Visualization of the externalized VP2 N termini of infectious human parvovirus B19

Studying the ssDNA loaded adeno-associated virus aggregation using coarse-grained molecular dynamics simulations

The aggregation of adeno-associated viral (AAV) capsids in an aqueous environment was investigated via coarse-grained molecular dynamics (CG-MD) simulations. The primary driving force and mechanism of the aggregation were investigated with or without single-strand DNA (ssDNA) loaded at various process temperatures. Capsid aggregation appeared to involve multiple residue interactions (i.e., hydrophobic, polar and charged residues) leading to complex protein aggregation. In addition, two aggregation mechanisms (i.e., the fivefold face-to-face contact and the edge-to-edge contact) were identified from this study. The ssDNA with its asymmetric structure could be the reason for destabilizing protein subunits and enhancing the interaction between the charged residues, and further result in the non-reversible face-to-face contact. At higher temperature, the capsid structure was found to be unstable with the significant size expansion of the loaded ssDNA which could be attributed to reduced number of intramolecular hydrogen bonds, the increased conformational deviations of protein subunits and the higher residue fluctuations. The CG-MD model was further validated with previous experimental and simulation data, including the full capsid size measurement and the capsid internal pressure. Thus, a good understanding of AAV capsid aggregation, instability and the role of ssDNA were revealed by applying the developed computational model.

Therapeutic Potential of Engineered Virus-like Particles of Parvovirus B19

Virus-like particles (VLPs) comprise one or many structural components of virions, except their genetic material. Thus, VLPs keep their structural properties of cellular recognition while being non-infectious. VLPs of Parvovirus B19 (B19V) can be produced by the heterologous expression of their structural proteins VP1 and VP2 in bacteria. These proteins are purified under denaturing conditions, refolded, and assembled into VLPs. Moreover, chimeric forms of VP2 have been constructed to harbor peptides or functional proteins on the surface of the particles without dropping their competence to form VLPs, serving as presenting nanoparticles. The in-vitro assembly approach offers exciting possibilities for the composition of VLPs, as more than one chimeric form of VP2 can be included in the assembly stage, producing multifunctional VLPs. Here, the heterologous expression and in-vitro assembly of B19V structural proteins and their chimeras are reviewed. Considerations for the engineering of the structural proteins of B19V are also discussed. Finally, the construction of multifunctional VLPs and their future potential as innovative medical tools are examined.

Bipartite genome and structural organization of the parvovirus Acheta domesticus segmented densovirus

Parvoviruses (family Parvoviridae) are currently defined by a linear monopartite ssDNA genome, T = 1 icosahedral capsids, and distinct structural (VP) and non-structural (NS) protein expression cassettes within their genome. We report the discovery of a parvovirus with a bipartite genome, Acheta domesticus segmented densovirus (AdSDV), isolated from house crickets (Acheta domesticus), in which it is pathogenic. We found that the AdSDV harbors its NS and VP cassettes on two separate genome segments. Its vp segment acquired a phospholipase A2-encoding gene, vpORF3, via inter-subfamily recombination, coding for a non-structural protein. We showed that the AdSDV evolved a highly complex transcription profile in response to its multipartite replication strategy compared to its monopartite ancestors. Our structural and molecular examinations revealed that the AdSDV packages one genome segment per particle. The cryo-EM structures of two empty- and one full-capsid population (3.3, 3.1 and 2.3 Å resolution) reveal a genome packaging mechanism, which involves an elongated C-terminal tail of the VP, "pinning" the ssDNA genome to the capsid interior at the twofold symmetry axis. This mechanism fundamentally differs from the capsid-DNA interactions previously seen in parvoviruses. This study provides new insights on the mechanism behind ssDNA genome segmentation and on the plasticity of parvovirus biology.

