In vitro-reassembled plant virus-like particles for loading of polyacids

Size-selective downstream processing of virus particles and non-enveloped virus-like particles

Non-enveloped virus-like particles (VLPs) are versatile protein nanoparticles with great potential for biopharmaceutical applications. However, conventional protein downstream processing (DSP) and platform processes are often not easily applicable due to the large size of VLPs and virus particles (VPs) in general. The application of size-selective separation techniques offers to exploit the size difference between VPs and common host-cell impurities. Moreover, size-selective separation techniques offer the potential for wide applicability across different VPs. In this work, basic principles and applications of size-selective separation techniques are reviewed to highlight their potential in DSP of VPs. Finally, specific DSP steps for non-enveloped VLPs and their subunits are reviewed as well as the potential applications and benefits of size-selective separation techniques are shown.

Virus-like particle-based nanocarriers as an emerging platform for drug delivery

New nanocarrier technologies are emerging, and they have great potential for improving drug delivery, targeting efficiency, and bioavailability. Virus-like particles (VLPs) are natural nanoparticles from animal and plant viruses and bacteriophages. Hence, VLPs present several great advantages, such as morphological uniformity, biocompatibility, reduced toxicity, and easy functionalisation. VLPs can deliver many active ingredients to the target tissue and have great potential as a nanocarrier to overcome the limitations associated with other nanoparticles. This review will focus primarily on the construction and applications of VLPs, particularly as a novel nanocarrier to deliver active ingredients. Herein, the main methods for the construction, purification, and characterisation of VLPs, as well as various VLP-based materials used in delivery systems are summarised. The biological distribution of VLPs in drug delivery, phagocyte-mediated clearance, and toxicity are also discussed.

Electrostatic Theory of the Acidity of the Solution in the Lumina of Viruses and Virus-Like Particles

Recently, Maassen et al. measured an appreciable pH difference between the bulk solution and the solution in the lumen of virus-like particles, self-assembled in an aqueous buffer solution containing the coat proteins of a simple plant virus and polyanions (Maassen, S. J.; et al. Small 2018, 14, 1802081). They attribute this to the Donnan effect, caused by an imbalance between the number of negative charges on the encapsulated polyelectrolyte molecules and the number of positive charges on the RNA binding domains of the coat proteins that make up the virus shell or capsid. By applying Poisson-Boltzmann theory, we confirm this conclusion and show that simple Donnan theory is accurate even for the smallest of viruses and virus-like particles. This, in part, is due to the additional screening caused by the presence of a large number of immobile charges in the cavity of the shell. The presence of a net charge on the outer surface of the capsid we find in practice to not have a large effect on the pH shift. Hence, Donnan theory can indeed be applied to connect the local pH and the amount of encapsulated material. The large shifts up to a full pH unit that we predict must have consequences for applications of virus capsids as nanocontainers in bionanotechnology and artificial cell organelles.

The diversity of protein-protein interaction interfaces within T=3 icosahedral viral capsids

Some non-enveloped virus capsids assemble from multiple copies of a single type of coat-protein (CP). The comparative energetics of the diverse CP-CP interfaces present in such capsids likely govern virus assembly-disassembly mechanisms. The T = 3 icosahedral capsids comprise 180 CP copies arranged about two-, three-, five- and six-fold axes of (quasi-)rotation symmetry. Structurally diverse CPs can assemble into T = 3 capsids. Specifically, the Leviviridae CPs are structurally distinct from the Bromoviridae, Tombusviridae and Tymoviridae CPs which fold into the classic “jelly-roll” fold. However, capsids from across the four families are known to disassemble into dimers. To understand whether the overall symmetry of the capsid or the structural details of the CP determine virus assembly-disassembly mechanisms, we analyze the different CP-CP interfaces that occur in the four virus families. Previous work studied protein homodimer interfaces using interface size (relative to the monomer) and hydrophobicity. Here, we analyze all CP-CP interfaces using these two parameters and find that the dimerization interface (present between two CPs congruent through a two-fold axis of rotation) has a larger relative size in the Leviviridae than in the other viruses. The relative sizes of the other Leviviridae interfaces and all the jelly-roll interfaces are similar. However, the dimerization interfaces across families have slightly higher hydrophobicity, potentially making them stronger than other interfaces. Finally, although the CP-monomers of the jelly-roll viruses are structurally similar, differences in their dimerization interfaces leads to varied dimer flexibility. Overall, differences in CP-structures may induce different modes of swelling and assembly-disassembly in the T = 3 viruses.

Innovative Applications of Plant Viruses in Drug Targeting and Molecular Imaging- A Review

BACKGROUND

Nature had already engineered various types of nanoparticles (NPs), especially viruses, which can deliver their cargo to the host/targeted cells. The ability to selectively target specific cells offers a significant advantage over the conventional approach. Numerous organic NPs, including native protein cages, virus-like particles, polymeric saccharides, and liposomes, have been used for the preparation of nanoparticles. Such nanomaterials have demonstrated better performance as well as improved biocompatibility, devoid of side effects, and stable without any deterioration.

OBJECTIVE

This review discusses current clinical and scientific research on naturally occurring nanomaterials. It also illustrates and updates the tailor-made approaches for selective delivery and targeted medications that require a high-affinity interconnection to the targeted cells.

METHODS

A comprehensive search was performed using keywords for viral nanoparticles, viral particles for drug delivery, viral nanoparticles for molecular imaging, theranostics applications of viral nanoparticles and plant viruses in nanomedicine. We searched on Google Scholar, PubMed, Springer, Medline, and Elsevier from 2000 till date and by the bibliographic review of all identified articles.

RESULTS

The findings demonstrated that structures dependent on nanomaterials might have potential applications in diagnostics, cell marking, comparing agents (computed tomography and magnetic resonance imaging), and antimicrobial drugs, as well as drug delivery structures. However, measures should be taken in order to prevent or mitigate, in pharmaceutical or medical applications, the toxic impact or incompatibility of nanoparticle-based structures with biological systems.

CONCLUSION

The review provided an overview of the latest advances in nanotechnology, outlining the difficulties and the advantages of in vivo and in vitro structures that are focused on a specific subset of the natural nanomaterials.

