their main applications in the areas of bioengineering, biotechnology, and biomedicine. Afterwards, we focus on a description of the current processes used in the manufacturing of protein nanocages with particular emphasis on the most relevant aspects of production and purification. The state-of-the-art on current characterization techniques is then described and future alternative or complementary approaches in development are also discussed. Finally, a critical analysis of the limitations and drawbacks of the current manufacturing strategies is presented, alongside with the identification of the major challenges and bottlenecks.

Molecular Engineering of E. coli Bacterioferritin: A Versatile Nanodimensional Protein Cage

Currently, intense interest is focused on the discovery and application of new multisubunit cage proteins and spherical virus capsids to the fields of bionanotechnology, drug delivery, and diagnostic imaging as their internal cavities can serve as hosts for fluorophores or bioactive molecular cargo. Bacterioferritin is unusual in the ferritin protein superfamily of iron-storage cage proteins in that it contains twelve heme cofactors and is homomeric. The goal of the present study is to expand the capabilities of ferritins by developing new approaches to molecular cargo encapsulation employing bacterioferritin. Two strategies were explored to control the encapsulation of a diverse range of molecular guests compared to random entrapment, a predominant strategy employed in this area. The first was the inclusion of histidine-tag peptide fusion sequences within the internal cavity of bacterioferritin. This approach allowed for the successful and controlled encapsulation of a fluorescent dye, a protein (fluorescently labeled streptavidin), or a 5 nm gold nanoparticle. The second strategy, termed the heme-dependent cassette strategy, involved the substitution of the native heme with heme analogs attached to (i) fluorescent dyes or (ii) nickel-nitrilotriacetate (NTA) groups (which allowed for controllable encapsulation of a histidine-tagged green fluorescent protein). An in silico docking approach identified several small molecules able to replace the heme and capable of controlling the quaternary structure of the protein. A transglutaminase-based chemoenzymatic approach to surface modification of this cage protein was also accomplished, allowing for future nanoparticle targeting. This research presents novel strategies to control a diverse set of molecular encapsulations and adds a further level of sophistication to internal protein cavity engineering.

Ferritin nanocages: a versatile platform for nanozyme design

Nanozymes are a class of nanomaterials with enzyme-like activities and have attracted increasing attention due to their potential applications in biomedicine. However, nanozyme design incorporating the desired properties remains challenging. Natural or genetically engineered protein scaffolds, such as ferritin nanocages, have emerged as a promising platform for nanozyme design due to their unique protein structure, natural biomineralization capacity, self-assembly properties, and high biocompatibility. In this review, we highlight the intrinsic properties of ferritin nanocages, especially for nanozyme design. We also discuss the advantages of genetically engineered ferritin in the versatile design of nanozymes over natural ferritin. Additionally, we summarize the bioapplications of ferritin-based nanozymes based on their enzyme-mimicking activities. In this perspective, we mainly provide potential insights into the utilization of ferritin nanocages for nanozyme design.

Protein-based nanomaterials: a new tool for targeted drug delivery

Protein nanomaterials are well-defined, hollow protein nanoparticles comprised of virus capsids, virus-like particles, ferritin, heat shock proteins, chaperonins and many more. Protein-based nanomaterials are formed by the self-assembly of protein subunits and have numerous desired properties as drug-delivery vehicles, including being optimally sized for endocytosis, nontoxic, biocompatible, biodegradable and functionalized at three separate interfaces (external, internal and intersubunit). As a result, protein nanomaterials have been intensively investigated as functional entities in bionanotechnology, including drug delivery, nanoreactors and templates for organic and inorganic nanomaterials. Several variables influence efficient administration, particularly active targeting, cellular uptake, the kinetics of the release and systemic elimination. This review examines the wide range of medicines, loading/release processes, targeted therapies and treatment effectiveness.