Tracking of Human Parvovirus B19 Virus-Like Particles Using Short Peptide Tags Reveals a Membrane-Associated Extracellular Release of These Particles

B19 virus (B19V) is a pathogenic human parvovirus that infects erythroid progenitor cells. Because there are limited in vitro culture systems to propagate this virus, little is known about the molecular mechanisms by which it propagates in cells. In this study, we introduced a HiBiT peptide tag into various loops of VP2 located on the surface of B19V particles and evaluated their ability to form virus-like particles (VLPs). Three independent sites were identified as permissive sites for peptide tag insertion without affecting VLP formation. When the HiBiT tag was introduced into B19V clones (pB19-M20) and transfected into a semipermissive erythroleukemia cell line (UT7/Epo-S1), HiBiT-dependent luciferase activities (HiBiT activities) increased depending on helicase activity of viral NS1. Furthermore, we used a GFP11 tag-split system to visualize VLPs in the GFP1-10-expressing live cells. Time-lapse imaging of green fluorescent protein (GFP)-labeled VLPs revealed that nuclear VLPs were translocated into the cytoplasm only after cell division, suggesting that the breakdown of the nuclear envelope during mitosis contributes to VLP nuclear export. Moreover, HiBiT activities of culture supernatants were dependent on the presence of a detergent, and the released VLPs were associated with extracellular vesicles, as observed under electron microscopy. Treatment with an antimitotic agent (nocodazole) enhanced the release of VLPs. These results suggest that the virions accumulated in the cytoplasm are constitutively released from the cell as membrane-coated vesicles. These properties are likely responsible for viral escape from host immune responses and enhance membrane fusion-mediated transmission. IMPORTANCE Parvovirus particles are expected to be applied as nanoparticles in drug delivery systems. However, little is known about how nuclear-assembled B19 virus (B19V) virions are released from host cells. This study provides evidence of mitosis-dependent nuclear export of B19V and extracellular vesicle-mediated virion release. Moreover, this study provides methods for modifying particle surfaces with various exogenous factors and contributes to the development of fine nanoparticles with novel valuable functions. The pB19-M20 plasmid expressing HiBiT-tagged VP2 is a novel tool to easily quantify VP2 expression. Furthermore, this system can be applied in high-throughput screening of reagents that affect VP2 expression, which might be associated with viral propagation.

Capsid Structure of Aleutian Mink Disease Virus and Human Parvovirus 4: New Faces in the Parvovirus Family Portrait

Parvoviruses are small, single-stranded DNA viruses with non-enveloped capsids. Determining the capsid structures provides a framework for annotating regions important to the viral life cycle. Aleutian mink disease virus (AMDV), a pathogen in minks, and human parvovirus 4 (PARV4), infecting humans, are parvoviruses belonging to the genera Amdoparvovirus and Tetraparvovirus, respectively. While Aleutian mink disease caused by AMDV is a major threat to mink farming, no clear clinical manifestations have been established following infection with PARV4 in humans. Here, the capsid structures of AMDV and PARV4 were determined via cryo-electron microscopy at 2.37 and 3.12 Å resolutions, respectively. Despite low amino acid sequence identities (10-30%) both viruses share the icosahedral nature of parvovirus capsids, with 60 viral proteins (VPs) assembling the capsid via two-, three-, and five-fold symmetry VP-related interactions, but display major structural variabilities in the surface loops when the capsid structures are superposed onto other parvoviruses. The capsid structures of AMDV and PARV4 will add to current knowledge of the structural platform for parvoviruses and permit future functional annotation of these viruses, which will help in understanding their infection mechanisms at a molecular level for the development of diagnostics and therapeutics.

Cryo-electron Microscopy of Adeno-associated Virus

Adeno-associated virus (AAV) has a single-stranded DNA genome encapsidated in a small icosahedrally symmetric protein shell with 60 subunits. AAV is the leading delivery vector in emerging gene therapy treatments for inherited disorders, so its structure and molecular interactions with human hosts are of intense interest. A wide array of electron microscopic approaches have been used to visualize the virus and its complexes, depending on the scientific question, technology available, and amenability of the sample. Approaches range from subvolume tomographic analyses of complexes with large and flexible host proteins to detailed analysis of atomic interactions within the virus and with small ligands at resolutions as high as 1.6 Å. Analyses have led to the reclassification of glycan receptors as attachment factors, to structures with a new-found receptor protein, to identification of the epitopes of antibodies, and a new understanding of possible neutralization mechanisms. AAV is now well-enough characterized that it has also become a model system for EM methods development. Heralding a new era, cryo-EM is now also being deployed as an analytic tool in the process development and production quality control of high value pharmaceutical biologics, namely AAV vectors.