Inorganic-organic Nanomaterials for Therapeutics and Molecular Imaging Applications

Background::

Surface modification of nanoparticles with targeting moieties can be achieved through bioconjugation chemistries to impart new Functionalities. Various polymeric nanoparticles have been used for the formulation of nanoparticles such as naturally-occurring protein cages, virus-like particles, polymeric saccharides, and liposomes. These polymers have been proven to be biocompatible, side effects free and degradable with no toxicity.

Objectives::

This paper reviews available literature on the nanoparticles pharmaceutical and medical applications. The review highlights and updates the customized solutions for selective drug delivery systems that allow high-affinity binding between nanoparticles and the target receptors.

Methods::

Bibliographic databases and web-search engines were used to retrieve studies that assessed the usability of nanoparticles in the pharmaceutical and medical fields. Data were extracted on each system in vivo and in vitro applications, its advantages and disadvantages, and its ability to be chemically and genetically modified to impart new functionalities. Finally, a comparison between naturally occurring and their synthetic counterparts was carried out.

Results::

The results showed that nanoparticles-based systems could have promising applications in diagnostics, cell labeling, contrast agents (Magnetic Resonance Imaging and Computed Tomography), antimicrobial agents, and as drug delivery systems. However, precautions should be taken to avoid or minimize toxic effect or incompatibility of nanoparticles-based systems with the biological systems in case of pharmaceutical or medical applications.

Conclusion::

This review presented a summary of recent developments in the field of pharmaceutical nanotechnology and highlighted the challenges and the merits that some of the nanoparticles- based systems both in vivo and in vitro systems.

Process monitoring of virus-like particle reassembly by diafiltration with UV/Vis spectroscopy and light scattering

Virus-like particles (VLPs) have shown great potential as biopharmaceuticals in the market and in clinics. Nonenveloped, in vivo assembled VLPs are typically disassembled and reassembled in vitro to improve particle stability, homogeneity, and immunogenicity. At the industrial scale, cross-flow filtration (CFF) is the method of choice for performing reassembly by diafiltration. Here, we developed an experimental CFF setup with an on-line measurement loop for the implementation of process analytical technology (PAT). The measurement loop included an ultraviolet and visible (UV/Vis) spectrometer as well as a light scattering photometer. These sensors allowed for monitoring protein concentration, protein tertiary structure, and protein quaternary structure. The experimental setup was tested with three Hepatitis B core Antigen (HBcAg) variants. With each variant, three reassembly processes were performed at different transmembrane pressures (TMPs). While light scattering provided information on the assembly progress, UV/Vis allowed for monitoring the protein concentration and the rate of VLP assembly based on the microenvironment of Tyrosine-132. VLP formation was verified by off-line dynamic light scattering (DLS) and transmission electron microscopy (TEM). Furthermore, the experimental results provided evidence of aggregate-related assembly inhibition and showed that off-line size-exclusion chromatography does not provide a complete picture of the particle content. Finally, a Partial-Least Squares (PLS) model was calibrated to predict VLP concentrations in the process solution. Q 2 values of 0.947-0.984 were reached for the three HBcAg variants. In summary, the proposed experimental setup provides a powerful platform for developing and monitoring VLP reassembly steps by CFF.

Plant/Bacterial Virus-Based Drug Discovery, Drug Delivery, and Therapeutics

Viruses have recently emerged as promising nanomaterials for biotechnological applications. One of the most important applications of viruses is phage display, which has already been employed to identify a broad range of potential therapeutic peptides and antibodies, as well as other biotechnologically relevant polypeptides (including protease inhibitors, minimizing proteins, and cell/organ targeting peptides). Additionally, their high stability, easily modifiable surface, and enormous diversity in shape and size, distinguish viruses from synthetic nanocarriers used for drug delivery. Indeed, several plant and bacterial viruses (e.g., phages) have been investigated and applied as drug carriers. The ability to remove the genetic material within the capsids of some plant viruses and phages produces empty viral-like particles that are replication-deficient and can be loaded with therapeutic agents. This review summarizes the current applications of plant viruses and phages in drug discovery and as drug delivery systems and includes a discussion of the present status of virus-based materials in clinical research, alongside the observed challenges and opportunities.

Physical, chemical, and synthetic virology: Reprogramming viruses as controllable nanodevices

The fields of physical, chemical, and synthetic virology work in partnership to reprogram viruses as controllable nanodevices. Physical virology provides the fundamental biophysical understanding of how virus capsids assemble, disassemble, display metastability, and assume various configurations. Chemical virology considers the virus capsid as a chemically addressable structure, providing chemical pathways to modify the capsid exterior, interior, and subunit interfaces. Synthetic virology takes an engineering approach, modifying the virus capsid through rational, combinatorial, and bioinformatics-driven design strategies. Advances in these three subfields of virology aim to develop virus-based materials and tools that can be applied to solve critical problems in biomedicine and biotechnology, including applications in gene therapy and drug delivery, diagnostics, and immunotherapy. Examples discussed include mammalian viruses, such as adeno-associated virus (AAV), plant viruses, such as cowpea mosaic virus (CPMV), and bacterial viruses, such as Qβ bacteriophage. Importantly, research efforts in physical, chemical, and synthetic virology have further unraveled the design principles foundational to the form and function of viruses. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Biology-Inspired Nanomaterials > Protein and Virus-Based Structures.

Self consistent field theory of virus assembly

The ground state dominance approximation (GSDA) has been extensively used to study the assembly of viral shells. In this work we employ the self-consistent field theory (SCFT) to investigate the adsorption of RNA onto positively charged spherical viral shells and examine the conditions when GSDA does not apply and SCFT has to be used to obtain a reliable solution. We find that there are two regimes in which GSDA does work. First, when the genomic RNA length is long enough compared to the capsid radius, and second, when the interaction between the genome and capsid is so strong that the genome is basically localized next to the wall. We find that for the case in which RNA is more or less distributed uniformly in the shell, regardless of the length of RNA, GSDA is not a good approximation. We observe that as the polymer-shell interaction becomes stronger, the energy gap between the ground state and first excited state increases and thus GSDA becomes a better approximation. We also present our results corresponding to the genome persistence length obtained through the tangent-tangent correlation length and show that it is zero in case of GSDA but is equal to the inverse of the energy gap when using SCFT.