Emerging prospects of protein/peptide-based nanoassemblies for drug delivery and vaccine development

Proteins have been widely used in the biomedical field because of their well-defined architecture, accurate molecular weight, excellent biocompatibility and biodegradability, and easy-to-functionalization. Inspired by the wisdom of nature, increasing proteins/peptides that possess self-assembling capabilities have been explored and designed to generate nanoassemblies with unique structure and function, including spatially organized conformation, passive and active targeting, stimuli-responsiveness, and high stability. These characteristics make protein/peptide-based nanoassembly an ideal platform for drug delivery and vaccine development. In this review, we focus on recent advances in subsistent protein/peptide-based nanoassemblies, including protein nanocages, virus-like particles, self-assemblable natural proteins, and self-assemblable artificial peptides. The origin and characteristics of various protein/peptide-based assemblies and their applications in drug delivery and vaccine development are summarized. In the end, the prospects and challenges are discussed for the further development of protein/peptide-based nanoassemblies.

Bioengineering bacterial encapsulin nanocompartments as targeted drug delivery system

The development of Drug Delivery Systems (DDS) has led to increasingly efficient therapies for the treatment and detection of various diseases. DDS use a range of nanoscale delivery platforms produced from polymeric of inorganic materials, such as micelles, and metal and polymeric nanoparticles, but their variant chemical composition make alterations to their size, shape, or structures inherently complex. Genetically encoded protein nanocages are highly promising DDS candidates because of their modular composition, ease of recombinant production in a range of hosts, control over assembly and loading of cargo molecules and biodegradability. One example of naturally occurring nanocompartments are encapsulins, recently discovered bacterial organelles that have been shown to be reprogrammable as nanobioreactors and vaccine candidates. Here we report the design and application of a targeted DDS platform based on the Thermotoga maritima encapsulin reprogrammed to display an antibody mimic protein called Designed Ankyrin repeat protein (DARPin) on the outer surface and to encapsulate a cytotoxic payload. The DARPin9.29 chosen in this study specifically binds to human epidermal growth factor receptor 2 (HER2) on breast cancer cells, as demonstrated in an in vitro cell culture model. The encapsulin-based DDS is assembled in one step in vivo by co-expressing the encapsulin-DARPin9.29 fusion protein with an engineered flavin-binding protein mini-singlet oxygen generator (MiniSOG), from a single plasmid in Escherichia coli. Purified encapsulin-DARPin_miniSOG nanocompartments bind specifically to HER2 positive breast cancer cells and trigger apoptosis, indicating that the system is functional and specific. The DDS is modular and has the potential to form the basis of a multi-receptor targeted system by utilising the DARPin screening libraries, allowing use of new DARPins of known specificities, and through the proven flexibility of the encapsulin cargo loading mechanism, allowing selection of cargo proteins of choice.

Redesign of protein nanocages: the way from 0D, 1D, 2D to 3D assembly

Compartmentalization is a hallmark of living systems. Through compartmentalization, ubiquitous protein nanocages such as viral capsids, ferritin, small heat shock proteins, and DNA-binding proteins from starved cells fulfill a variety of functions, while their shell-like structures hold great promise for various applications in the field of nanomedicine and nanotechnology. However, the number and structure of natural protein nanocages are limited, and these natural protein nanocages may not be suited for a given application, which might impede their further application as nanovehicles, biotemplates or building blocks. To overcome these shortcomings, different strategies have been developed by scientists to construct artificial protein nanocages, and 1D, 2D and 3D protein arrays with protein nanocages as building blocks through genetic and chemical modification to rival the size and functionality of natural protein nanocages. This review outlines the recent advances in the field of the design and construction of artificial protein nanocages and their assemblies with higher order, summarizes the strategies for creating the assembly of protein nanocages from zero-dimension to three dimensions, and introduces their corresponding applications in the preparation of nanomaterials, electrochemistry, and drug delivery. The review will highlight the roles of both the inter-subunit/intermolecular interactions at the key interface and the protein symmetry in constructing and controlling protein nanocage assemblies with different dimensions.

Controlling Protein Nanocage Assembly with Hydrostatic Pressure

Controlling the assembly and disassembly of nanoscale protein cages for the capture and internalization of protein or non-proteinaceous components is fundamentally important to a diverse range of bionanotechnological applications. Here, we study the reversible, pressure-induced dissociation of a natural protein nanocage, E. coli bacterioferritin (Bfr), using synchrotron radiation small-angle X-ray scattering (SAXS) and circular dichroism (CD). We demonstrate that hydrostatic pressures of 450 MPa are sufficient to completely dissociate the Bfr 24-mer into protein dimers, and the reversibility and kinetics of the reassembly process can be controlled by selecting appropriate buffer conditions. We also demonstrate that the heme B prosthetic group present at the subunit dimer interface influences the stability and pressure lability of the cage, despite its location being discrete from the interdimer interface that is key to cage assembly. This indicates a major cage-stabilizing role for heme within this family of ferritins.