Structural Dynamics and Activity of B19V VP1u during the pHs of Cell Entry and Endosomal Trafficking

Parvovirus B19 (B19V) is a human pathogen that is the causative agent of fifth disease in children. It is also known to cause hydrops in fetuses, anemia in AIDS patients, and transient aplastic crisis in patients with sickle cell disease. The unique N-terminus of Viral Protein 1 (VP1u) of parvoviruses, including B19V, exhibits phospholipase A2 (PLA2) activity, which is required for endosomal escape. Presented is the structural dynamics of B19V VP1u under conditions that mimic the pHs of cell entry and endosomal trafficking to the nucleus. Using circular dichroism spectroscopy, the receptor-binding domain of B19V VP1u is shown to exhibit an α-helical fold, whereas the PLA2 domain exhibits a probable molten globule state, both of which are pH invariant. Differential scanning calorimetry performed at endosomal pHs shows that the melting temperature (Tm) of VP1u PLA2 domain is tuned to body temperature (37 °C) at pH 7.4. In addition, PLA2 assays performed at temperatures ranging from 25-45 °C show both a temperature and pH-dependent change in activity. We hypothesize that VP1u PLA2 domain differences in Tm at differing pHs have enabled the virus to "switch on/off" the phospholipase activity during capsid trafficking. Furthermore, we propose the environment of the early endosome as the optimal condition for endosomal escape leading to B19V infection.

Adeno-Associated Virus Receptor-Binding: Flexible Domains and Alternative Conformations through Cryo-Electron Tomography of Adeno-Associated Virus 2 (AAV2) and AAV5 Complexes

Recombinant forms of adeno-associated virus (rAAV) are vectors of choice in the development of treatments for a number of genetic dispositions. Greater understanding of AAV’s molecular virology is needed to underpin needed improvements in efficiency and specificity. Recent advances have included identification of a near-universal entry receptor, AAVR, and structures detected by cryo-electron microscopy (EM) single particle analysis (SPA) that revealed, at high resolution, only the domains of AAVR most tightly bound to AAV. Here, cryogenic electron tomography (cryo-ET) is applied to reveal the neighboring domains of the flexible receptor. For AAV5, where the PKD1 domain is bound strongly, PKD2 is seen in three configurations extending away from the virus. AAV2 binds tightly to the PKD2 domain at a distinct site, and cryo-ET now reveals four configurations of PKD1, all different from that seen in AAV5. The AAV2 receptor complex also shows unmodeled features on the inner surface that appear to be an equilibrium alternate configuration. Other AAV structures start near the 5-fold axis, but now β-strand A is the minor conformer and, for the major conformer, partially ordered N termini near the 2-fold axis join the canonical capsid jellyroll fold at the βA-βB turn. The addition of cryo-ET is revealing unappreciated complexity that is likely relevant to viral entry and to the development of improved gene therapy vectors.

IMPORTANCE With 150 clinical trials for 30 diseases under way, AAV is a leading gene therapy vector. Immunotoxicity at high doses used to overcome inefficient transduction has occasionally proven fatal and highlighted gaps in fundamental virology. AAV enters cells, interacting through distinct sites with different domains of the AAVR receptor, according to AAV clade. Single domains are resolved in structures by cryogenic electron microscopy. Here, the adjoining domains are revealed by cryo-electron tomography of AAV2 and AAV5 complexes. They are in flexible configurations interacting minimally with AAV, despite measurable dependence of AAV2 transduction on both domains.