The different faces of mass action in virus assembly

The spontaneous encapsulation of genomic and non-genomic polyanions by coat proteins of simple icosahedral viruses is driven, in the first instance, by electrostatic interactions with polycationic RNA binding domains on these proteins. The efficiency with which the polyanions can be encapsulated in vitro, and presumably also in vivo, must in addition be governed by the loss of translational and mixing entropy associated with co-assembly, at least if this co-assembly constitutes a reversible process. These forms of entropy counteract the impact of attractive interactions between the constituents and hence they counteract complexation. By invoking mass action-type arguments and a simple model describing electrostatic interactions, we show how these forms of entropy might settle the competition between negatively charged polymers of different molecular weights for co-assembly with the coat proteins. In direct competition, mass action turns out to strongly work against the encapsulation of RNAs that are significantly shorter, which is typically the case for non-viral (host) RNAs. We also find that coat proteins favor forming virus particles over nonspecific binding to other proteins in the cytosol even if these are present in vast excess. Our results rationalize a number of recent in vitro co-assembly experiments showing that short polyanions are less effective at attracting virus coat proteins to form virus-like particles than long ones do, even if both are present at equal weight concentrations in the assembly mixture.

Review on nanoparticles and nanostructured materials: history, sources, toxicity and regulations

Nanomaterials (NMs) have gained prominence in technological advancements due to their tunable physical, chemical and biological properties with enhanced performance over their bulk counterparts. NMs are categorized depending on their size, composition, shape, and origin. The ability to predict the unique properties of NMs increases the value of each classification. Due to increased growth of production of NMs and their industrial applications, issues relating to toxicity are inevitable. The aim of this review is to compare synthetic (engineered) and naturally occurring nanoparticles (NPs) and nanostructured materials (NSMs) to identify their nanoscale properties and to define the specific knowledge gaps related to the risk assessment of NPs and NSMs in the environment. The review presents an overview of the history and classifications of NMs and gives an overview of the various sources of NPs and NSMs, from natural to synthetic, and their toxic effects towards mammalian cells and tissue. Additionally, the types of toxic reactions associated with NPs and NSMs and the regulations implemented by different countries to reduce the associated risks are also discussed.

Plant virus nanoparticles: Novel and robust nanocarriers for drug delivery and imaging

Nanoparticles have been gained much attention for biomedical applications. A promising type of nanocarriers is viral nanoparticles (VNPs) which are natural bio-nanomaterials derived from different type of viruses. Amongst VNPs, plant VNPs present several pros over general nanoparticles such as liposomes, dendrimers or quantum dots. Some of these advantages include: degradability, safety for human, known structures to atomic level, possibility of attaching ligand with vigorous control on structure, availability for genetic and chemical manipulations and very flexible methods to prepare them. Variety of plant viruses have been modified by chemical and genetic modification of their inner cavities and their outer-surfaces. These modifications provide suitable sites for attachment of markers and drug molecules for vascular imaging and tumor targeting. In this review a brief description of plant virus nanoparticles and their biomedical applications especially in drug delivery is provided. The methods of loading cargos in these VNPs and their final biofate are also reviewed.

The effect of RNA stiffness on the self-assembly of virus particles

Under many in vitro conditions, some small viruses spontaneously encapsidate a single stranded (ss) RNA into a protein shell called the capsid. While viral RNAs are found to be compact and highly branched because of long distance base-pairing between nucleotides, recent experiments reveal that in a head-to-head competition between an ssRNA with no secondary or higher order structure and a viral RNA, the capsid proteins preferentially encapsulate the linear polymer! In this paper, we study the impact of genome stiffness on the encapsidation free energy of the complex of RNA and capsid proteins. We show that an increase in effective chain stiffness because of base-pairing could be the reason why under certain conditions linear chains have an advantage over branched chains when it comes to encapsidation efficiency. While branching makes the genome more compact, RNA base-pairing increases the effective Kuhn length of the RNA molecule, which could result in an increase of the free energy of RNA confinement, that is, the work required to encapsidate RNA, and thus less efficient packaging.

In Vitro-Reassembled Plant Virus-Like Particles of Hibiscus Chlorotic Ringspot Virus (HCRSV) as Nano-Protein Cages for Drugs

Spherical shaped plant viruses require a precise quantity, size, and shape of their coat protein subunits to assemble into virions of identical dimensions. The capsid of spherical plant virus particles typically consists of a precisely shaped protein cage, which in many cases is assembled from identical coat protein subunits. In addition to packaging the viral genome, such protein cages may have the capacity to load foreign compounds, either large molecules (e.g., polymers) or small molecules (e.g., anticancer chemotherapy drugs). Therefore, reassembled protein cages of suitable viruses can serve as carriers for cargo loading, which is what makes them an attractive platform for drug delivery. Here we describe methods to reassemble plant virus-like particles of hibiscus chlorotic ringspot virus (HCRSV) as nano-protein cages including the techniques to purify coat protein, prepare virus-like particles, and load them with foreign compounds.

Physalis Mottle Virus-Like Particles as Nanocarriers for Imaging Reagents and Drugs

Platform technologies based on plant virus nanoparticles (VNPs) and virus-like particles (VLPs) are attracting the attention of researchers and clinicians because the particles are biocompatible, biodegradable, noninfectious in mammals, and can readily be chemically and genetically engineered to carry imaging agents and drugs. When the Physalis mottle virus (PhMV) coat protein is expressed in Escherichia coli, the resulting VLPs are nearly identical to the viruses formed in vivo. Here, we isolated PhMV-derived VLPs from ClearColi cells and carried out external and internal surface modification with fluorophores using reactive lysine-N-hydroxysuccinimide ester and cysteine-maleimide chemistries, respectively. The uptake of dye-labeled particles was tested in a range of cancer cells and monitored by confocal microscopy and flow cytometry. VLPs labeled internally on cysteine residues were taken up with high efficiency by several cancer cell lines and were colocalized with the endolysosomal marker LAMP-1 within 6 h, whereas VLPs labeled externally on lysine residues were taken up with lower efficiency, probably reflecting differences in surface charge and the propensity to bind to the cell surface. The infusion of dye and drug molecules into the cavity of the VLPs revealed that the photosensitizer (PS), Zn-EpPor, and the drugs crystal violet, mitoxantrone (MTX), and doxorubicin (DOX) associated stably with the carrier via noncovalent interactions. We confirmed the cytotoxicity of the PS-PhMV and DOX-PhMV particles against prostate cancer, ovarian and breast cancer cell lines, respectively. Our results show that PhMV-derived VLPs provide a new platform technology for the delivery of imaging agents and drugs, with preferential uptake into cancer cells. These particles could therefore be developed as multifunctional tools for cancer diagnosis and therapy.