Small Heat Shock Proteins in Cancers: Functions and Therapeutic Potential for Cancer Therapy

Small heat shock proteins (sHSPs) are ubiquitous ATP-independent chaperones that play essential roles in response to cellular stresses and protein homeostasis. Investigations of sHSPs reveal that sHSPs are ubiquitously expressed in numerous types of tumors, and their expression is closely associated with cancer progression. sHSPs have been suggested to control a diverse range of cancer functions, including tumorigenesis, cell growth, apoptosis, metastasis, and chemoresistance, as well as regulation of cancer stem cell properties. Recent advances in the field indicate that some sHSPs have been validated as a powerful target in cancer therapy. In this review, we present and highlight current understanding, recent progress, and future challenges of sHSPs in cancer development and therapy.

The biomedical and bioengineering potential of protein nanocompartments

Protein nanocompartments (PNCs) are self-assembling biological nanocages that can be harnessed as platforms for a wide range of nanobiotechnology applications. The most widely studied examples of PNCs include virus-like particles, bacterial microcompartments, encapsulin nanocompartments, enzyme-derived nanocages (such as lumazine synthase and the E2 component of the pyruvate dehydrogenase complex), ferritins and ferritin homologues, small heat shock proteins, and vault ribonucleoproteins. Structural PNC shell proteins are stable, biocompatible, and tolerant of both interior and exterior chemical or genetic functionalization for use as vaccines, therapeutic delivery vehicles, medical imaging aids, bioreactors, biological control agents, emulsion stabilizers, or scaffolds for biomimetic materials synthesis. This review provides an overview of the recent biomedical and bioengineering advances achieved with PNCs with a particular focus on recombinant PNC derivatives.

Structural nanotechnology: three-dimensional cryo-EM and its use in the development of nanoplatforms for in vitro catalysis

The organization of enzymes into different subcellular compartments is essential for correct cell function. Protein-based cages are a relatively recently discovered subclass of structurally dynamic cellular compartments that can be mimicked in the laboratory to encapsulate enzymes. These synthetic structures can then be used to improve our understanding of natural protein-based cages, or as nanoreactors in industrial catalysis, metabolic engineering, and medicine. Since the function of natural protein-based cages is related to their three-dimensional structure, it is important to determine this at the highest possible resolution if viable nanoreactors are to be engineered. Cryo-electron microscopy (cryo-EM) is ideal for undertaking such analyses within a feasible time frame and at near-native conditions. This review describes how three-dimensional cryo-EM is used in this field and discusses its advantages. An overview is also given of the nanoreactors produced so far, their structure, function, and applications.

Structural Characterization of Native and Modified Encapsulins as Nanoplatforms for in Vitro Catalysis and Cellular Uptake

Recent years have witnessed the emergence of bacterial semiorganelle encapsulins as promising platforms for bio-nanotechnology. To advance the development of encapsulins as nanoplatforms, a functional and structural basis of these assemblies is required. Encapsulin from Brevibacterium linens is known to be a protein-based vessel for an enzyme cargo in its cavity, which could be replaced with a foreign cargo, resulting in a modified encapsulin. Here, we characterize the native structure of B. linens encapsulins with both native and foreign cargo using cryo-electron microscopy (cryo-EM). Furthermore, by harnessing the confined enzyme (i.e., a peroxidase), we demonstrate the functionality of the encapsulin for an in vitro surface-immobilized catalysis in a cascade pathway with an additional enzyme, glucose oxidase. We also demonstrate the in vivo functionality of the encapsulin for cellular uptake using mammalian macrophages. Unraveling both the structure and functionality of the encapsulins allows transforming biological nanocompartments into functional systems.