Comparative structural, biophysical, and receptor binding study of true type and wild type AAV2

Adeno-associated viruses (AAV) are utilized as gene transfer vectors in the treatment of monogenic disorders. A variant, rationally engineered based on natural AAV2 isolates, designated AAV-True Type (AAV-TT), is highly neurotropic compared to wild type AAV2 in vivo, and vectors based on it, are currently being evaluated for central nervous system applications. AAV-TT differs from AAV2 by 14 amino acids, including R585S and R588T, two residues previously shown to be essential for heparan sulfate binding of AAV2. The capsid structures of AAV-TT and AAV2 visualized by cryo-electron microscopy at 3.4 and 3.0 Å resolution, respectively, highlighted structural perturbations at specific amino acid differences. Differential scanning fluorimetry (DSF) performed at different pH conditions demonstrated that the melting temperature (Tm) of AAV2 was consistently ∼5 °C lower than AAV-TT, but both showed maximal stability at pH 5.5, corresponding to the pH in the late endosome, proposed as required for VP1u externalization to facilitate endosomal escape. Reintroduction of arginines at positions 585 and 588 in AAV-TT caused a reduction in Tm, demonstrating that the lack of basic amino acids at these positions are associated with capsid stability. These results provide structural and thermal annotation of AAV2/AAV-TT residue differences, that account for divergent cell binding, transduction, antigenic reactivity, and transduction of permissive tissues between the two viruses. Specifically, these data indicate that AAV-TT may not utilize a glycan receptor mediated pathway to enter cells and may have lower antigenic properties as compared to AAV2.

The N-terminal 5-68 amino acids domain of the minor capsid protein VP1 of human parvovirus B19 enters human erythroid progenitors and inhibits B19 infection

Parvovirus B19 (B19V) infection causes diseases in humans ranging from the mild erythema infectiosum to severe hematological disorders. The unique region of the minor structural protein VP1 (VP1u) of 227 amino acids harbors strong neutralizing epitopes which elicit dominant immune responses in patients. Recent studies have shown that the VP1u selectively binds to and enters B19V permissive cells through an unknown cellular proteinaceous receptor. In the present study, we demonstrated that purified recombinant VP1u effectively inhibits B19V infection of ex vivo expanded primary human erythroid progenitors. Furthermore, we identified the amino acid sequence 5-68 of the VP1 (VP1u^5-68aa) is sufficient to confer the inhibition of B19V infection at a level similar to that of the full-length VP1u. In silico structure prediction suggests that the VP1u^5-68aa contains three α-helices. Importantly, we found that the inhibition capability of the minimal domain VP1u^5-68aa is independent of its dimerization but is likely dependent on the structure of the three predicated α-helices. As VP1u^5-68aa outcompetes the full-length VP1u in entering cells, we believe that VP1u^5-68aa functions as a receptor-binding ligand during virus entry. Finally, we determined the effective inhibition potency of VP1u^5-68aa in B19V infection of human erythroid progenitors, which has a half maximal effective concentration (EC₅₀) of 67 nM, suggesting an anti-viral peptide candidate to combat B19V infection.IMPORTANCEHuman parvovirus B19 infection causes severe hematological disorders, including transient aplastic crisis, pure red cell aplasia, and hydrops fetalis. A productive B19 infection is highly restricted to human erythroid progenitors in human bone marrow and fetal liver. In the current study, we identified that the N-terminal 5-68 amino acids domain of the minor viral capsid protein VP1 enters ex vivo expanded human erythroid progenitors, which is nearly 5 times more efficient than the full-length VP1 unique region (1-227aa). Importantly, purified recombinant 5-68aa of the VP1 has a high efficiency in inhibition of parvovirus B19 infection of human erythroid progenitors, which has a half maximal effective concentration (EC₅₀) of 67 nM and a low cytotoxicity. The N-terminal 5-68 amino acids holds the potential as an effective antiviral of parvovirus B19 caused hematological disorders, as well as a carrier to deliver proteins to human erythroid progenitors.

The VP1u of Human Parvovirus B19: A Multifunctional Capsid Protein with Biotechnological Applications

The viral protein 1 unique region (VP1u) of human parvovirus B19 (B19V) is a multifunctional capsid protein with essential roles in virus tropism, uptake, and subcellular trafficking. These functions reside on hidden protein domains, which become accessible upon interaction with cell membrane receptors. A receptor-binding domain (RBD) in VP1u is responsible for the specific targeting and uptake of the virus exclusively into cells of the erythroid lineage in the bone marrow. A phospholipase A₂ domain promotes the endosomal escape of the incoming virus. The VP1u is also the immunodominant region of the capsid as it is the target of neutralizing antibodies. For all these reasons, the VP1u has raised great interest in antiviral research and vaccinology. Besides the essential functions in B19V infection, the remarkable erythroid specificity of the VP1u makes it a unique erythroid cell surface biomarker. Moreover, the demonstrated capacity of the VP1u to deliver diverse cargo specifically to cells around the proerythroblast differentiation stage, including erythroleukemic cells, offers novel therapeutic opportunities for erythroid-specific drug delivery. In this review, we focus on the multifunctional role of the VP1u in B19V infection and explore its potential in diagnostics and erythroid-specific therapeutics.