Icosahedral plant viral nanoparticles - bioinspired synthesis of nanomaterials/nanostructures

Viral nanotechnology utilizes virus nanoparticles (VNPs) and virus-like nanoparticles (VLPs) of plant viruses as highly versatile platforms for materials synthesis and molecular entrapment that can be used in the nanotechnological fields, such as in next-generation nanoelectronics, nanocatalysis, biosensing and optics, and biomedical applications, such as for targeting, therapeutic delivery, and non-invasive in vivo imaging with high specificity and selectivity. In particular, plant virus capsids provide biotemplates for the production of novel nanostructured materials with organic/inorganic moieties incorporated in a very precise and controlled manner. Interestingly, capsid proteins of spherical plant viruses can self-assemble into well-organized icosahedral three-dimensional (3D) nanoscale multivalent architectures with high monodispersity and structural symmetry. Using viral genetic and protein engineering of icosahedral viruses with a variety of sizes, the interior, exterior and the interfaces between coat protein (CP) subunits can be manipulated to fabricate materials with a wide range of desirable properties allowing for biomineralization, encapsulation, infusion, controlled self-assembly, and multivalent ligand display of nanoparticles or molecules for varied applications. In this review, we discuss the various functional nanomaterials/nanostructures developed using the VNPs and VLPs of different icosahedral plant viruses and their nano(bio)technological and nanomedical applications.

Impact of a nonuniform charge distribution on virus assembly

Many spherical viruses encapsulate their genomes in protein shells with icosahedral symmetry. This process is spontaneous and driven by electrostatic interactions between positive domains on the virus coat proteins and the negative genomes. We model the effect of the nonuniform icosahedral charge distribution from the protein shell instead using a mean-field theory. We find that this nonuniform charge distribution strongly affects the optimal genome length and that it can explain the experimentally observed phenomenon of overcharging of virus and viruslike particles.

Structural basis for biologically relevant mechanical stiffening of a virus capsid by cavity-creating or spacefilling mutations

Recent studies reveal that the mechanical properties of virus particles may have been shaped by evolution to facilitate virus survival. Manipulation of the mechanical behavior of virus capsids is leading to a better understanding of viral infection, and to the development of virus-based nanoparticles with improved mechanical properties for nanotechnological applications. In the minute virus of mice (MVM), deleterious mutations around capsid pores involved in infection-related translocation events invariably increased local mechanical stiffness and interfered with pore-associated dynamics. To provide atomic-resolution insights into biologically relevant changes in virus capsid mechanics, we have determined by X-ray crystallography the structural effects of deleterious, mechanically stiffening mutations around the capsid pores. Data show that the cavity-creating N170A mutation at the pore wall does not induce any dramatic structural change around the pores, but instead generates subtle rearrangements that propagate throughout the capsid, resulting in a more compact, less flexible structure. Analysis of the spacefilling L172W mutation revealed the same relationship between increased stiffness and compacted capsid structure. Implications for understanding connections between virus mechanics, structure, dynamics and infectivity, and for engineering modified virus-based nanoparticles, are discussed.

Viral Delivery of dsRNA for Control of Insect Agricultural Pests and Vectors of Human Disease: Prospects and Challenges

RNAi is applied as a new and safe method for pest control in agriculture but efficiency and specificity of delivery of dsRNA trigger remains a critical issue. Various agents have been proposed to augment dsRNA delivery, such as engineered micro-organisms and synthetic nanoparticles, but the use of viruses has received relatively little attention. Here we present a critical view of the potential of the use of recombinant viruses for efficient and specific delivery of dsRNA. First of all, it requires the availability of plasmid-based reverse genetics systems for virus production, of which an overview is presented. For RNA viruses, their application seems to be straightforward since dsRNA is produced as an intermediate molecule during viral replication, but DNA viruses also have potential through the production of RNA hairpins after transcription. However, application of recombinant virus for dsRNA delivery may not be straightforward in many cases, since viruses can encode RNAi suppressors, and virus-induced silencing effects can be determined by the properties of the encoded RNAi suppressor. An alternative is virus-like particles that retain the efficiency and specificity determinants of natural virions but have encapsidated non-replicating RNA. Finally, the use of viruses raises important safety issues which need to be addressed before application can proceed.

Exploiting virus-like particles as innovative vaccines against emerging viral infections

Emerging viruses pose a major threat to humans and livestock with global public health and economic burdens. Vaccination remains an effective tool to reduce this threat, and yet, the conventional cell culture often fails to produce sufficient vaccine dose. As an alternative to cell-culture based vaccine, virus-like particles (VLPs) are considered as a highpriority vaccine strategy against emerging viruses. VLPs represent highly ordered repetitive structures via macromolecular assemblies of viral proteins. The particulate nature allows efficient uptake into antigen presenting cells stimulating both innate and adaptive immune responses towards enhanced vaccine efficacy. Increasing research activity and translation opportunity necessitate the advances in the design of VLPs and new bioprocessing modalities for efficient and cost-effective production. Herein, we describe major achievements and challenges in this endeavor, with respect to designing strategies to harnessing the immunogenic potential, production platforms, downstream processes, and some exemplary cases in developing VLP-based vaccines.

