Controlling gold nanoparticle seeded growth in thermophilic ferritin protein templates

Engineered human H-chain ferritin with reversed charge of the internal cavity exhibits RNA-mediated spongelike effect for loading RNA/DNA-binding molecules

Ferritins are globular proteins with an internal cavity that enables the encapsulation of a plethora of low-mass compounds. Unfortunately, the overall negative surface charge of ferritin's internal cavity hampers efficient loading of negatively charged molecules. Therefore, we produced a genetically engineered human H-chain ferritin containing a cationic RKRK domain, reversing the natural net charge of the cavity to positive, thus allowing for efficient encapsulation of negatively charged siRNA. Due to the reversed, positive charge mediated by RKRK domains, the recombinant ferritin produced in E. coli inherently carries a load of bacterial RNA inside its cavity, turning the protein into an effective sponge possessing high affinity for DNA/RNA-binding substances that can be loaded with markedly higher efficiency compared to the wildtype protein. Using doxorubicin as payload, we show that due to its loading through the RNA sponge, doxorubicin is released in a sustained manner, with a cytotoxicity profile similar to the free drug. In summary, this is the first report demonstrating a ferritin/nucleic acid hybrid delivery vehicle with a broad spectrum of properties exploitable in various fields of biomedical applications.

Virus-like particles nanoreactors: from catalysis towards bio-applications

Virus-like particles (VLPs) are self-assembled supramolecular structures found in nature, often used for compartmentalization. Exploiting their inherent properties, including precise nanoscale structures, monodispersity, and high stability, these architectures have been widely used as nanocarriers to protect or enrich catalysts, facilitating catalytic reactions and avoiding interference from the bulk solutions. In this review, we summarize the current progress of virus-like particles (VLPs)-based nanoreactors. First, we briefly introduce the physicochemical properties of the most commonly used virus particles to understand their roles in catalytic reactions beyond the confined space. Next, we summarize the self-assembly of nanoreactors forming higher-order hierarchical structures, highlighting the emerging field of nanoreactors as artificial organelles and their potential biomedical applications. Finally, we discuss the current findings and future perspectives of VLPs-based nanoreactors.

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.

Enhanced Cellular Uptake of H-Chain Human Ferritin Containing Gold Nanoparticles

Gold nanoparticles (AuNP) capped with biocompatible layers have functional optical, chemical, and biological properties as theranostic agents in biomedicine. The ferritin protein containing in situ synthesized AuNPs has been successfully used as an effective and completely biocompatible nanocarrier for AuNPs in human cell lines and animal experiments in vivo. Ferritin can be uptaken by different cell types through receptor-mediated endocytosis. Despite these advantages, few efforts have been made to evaluate the toxicity and cellular internalization of AuNP-containing ferritin nanocages. In this work, we study the potential of human heavy-chain (H) and light-chain (L) ferritin homopolymers as nanoreactors to synthesize AuNPs and their cytotoxicity and cellular uptake in different cell lines. The results show very low toxicity of ferritin-encapsulated AuNPs on different human cell lines and demonstrate that efficient cellular ferritin uptake depends on the specific H or L protein chains forming the ferritin protein cage and the presence or absence of metallic cargo. Cargo-devoid apoferritin is poorly internalized in all cell lines, and the highest ferritin uptake was achieved with AuNP-loaded H-ferritin homopolymers in transferrin-receptor-rich cell lines, showing more than seven times more uptake than apoferritin.

Artificial Organelles: Towards Adding or Restoring Intracellular Activity

Compartmentalization is one of the main characteristics that define living systems. Creating a physically separated microenvironment allows nature a better control over biological processes, as is clearly specified by the role of organelles in living cells. Inspired by this phenomenon, researchers have developed a range of different approaches to create artificial organelles: compartments with catalytic activity that add new function to living cells. In this review we will discuss three complementary lines of investigation. First, orthogonal chemistry approaches are discussed, which are based on the incorporation of catalytically active transition metal-containing nanoparticles in living cells. The second approach involves the use of premade hybrid nanoreactors, which show transient function when taken up by living cells. The third approach utilizes mostly genetic engineering methods to create bio-based structures that can be ultimately integrated with the cell's genome to make them constitutively active. The current state of the art and the scope and limitations of the field will be highlighted with selected examples from the three approaches.

Ferritin Nanocages for Protein Delivery to Tumor Cells

The delivery of therapeutic proteins is one of the greatest challenges in the treatment of human diseases. In this frame, ferritins occupy a very special place. Thanks to their hollow spherical structure, they are used as modular nanocages for the delivery of anticancer drugs. More recently, the possibility of encapsulating even small proteins with enzymatic or cytotoxic activity is emerging. Among all ferritins, particular interest is paid to the Archaeoglobus fulgidus one, due to its peculiar ability to associate/dissociate in physiological conditions. This protein has also been engineered to allow recognition of human receptors and used in vitro for the delivery of cytotoxic proteins with extremely promising results.