Molecular biology and structure of a novel penaeid shrimp densovirus elucidate convergent parvoviral host capsid evolution

The giant tiger prawn (Penaeus monodon) is a decapod crustacean widely reared for human consumption. Currently, viruses of two distinct lineages of parvoviruses (PVs, family Parvoviridae; subfamily Hamaparvovirinae) infect penaeid shrimp. Here, a PV was isolated and cloned from Vietnamese P. monodon specimens, designated Penaeus monodon metallodensovirus (PmMDV). This is the first member of a third divergent lineage shown to infect penaeid decapods. PmMDV has a transcription strategy unique among invertebrate PVs, using extensive alternative splicing and incorporating transcription elements characteristic of vertebrate-infecting PVs. The PmMDV proteins have no significant sequence similarity with other PVs, except for an SF3 helicase domain in its nonstructural protein. Its capsid structure, determined by cryoelectron microscopy to 3-Å resolution, has a similar surface morphology to Penaeus stylirostris densovirus, despite the lack of significant capsid viral protein (VP) sequence similarity. Unlike other PVs, PmMDV folds its VP without incorporating a βA strand and displayed unique multimer interactions, including the incorporation of a Ca²⁺ cation, attaching the N termini under the icosahedral fivefold symmetry axis, and forming a basket-like pentamer helix bundle. While the PmMDV VP sequence lacks a canonical phospholipase A2 domain, the structure of an EDTA-treated capsid, determined to 2.8-Å resolution, suggests an alternative membrane-penetrating cation-dependent mechanism in its N-terminal region. PmMDV is an observed example of convergent evolution among invertebrate PVs with respect to host-driven capsid structure and unique as a PV showing a cation-sensitive/dependent basket structure for an alternative endosomal egress.

A new method for the construction of coarse-grained models of large biomolecules from low-resolution cryo-electron microscopy data

The rapid development of cryo-electron microscopy (cryo-EM) has led to the generation of significant low-resolution electron density data of biomolecules. However, the atomistic details of huge biomolecules usually cannot be obtained because it is very difficult to construct all-atom models for MD simulations. Thus, it is still a challenge to make use of the rich low-resolution cryo-EM data for computer simulation and functional study. In this study, we proposed a new method called Convolutional and K-means Coarse-Graining (CK-CG) for the efficient coarse-graining of large biological systems. Using the CK-CG method, we could directly map the cryo-EM data into coarse-grained (CG) beads. Furthermore, the CG beads were parameterized with an empirical harmonic potential to construct a new CG model. We subjected the CK-CG models of the fibrillar protein assemblies F-actin and collagen to external forces in pulling dynamic simulations to assess their mechanical response. The agreement between the estimated tensile stiffness between CG models and experiments demonstrates the validity of the CK-CG method. Thus, our method provides a practical strategy for the direct construction of a structural model from low-resolution data for biological function studies.

Parvovirus B19 Uncoating Occurs in the Cytoplasm without Capsid Disassembly and It Is Facilitated by Depletion of Capsid-Associated Divalent Cations