Amino Acid Side Chains Buried along Intersubunit Interfaces in a Viral Capsid Preserve Low Mechanical Stiffness Associated with Virus Infectivity

Single-molecule experimental techniques and theoretical approaches reveal that important aspects of virus biology can be understood in biomechanical terms at the nanoscale. A detailed knowledge of the relationship in virus capsids between small structural changes caused by single-point mutations and changes in mechanical properties may provide further physics-based insights into virus function; it may also facilitate the engineering of viral nanoparticles with improved mechanical behavior. Here, we used the minute virus of mice to undertake a systematic experimental study on the contribution to capsid stiffness of amino acid side chains at interprotein interfaces and the specific noncovalent interactions they establish. Selected side chains were individually truncated by introducing point mutations to alanine, and the effects on local and global capsid stiffness were determined using atomic force microscopy. The results revealed that, in the natural virus capsid, multiple, mostly hydrophobic, side chains buried along the interfaces between subunits preserve a comparatively low stiffness of most (S2 and S3) regions. Virtually no point mutation tested substantially reduced stiffness, whereas most mutations increased stiffness of the S2/S3 regions. This stiffening was invariably associated with reduced virus yields during cell infection. The experimental evidence suggests that a comparatively low stiffness at S3/S2 capsid regions may have been biologically selected because it facilitates capsid assembly, increasing infectious virus yields. This study demonstrated also that knowledge of individual amino acid side chains and biological pressures that determine the physical behavior of a protein nanoparticle may be used for engineering its mechanical properties.

Combining Protein Cages and Polymers: from Understanding Self-Assembly to Functional Materials

Protein cages, such as viruses, are well-defined biological nanostructures which are highly symmetrical and monodisperse. They are found in various shapes and sizes and can encapsulate or template non-native materials. Furthermore, the proteins can be chemically or genetically modified giving them new properties. For these reasons, these protein structures have received increasing attention in the field of polymer-protein hybrid materials over the past years, however, advances are still to be made. This Viewpoint highlights the different ways polymers and protein cages or their subunits have been combined to understand self-assembly and create functional materials.

The Generation of Turnip Crinkle Virus-Like Particles in Plants by the Transient Expression of Wild-Type and Modified Forms of Its Coat Protein

Turnip crinkle virus (TCV), a member of the genus carmovirus of the Tombusviridae family, has a genome consisting of a single positive-sense RNA molecule that is encapsidated in an icosahedral particle composed of 180 copies of a single type of coat protein. We have employed the CPMV-HT transient expression system to investigate the formation of TCV-like particles following the expression of the wild-type coat protein or modified forms of it that contain either deletions and/or additions. Transient expression of the coat protein in plants results in the formation of capsid structures that morphologically resemble TCV virions (T = 3 structure) but encapsidate heterogeneous cellular RNAs, rather than the specific TCV coat protein messenger RNA. Expression of an amino-terminal deleted form of the coat protein resulted in the formation of smaller T = 1 structures that are free of RNA. The possibility of utilizing TCV as a carrier for the presentation of foreign proteins on the particle surface was also explored by fusing the sequence of GFP to the C-terminus of the coat protein. The expression of coat protein-GFP hybrids permitted the formation of VLPs but the yield of particles is diminished compared to the yield obtained with unmodified coat protein. Our results confirm the importance of the N-terminus of the coat protein for the encapsidation of RNA and show that the coat protein's exterior P domain plays a key role in particle formation.

Virus-Based Nanoparticles as Versatile Nanomachines

Nanoscale engineering is revolutionizing the way we prevent, detect, and treat diseases. Viruses have played a special role in these developments because they can function as prefabricated nanoscaffolds that have unique properties and are easily modified. The interiors of virus particles can encapsulate and protect sensitive compounds, while the exteriors can be altered to display large and small molecules in precisely defined arrays. These properties of viruses, along with their innate biocompatibility, have led to their development as actively targeted drug delivery systems that expand on and improve current pharmaceutical options. Viruses are naturally immunogenic, and antigens displayed on their surface have been used to create vaccines against pathogens and to break self-tolerance to initiate an immune response to dysfunctional proteins. Densely and specifically aligned imaging agents on viruses have allowed for high-resolution and noninvasive visualization tools to detect and treat diseases earlier than previously possible. These and future applications of viruses have created an exciting new field within the disciplines of both nanotechnology and medicine.

Virus-based nanomaterials as positron emission tomography and magnetic resonance contrast agents: from technology development to translational medicine

Viruses have recently emerged as ideal protein scaffolds for a new class of contrast agents that can be used in medical imaging procedures such as positron emission tomography (PET) and magnetic resonance imaging (MRI). Whereas synthetic nanoparticles are difficult to produce as homogeneous formulations due to the inherently stochastic nature of the synthesis process, virus-based nanoparticles are genetically encoded and are therefore produced as homogeneous and monodisperse preparations with a high degree of quality control. Because the virus capsids have a defined chemical structure that has evolved to carry cargoes of nucleic acids, they can be modified to carry precisely defined cargoes of contrast agents and can be decorated with spatially defined contrast reagents on the internal or external surfaces. Viral nanoparticles can also be genetically programed or conjugated with targeting ligands to deliver contrast agents to specific cells, and the natural biocompatibility of viruses means that they are cleared rapidly from the body. Nanoparticles based on bacteriophages and plant viruses are safe for use in humans and can be produced inexpensively in large quantities as self-assembling recombinant proteins. Based on these considerations, a new generation of contrast agents has been developed using bacteriophages and plant viruses as scaffolds to carry positron-emitting radioisotopes such as [(18) F] fluorodeoxyglucose for PET imaging and iron oxide or Gd(3+) for MRI. Although challenges such as immunogenicity, loading efficiency, and regulatory compliance remain to be address, virus-based nanoparticles represent a promising new enabling technology for a new generation of highly biocompatible and biodegradable targeted imaging reagents.

In vitro and in vivo toxicity evaluation of plant virus nanocarriers

The use of biological self-assembling materials, plant virus nanoparticles in particular, appears very intriguing as it allows a great choice of symmetries and dimensions, easy chemical and biological engineering of both surface and/or internal cavity as well as safe and rapid production in plants. In this perspective, we present an initial evaluation of the safety profile of two structurally different plant viruses produced in Nicotiana benthamiana L. plants: the filamentous Potato virus X and the icosahedral Tomato bushy stunt virus. In vitro haemolysis assay was used to test the cytotoxic effects, which could arise by pVNPs interaction with cellular membranes, while early embryo assay was used to evaluate toxicity and teratogenicity in vivo. Data indicates that these structurally robust particles, still able to infect plants after incubation in serum up to 24h, have neither toxic nor teratogenic effects in vitro and in vivo. This work represents the first safety-focused characterization of pVNPs in view of their possible use as drug delivery carriers.