Thermostable iron oxide nanoparticle synthesis within recombinant ferritins from the hyperthermophile Pyrococcus yayanosii CH1

Thermostable nanoparticles have numerous applications in catalysis and in the oil/gas industry. However, synthesizing these nanoparticles requires expensive polymers. Here, a novel thermostable ferritin named PcFn, originally from the hyperthermophilic archaeon Pyrococcus yayanosii CH1, was overexpressed in Escherichia coli, purified and characterized, which could successfully direct the synthesis of thermostable magnetoferritins (M-PcFn) with monodispersed iron oxide nanoparticles in one step. Transmission electron microscopy and magnetic measurements show that the cores of the M-PcFn have an average diameter of 4.7 nm, are well-crystalline and superparamagnetic. Both the PcFn and M-PcFn can resist temperatures up to 110 °C, which is significantly higher than for human H-chain ferritin (HFn) and M-HFn, and comparable to temperatures previously reported for Pyrococcus furiosus ferritin (PfFn) and M-PfFn. After heating at 110 °C for 30 minutes, PcFn and M-PcFn maintained their secondary structures and PcFn retained 87.4% of its iron uptake activity. This remarkable thermostability of PcFn and M-PcFn suggests potential applications in elevated temperature environments.

A protein-protein host-guest complex: Thermostable ferritin encapsulating positively supercharged green fluorescent protein

We characterize the encapsulation of supercharged green fluorescent protein, GFP(+36), by thermophilic ferritin from Archaeoglobus fulgidus (AfFtn). The AfFtn-GFP(+36) assembly is rapid, nearly stoichiometric, and robust. Using a more stably assembled mutant AfFtn, we show that encapsulation can occur in the presence of mostly assembled cages, in addition to encapsulation starting from AfFtn individual subunits. Assembly and encapsulation do not occur with non-supercharged GFP or the alternately supercharged GFP(-30), highlighting the role of complementary electrostatic interactions between the cargo and AfFtn cage interior. We also present a method for verifying protein-protein encapsulation, using nickel nitrilotriacetic acid agarose resin. AfFtn-supercharged protein host-guest complexes could find applications in enzyme studies, protein separations, and in vivo protein stabilization and targeted delivery.

Spatially controlled assembly of affinity ligand and enzyme cargo enables targeting ferritin nanocarriers to caveolae

One of the goals of nanomedicine is targeted delivery of therapeutic enzymes to the sub-cellular compartments where their action is needed. Endothelial caveolae-derived endosomes represent an important yet challenging destination for targeting, in part due to smaller size of the entry aperture of caveolae (ca. 30-50 nm). Here, we designed modular, multi-molecular, ferritin-based nanocarriers with uniform size (20 nm diameter) for easy drug-loading and targeted delivery of enzymatic cargo to these specific vesicles. These nanocarriers targeted to caveolar Plasmalemmal Vesicle-Associated Protein (Plvap) deliver superoxide dismutase (SOD) into endosomes in endothelial cells, the specific site of influx of superoxide mediating by such pro-inflammatory signaling as some cytokines and lipopolysaccharide (LPS). Cell studies showed efficient internalization of Plvap-targeted SOD-loaded nanocarriers followed by dissociation from caveolin-containing vesicles and intracellular transport to endosomes. The nanocarriers had a profound protective anti-inflammatory effect in an animal model of LPS-induced inflammation, in agreement with the characteristics of their endothelial uptake and intracellular transport, indicating that these novel, targeted nanocarriers provide an advantageous platform for caveolae-dependent delivery of biotherapeutics.

Ferritin-based drug delivery systems: Hybrid nanocarriers for vascular immunotargeting

Ferritin subunits of heavy and light polypeptide chains self-assemble into a spherical nanocage that serves as a natural transport vehicle for metals but can include diverse cargoes. Ferritin nanoparticles are characterized by remarkable stability, small and uniform size. Chemical modifications and molecular re-engineering of ferritin yield a versatile platform of nanocarriers capable of delivering a broad range of therapeutic and imaging agents. Targeting moieties conjugated to the ferritin external surface provide multivalent anchoring of biological targets. Here, we highlight some of the current work on ferritin as well as examine potential strategies that could be used to functionalize ferritin via chemical and genetic means to enable its utility in vascular drug delivery.

Ferritin Nanocages with Biologically Orthogonal Conjugation for Vascular Targeting and Imaging

Genetic incorporation of biologically orthogonal functional groups into macromolecules has the potential to yield efficient, controlled, reproducible, site-specific conjugation of affinity ligands, contrast agents, or therapeutic cargoes. Here, we applied this approach to ferritin, a ubiquitous iron-storage protein that self-assembles into multimeric nanocages with remarkable stability, size uniformity (12 nm), and endogenous capacity for loading and transport of a variety of inorganic and organic cargoes. The unnatural amino acid, 4-azidophenylalanine (4-AzF), was incorporated at different sites in the human ferritin light chain (hFTL) to allow site-specific conjugation of alkyne-containing small molecules or affinity ligands to the exterior surface of the nanocage. The optimal positioning of the 4-AzF residue was evaluated by screening a library of variants for the efficiency of copper-free click conjugation. One of the engineered ferritins, hFTL-5X, was found to accommodate ∼14 small-molecule fluorophores (AlexaFluor 488) and 3-4 IgG molecules per nanocage. Intravascular injection in mice of radiolabeled hFTL-5X carrying antibody to cell adhesion molecule ICAM-1, but not control IgG, enabled specific targeting to the lung due to high basal expression of ICAM-1 (43.3 ± 6.99 vs 3.48 ± 0.14%ID/g for Ab vs IgG). Treatment of mice with endotoxin known to stimulate inflammatory ICAM-1 overexpression resulted in 2-fold enhancement of pulmonary targeting (84.4 ± 12.89 vs 43.3 ± 6.99%ID/g). Likewise, injection of fluorescent, ICAM-targeted hFTL-5X nanocages revealed the effect of endotoxin by enhancement of near-infrared signal, indicating potential utility of this approach for both vascular targeting and imaging.