Chaperonin-Dendrimer Conjugates for siRNA Delivery

The group II chaperonin thermosome (THS) is a hollow protein nanoparticle that can encapsulate macromolecular guests. Two large pores grant access to the interior of the protein cage. Poly(amidoamine) (PAMAM) is conjugated into THS to act as an anchor for small interfering RNA (siRNA), allowing to load the THS with therapeutic payload. THS-PAMAM protects siRNA from degradation by RNase A and traffics KIF11 and GAPDH siRNA into U87 cancer cells. By modification of the protein cage with the cell-penetrating peptide TAT, RNA interference is also induced in PC-3 cells. THS-PAMAM protein-polymer conjugates are therefore promising siRNA transfection reagents and greatly expand the scope of protein cages in drug delivery applications.

Functionalization of protein-based nanocages for drug delivery applications

Traditional drug delivery strategies involve drugs which are not targeted towards the desired tissue. This can lead to undesired side effects, as normal cells are affected by the drugs as well. Therefore, new systems are now being developed which combine targeting functionalities with encapsulation of drug cargo. Protein nanocages are highly promising drug delivery platforms due to their perfectly defined structures, biocompatibility, biodegradability and low toxicity. A variety of protein nanocages have been modified and functionalized for these types of applications. In this review, we aim to give an overview of different types of modifications of protein-based nanocontainers for drug delivery applications.

Lumazine synthase protein cage nanoparticles as modular delivery platforms for targeted drug delivery

Lumazine synthase protein cage nanoparticle is developed as a modular delivery nanoplatform that delivers drugs to their target cancer cells.

Tumor fibroblast specific activation of a hybrid ferritin nanocage-based optical probe for tumor microenvironment imaging

Ferritin-based FAP-α-responsive fluorescence nanoprobes could be activated immediately as they penetrate the tumor blood vessels and come across the FAP-α molecules on the membrane of CAFs in the tumor microenvironment instead of reaching a certain depth into tumor tissue to interact with tumor cells. This probe may become a promising nanomaterial for highly specific and sensitive tumor imaging through responding to the enzymes in the tumor microenvironment.

Hyperbranched conjugated polyelectrolyte for dual-modality fluorescence and magnetic resonance cancer imaging

Herein is reported the synthesis of gadolinium ion (Gd(III))-chelated hyperbranched conjugated polyelectrolyte (HCPE-Gd) and its application in fluorescence and magnetic resonance (MR) dual imaging in live animals. The synthesized HCPE-Gd forms nanospheres with an average diameter of ∼42 nm measured by laser light scattering and a quantum yield of 10% in aqueous solution. The absorption spectrum of HCPE-Gd has two maxima at 318 and 417 nm, and its photoluminescence maximum centers at 591 nm. Confocal laser scanning microscopy studies indicate that the HCPE-Gd is internalized in MCF-7 cancer cell cytoplasm with good photostability and low cytotoxicity. Further fluorescence and MR imaging studies on hepatoma H22 tumor-bearing mouse model reveal that HCPE-Gd can serve as an efficient optical/MR dual-modal imaging nanoprobe for in vivo cancer diagnosis.

Systemic delivery of siRNA by chimeric capsid protein: tumor targeting and RNAi activity in vivo

Recently, we reported that a chimeric capsid protein assembled into a macromolecular container-like structure with capsid shell and the resulting siRNA/capsid nanocarrier complexes efficiently suppressed RFP gene expression in the cell culture system. To extend RNAi to the in vivo applications, we here demonstrated that the siRNA/capsid nanocarrier complexes could have tumor-specific targeting ability in vivo as well as the increased stability of siRNA during body circulation. When systemically administered, our siRNA/capsid nanocarrier complexes delivered siRNA to tumor tissues and efficiently suppressed RFP gene expression in tumor-bearing mice. The enhanced longevity of siRNA in vivo could be explained by shielding effect derived from the capsid shell, where the encapsulated siRNAs are protected from nucleases in plasma. The multivalent RGD peptides on shell surface, as a result of self-assembling of capsid protein subunits, showed efficient delivery of siRNA to the tumor tissues in vivo, due to the RGD-mediated binding to integrin receptors overexpressed on tumor cells. Moreover, the prolonged in vivo circulation time of our siRNA/capsid nanocarrier complexes increased the potential to serve as siRNA carriers for optimal in vivo RNAi. These results provide an alternative approach to systemically deliver siRNA to the tumor sites as well as to enhance the stability of siRNA in vivo. Therefore, our results revealed the promising potential of our capsid nanocarrier system as a therapeutic siRNA carrier for cancer treatment.