Next generation vaccines and vectors: Designing downstream processes for recombinant protein-based virus-like particles

In recent years, the development of novel recombinant virus-like particles (VLPs) has been generating new perspectives for the prevention of untreated and arising infectious diseases. However, cost-reduction and acceleration of manufacturing processes for VLP-based vaccines or vectors are key challenges for the global health system. In particular, the design of rapid and cost-efficient purification processes is a critical bottleneck. In this review, we describe and evaluate new concepts, development strategies and unit operations for the downstream processing of VLPs. A special focus is placed on purity requirements and current trends, as well as chances and limitations of novel technologies. The discussed methods and case studies demonstrate the advances and remaining challenges in both rational process development and purification tools for large biomolecules. The potential of a new era of VLP-based products is highlighted by the progress of various VLPs in clinical phases.

Weighing polyelectrolytes packaged in viruslike particles

This Letter reports on the remarkable selectivity of capsid proteins for packaging synthetic polyelectrolytes in viruslike particles. By applying the contrast variation method in small-angle neutron scattering, we accurately estimated the mean mass of packaged polyelectrolytes ⟨Mp⟩ and that of the surrounding capsid ⟨Mcap⟩. Remarkably, the mass ratio ⟨Mp⟩/⟨Mcap⟩ was invariant for polyelectrolyte molecular weights spanning more than 2 orders of magnitude. To do so, capsids either packaged several chains simultaneously or selectively retained the shortest chains that could fit the capsid interior. Our data are in qualitative agreement with theoretical predictions based on free energy minimization and emphasize the importance of protein self-energy. These findings may give new insights into the nonspecific origin of genome selectivity for a number of viral systems.

RNA topology remolds electrostatic stabilization of viruses

Simple RNA viruses efficiently encapsulate their genome into a nano-sized protein shell: the capsid. Spontaneous coassembly of the genome and the capsid proteins is driven predominantly by electrostatic interactions between the negatively charged RNA and the positively charged inner capsid wall. Using field theoretic formulation we show that the inherently branched RNA secondary structure allows viruses to maximize the amount of encapsulated genome and make assembly more efficient, allowing viral RNAs to out-compete cellular RNAs during replication in infected host cells.

Exploiting plant virus-derived components to achieve in planta expression and for templates for synthetic biology applications

This review discusses the varying roles that have been played by many plant-viral regulatory sequences and proteins in the creation of plant-based expression systems and virus particles for use in nanotechnology. Essentially, there are two ways of expressing an exogenous protein: the creation of transgenic plants possessing a stably integrated gene construction, or the transient expression of the desired gene following the infiltration of the gene construct. Both depend on disarmed strains of Agrobacterium tumefaciens to deliver the created gene construction into cell nuclei, usually through the deployment of virus-derived components. The importance of efficient mRNA translation in the latter process is highlighted. Plant viruses replicate to sustain an infection to promote their survival. The major product of this, the virus particle, is finding increasing roles in the emerging field of bionanotechnology. One of the major products of plant-viral expression is the virus-like particle (VLP). These are increasingly playing a role in vaccine development. Similarly, many VLPs are suitable for the investigation of the many facets of the emerging field of synthetic biology, which encompasses the design and construction of new biological functions and systems not found in nature. Genetic and chemical modifications to plant-generated VLPs serve as ideal starter templates for many downstream synthetic biology applications.

A plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus

Structure, size, physicochemical properties and production strategies make many plant viruses ideal protein based nanoscaffolds, nanocontainers and nano-building blocks expected to deliver a multitude of applications in different fields such as biomedicine, pharmaceutical chemistry, separation science, catalytic chemistry, crop pest control and biomaterials science. Functionalization of viral nanoparticles through modification by design of their external and internal surfaces is essential to fully exploit the potentiality of these objects. In the present paper we describe the development of a plant derived multifunctional tool for nanobiotechnology based on Tomato bushy stunt virus. We demonstrate the ability of this system to remarkably sustain genetic modifications and in vitro chemical derivatizations of its outer surface, which resulted in the successful display of large chimeric peptides fusions and small chemical molecules, respectively. Moreover, we have defined physicochemical conditions for viral swelling and reversible viral pore gating that we have successfully employed for foreign molecules loading and retention in the inner cavity of this plant virus nanoparticles system. Finally, a production and purification strategy from Nicotiana benthamiana plants has been addressed and optimized.

Impact of the topology of viral RNAs on their encapsulation by virus coat proteins

Single-stranded RNAs of simple viruses seem to be topologically more compact than other types of single-stranded RNA. It has been suggested that this has an evolutionary purpose: more compact structures are more easily encapsulated in the limited space that the cavity of the virus capsid offers. We employ a simple Flory theory to calculate the optimal amount of polymers confined in a viral shell. We find that the free energy gain or more specifically the efficiency of RNA encapsidation increases substantially with topological compactness. We also find that the optimal length of RNA encapsidated in a capsid increases with the degree of branching of the genome even though this effect is very weak. Further, we show that if the structure of the branching of the polymer is allowed to anneal, the optimal loading increases substantially.

Design rules for nanomedical engineering: from physical virology to the applications of virus-based materials in medicine

Physical virology seeks to define the principles of physics underlying viral infections, traditionally focusing on the fundamental processes governing virus assembly, maturation, and disassembly. A detailed understanding of virus structure and assembly has facilitated the development and analysis of virus-based materials for medical applications. In this Physical Virology review article, we discuss the recent developments in nanomedicine that help us to understand how physical properties affect the in vivo fate and clinical impact of (virus-based) nanoparticles. We summarize and discuss the design rules that need to be considered for the successful development and translation of virus-based nanomaterials from bench to bedside.

Impact of charge variation on the encapsulation of nanoparticles by virus coat proteins

Electrostatic interaction is the driving force for the encapsulation by virus coat proteins of nanoparticles such as quantum dots, gold particles and magnetic beads for, e.g., imaging and therapeutic purposes. In recent experimental work, Daniel et al (2010 ACS Nano 4 3853-60) found the encapsulation efficiency to sensitively depend on the interplay between the surface charge density of negatively charged gold nanoparticles and the number of positive charges on the RNA binding domains of the proteins. Surprisingly, these experiments reveal that despite the highly cooperative nature of the co-assembly at low pH, the efficiency of encapsulation is a gradual function of their surface charge density. We present a simple all-or-nothing mass action law combined with an electrostatic interaction model to explain the experiments. We find quantitative agreement with experimental observations, supporting the existence of a natural statistical charge distribution between nanoparticles.

Increased tumor homing and tissue penetration of the filamentous plant viral nanoparticle Potato virus X

Nanomaterials with elongated architectures have been shown to possess differential tumor homing properties compared to their spherical counterparts. Here, we investigate whether this phenomenon is mirrored by plant viral nanoparticles that are filamentous (Potato virus X) or spherical (Cowpea mosaic virus). Our studies demonstrate that Potato virus X (PVX) and Cowpea mosaic virus (CPMV) show distinct biodistribution profiles and differ in their tumor homing and penetration efficiency. Analogous to what is seen with inorganic nanomaterials, PVX shows enhanced tumor homing and tissue penetration. Human tumor xenografts exhibit higher uptake of PEGylated filamentous PVX compared to CPMV, particularly in the core of the tumor. This is supported by immunohistochemical analysis of the tumor sections, which indicates greater penetration and accumulation of PVX within the tumor tissues. The enhanced tumor homing and retention properties of PVX along with its higher payload carrying capacity make it a potentially superior platform for applications in cancer drug delivery and imaging applications.

Nanoreactors by programmed enzyme encapsulation inside the capsid of the bacteriophage P22

The virus like particle (VLP) derived from bacteriophage P22 presents a unique platform for constructing catalytically functional nanomaterials by encapsulation of enzymes into its interior. Encapsulation has been engineered to be genetically programmed allowing "one pot" synthesis and incorporation of desired enzymes. The unique characteristic that separates P22 from other VLP systems is the ability to modulate the overall volume and porosity of the VLP structure, thus controlling substrate access to the encapsulated enzyme. The present study demonstrates incorporation of an enzyme, alcohol dehydrogenase D, with the highest internal loading for an active enzyme by any VLP described thus far. In addition, we show that not only does encapsulating AdhD inside P22 affect its kinetic parameters in comparison with the "free" enzyme, but transformation of P22 to different morphological states, which changes the internal volume of the VLP, yields changes in the overall activity of the encapsulated enzyme as well. The findings reported here clearly illustrate that P22 holds potential for synthetic approaches to create nanoreactors, by design, using the power of highly evolved enzymes for chemical transformations.

Virus-based nanocarriers for drug delivery

New nanocarrier platforms based on natural biological building blocks offer great promises in revolutionalizing medicine. The usage of specific protein cage structures: virus-like particles (VLPs) for drug packaging and targetted delivery is summarized here. Versatile chemical and genetic modifications on the outer surfaces and inner cavities of VLPs facilitate the preparation of new materials that could meet the biocompatibility, solubility and high uptake efficiency requirements for drug delivery. A full evaluation on the toxicity, bio-distribution and immunology of these materials are envisaged to boost their application potentials.

Development of viral nanoparticles for efficient intracellular delivery

Viral nanoparticles (VNPs) based on plant viruses such as Cowpea mosaic virus (CPMV) can be used for a broad range of biomedical applications because they present a robust scaffold that allows functionalization by chemical conjugation and genetic modification, thereby offering an efficient drug delivery platform that can target specific cells and tissues. VNPs such as CPMV show natural affinity to cells; however, cellular uptake is inefficient. Here we show that chemical modification of the CPMV surface with a highly reactive, specific and UV-traceable hydrazone linker allows bioconjugation of polyarginine (R5) cell penetrating peptides (CPPs), which can overcome these limitations. The resulting CPMV-R5 particles were taken up into a human cervical cancer cell line (HeLa) more efficiently than native particles. Uptake efficiency was dependent on the density of R5 peptides on the surface of the VNP; particles displaying 40 R5 peptides per CPMV (denoted as CPMV-R5H) interact strongly with the plasma membrane and are taken up into the cells via an energy-dependent mechanism whereas particles displaying 10 R5 peptides per CPMV (CPMV-R5L) are only slowly taken up. The fate of CPMV-R5 versus native CPMV particles within cells was evaluated in a co-localization time course study. It was indicated that the intracellular localization of CPMV-R5 and CPMV differs; CPMV remains trapped in Lamp-1 positive endolysosomes over long time frames; in contrast, 30-50% of the CPMV-R5 particles transitioned from the endosome into other cellular vesicles or compartments. Our data provide the groundwork for the development of efficient drug delivery formulations based on CPMV-R5.

Encapsidation of DNA, a protein and a fluorophore into virus-like particles by the capsid protein of cucumber mosaic virus

An important property of some spherical plant viruses is their ability to reassemble in vitro from native capsid protein (CP) and RNA into infectious virus-like particles (VLPs). Virions of cucumber mosaic virus (CMV) are stabilized by protein–RNA interactions and the nucleic acid is essential for assembly. This study demonstrated that VLPs will form in the presence of both ssDNA and dsDNA oligonucleotides, and with a lower size limit of 20 nt. Based on urea disruption assays, assembled VLPs from CMV CP and RNA (termed ReCMV) exhibited a level of stability similar to that of virions purified from plants, whilst VLPs from CMV CP and a 20mer exhibited comparable or greater stability. Fluorescent labelling of VLPs was achieved by the encapsidation of an Alexa Fluor 488-labelled 45mer oligonucleotide (ReCMV-Alexa488-45) and confirmed by transmission electron and confocal microscopy. Using ssDNA as a nucleating factor, encapsidation of fluorescently labelled streptavidin (53 kDa) conjugated to a biotinylated oligonucleotide was observed. The biological activity and stability of ReCMV and ReCMV-Alexa488-45 was confirmed in infectivity assays and insect vector feeding assays. This work demonstrates the utility of CMV CP as a protein cage for use in the growing repertoire of nanotechnological applications.

Maize rayado fino virus virus-like particles expressed in tobacco plants: A new platform for cysteine selective bioconjugation peptide display

Maize rayado fino virus (MRFV) virus-like-particles (VLPs) produced in tobacco plants were examined for their ability to serve as a novel platform to which a variety of peptides can be covalently displayed when expressed through a Potato virus X (PVX)-based vector. To provide an anchor for chemical modifications, three Cys-MRFV-VLPs mutants were created by substituting several of the amino acids present on the shell of the wild-type MRFV-VLPs with cysteine residues. The mutant designated Cys 2-VLPs exhibited, under native conditions, cysteine thiol reactivity in bioconjugation reactions with a fluorescent dye. In addition, this Cys 2-VLPs was cross-linked by NHS-PEG4-Maleimide to 17 (F) and 8 (HN) amino acid long peptides, corresponding to neutralizing epitopes of Newcastle disease virus (NDV). The resulting Cys 2-VLPs-F and Cys 2-VLPs-HN were recognized in Western blots by antibodies to MRFV as well as to F and HN. The results demonstrated that plant-produced MRFV-VLPs have the ability to function as a novel platform for the multivalent display of surface ligands.

Applications of plant viruses in bionanotechnology

The capsids of most plant viruses are simple and robust structures consisting of multiple copies of one or a few types of protein subunit arranged with either icosahedral or helical symmetry. In many cases, capsids can be produced in large quantities either by the infection of plants or by the expression of the subunit(s) in a variety of heterologous systems. In view of their relative simplicity, stability and ease of production, plant virus particles or virus-like particles (VLPs) have attracted attention as potential reagents for applications in bionanotechnology. As a result, plant virus particles have been subjected to both genetic and chemical modification, have been used to encapsulate foreign material and have, themselves, been incorporated into supramolecular structures.

Using polymer conformation to control architecture in semiconducting polymer/viral capsid assemblies

Cowpea chlorotic mottle virus is a single-stranded RNA plant virus with a diameter of 28 nm. The proteins comprising the capsid of this virus can be purified and reassembled either by themselves to form hollow structures or with polyanions such as double-stranded DNA or single-stranded RNA. Depending on pH and ionic strength, a diverse range of structures and shapes can form. The work presented here focuses on using these proteins to encapsulate a fluorescent polyanionic semiconducting polymer, MPS-PPV (poly-2-methoxy-5-propyloxy sulfonate phenylene vinlyene), in order to obtain optically active virus-like particles. After encapsulation, fluorescence from MPS-PPV shows two distinct peaks, which suggests the polymer may be in two conformations. A combination of TEM, fluorescence anisotropy, and sucrose gradient separation indicate that the blue peak arises from polymer encapsulated into spherical particles, while the redder peak corresponds to polymers contained in rod-like cages. Ionic strength during assembly can be used to tune the propensity to form rods or spheres. The results illustrate the synergy of hybrid synthetic/biological systems: polymer conformation drives the structure of this composite material, which in turn modifies the polymer optical properties. This synergy could be useful for the future development of synthetic/biological hybrid materials with designated functionality.

Applications of viral nanoparticles in medicine

Several nanoparticle platforms are currently being developed for applications in medicine, including both synthetic materials and naturally occurring bionanomaterials such as viral nanoparticles (VNPs) and their genome-free counterparts, virus-like particles (VLPs). A broad range of genetic and chemical engineering methods have been established that allow VNP/VLP formulations to carry large payloads of imaging reagents or drugs. Furthermore, targeted VNPs and VLPs can be generated by including peptide ligands on the particle surface. In this article, we highlight state-of-the-art virus engineering principles and discuss recent advances that bring potential biomedical applications a step closer. Viral nanotechnology has now come of age and it will not be long before these formulations assume a prominent role in the clinic.

The art of engineering viral nanoparticles

Viral nanotechnology is an emerging and highly interdisciplinary field in which viral nanoparticles (VNPs) are applied in diverse areas such as electronics, energy and next-generation medical devices. VNPs have been developed as candidates for novel materials, and are often described as "programmable" because they can be modified and functionalized using a number of techniques. In this review, we discuss the concepts and methods that allow VNPs to be engineered, including (i) bioconjugation chemistries, (ii) encapsulation techniques, (iii) mineralization strategies, and (iv) film and hydrogel development. With all these techniques in hand, the potential applications of VNPs are limited only by the imagination.

Expression and Self Assembly of Cowpea Chlorotic Mottle Virus Capsid Proteins inPichia pastorisand Encapsulation of Fluorescent Myoglobin

Cowpea chlorotic mottle virus (CCMV) has been a model system for virus studies for over 40 years and now is considered to be a perfect candidate as nanoplatform for applications in materials science and medicine. The ability of CCMV to self assemblein vitrointo virus-like particles (VLPs) or capsids makes an ideal reaction vessel for nanomaterial synthesis and entrapment. Here we report expression of codon optimized CCMV coat protein inPichia pastorisand production of self assembled CCMV VLPs by large-scale fermentation. CCMV coat protein gene (573 bp) was synthesized according to codon preference ofP. pastorisand cloned into pPICZA vector. The recombinant plasmid pPICZA-CP was transformed intoP. pastorisGS115 by electroporation. The resulting yeast colonies were screened by PCR and analyzed for protein expression by SDS-PAGE. After large-scale fermentation CCMV coat protein yields reached 4.8 g L−1. The CCMV VLPs were purified by modified PEG precipitation followed by cesium chloride density gradient ultracentrifugation, and then analyzed by size exclusion fast performance liquid chromatography (FPLC), UV spectrometry and transmission electron microscopy. Myoglobin was used as a model protein to be encapsulated in CCMV VLPs. The fluorescence spectroscopy showed that inclusion of myoglobin had occurred. The results indicated the production of CCMV capsids byP. pastorisfermentation now available for utilization in pharmacology or nanotechnology fields.

Viruses as nanomaterials for drug delivery

Virus delivery vectors are one among the many nanomaterials that are being developed as drug delivery materials. This chapter focuses on methods utilizing plant virus nanoparticles (PVNs) synthesized from the Red clover necrotic mosaic virus (RCNMV). A successful vector must be able to effectively carry and subsequently deliver a drug cargo to a specific target. In the case of the PVNs, we describe two types of ways cargo can be loaded within these structures: encapsidation and infusion. Several targeting approaches have been used for PVNs based on bioconjugate chemistry. Herein, examples of such approaches will be given that have been used for RCNMV as well as for other PVNs in the literature. Further, we describe characterization of PVNs, in vitro cell studies that can be used to test the efficacy of a targeting vector, and potential routes for animal administration.