Human parvovirus B19 (B19V) traffics to the cell nucleus where it delivers the genome for replication. The intracellular compartment where uncoating takes place, the required capsid structural rearrangements and the cellular factors involved remain unknown. We explored conditions that trigger uncoating in vitro and found that prolonged exposure of capsids to chelating agents or to buffers with chelating properties induced a structural rearrangement at 4 °C resulting in capsids with lower density. These lighter particles remained intact but were unstable and short exposure to 37 °C or to a freeze-thaw cycle was sufficient to trigger DNA externalization without capsid disassembly. The rearrangement was not observed in the absence of chelating activity or in the presence of MgCl₂ or CaCl₂, suggesting that depletion of capsid-associated divalent cations facilitates uncoating. The presence of assembled capsids with externalized DNA was also detected during B19V entry in UT7/Epo cells. Following endosomal escape and prior to nuclear entry, a significant proportion of the incoming capsids rearranged and externalized the viral genome without capsid disassembly. The incoming capsids with accessible genomes accumulated in the nuclear fraction, a process that was prevented when endosomal escape or dynein function was disrupted. In their uncoated conformation, capsids immunoprecipitated from cytoplasmic or from nuclear fractions supported in vitro complementary-strand synthesis at 37 °C. This study reveals an uncoating strategy of B19V based on a limited capsid rearrangement prior to nuclear entry, a process that can be mimicked in vitro by depletion of divalent cations.

Structure of Parvovirus B19 Decorated by Fabs from a Human Antibody

Parvovirus B19, one of the most common human pathogens, is a small DNA virus that belongs to the Parvoviridae As a result of previous infections, antibodies to B19 are present in most adults. B19 has a strong tropism to erythroid progenitor cells and is able to cause a series of medical conditions, including fifth disease, arthritis, myocarditis, hydrops fetalis, and aplastic crisis. No approved vaccine is currently available for B19, and there is a lack of structural characterization of any B19 epitopes. Here we present the first cryo-electron microscopy (cryo-EM) structure of a B19 virus-like particle (VLP) complexed with the antigen-binding fragment (Fab) of a human neutralizing antibody, 860-55D. A model was built into the 3.2-Å-resolution map, and the antigenic residues on the surface of the B19 capsid were identified. Antibody 860-55D bridges the capsid of B19 by binding to a quaternary structure epitope formed by residues from three neighboring VP2 capsid proteins.IMPORTANCE Parvovirus B19 is a common human pathogen and a particular threat to children, pregnant women, and patients with sickle cell disease or AIDS. Currently, neutralizing antibody is the most efficient treatment for acute B19 infections. Research on the antigenic properties of B19 will guide the usage of these antibodies and facilitate vaccine development. We have determined and report here the high-resolution structure of B19 virus-like particles (VLPs) complexed with the Fab of a human neutralizing antibody. The structure shows a quaternary structure epitope formed by three VP2 proteins and provides details on host recognition of human B19 virus.

Secondary structure of DNA released from purified capsids of human parvovirus B19 under moderate denaturing conditions

Parvovirus B19 (B19V) possesses a linear single-stranded DNA genome of either positive or negative polarity. Due to intramolecular sequence homologies, either strand may theoretically be folded in several alternative ways. Viral DNA, when extracted from virions by several procedures, presents as linear single-stranded and/or linear double-stranded molecules, except when one particular commercial kit is used. This protocol yields DNA with an aberrant electrophoretic mobility in addition to linear double-stranded molecules, but never any single-stranded molecules. This peculiar kind of DNA was found in all plasma or serum samples tested and so we decided to analyse its secondary structure. In line with our results for one- and two-dimensional electrophoresis, mobility shift assays, DNA preparation by an in-house extraction method with moderate denaturing conditions, density gradient ultracentrifugation, DNA digestion experiments and competition hybridization assays, we conclude that (i) the unique internal portions of this distinctive single-stranded molecules are folded into tight tangles and (ii) the two terminal redundant regions are associated with each other, yielding non-covalently closed pseudo-circular molecules stabilized by a short (18 nucleotides) intramolecular stem, whereas the extreme 3'- and 5'-ends are folded back on themselves, forming a structure resembling a twin hairpin. The question arises as to whether this fairly unstable structure represents the encapsidated genome structure. The answer to this question remains quite relevant in terms of comprehending the initiation and end of B19V genome replication.

Differential phosphorylation and n-terminal configuration of capsid subunits in parvovirus assembly and viral trafficking

The T1 parvovirus Minute Virus of Mice (MVM) was used to study the roles that phosphorylation and N-terminal domains (Nt) configuration of capsid subunits may play in icosahedral nuclear viruses assembly. In synchronous MVM infection, capsid subunits newly assembled as two types of cytoplasmic trimeric intermediates (3VP2, and 1VP1:2VP2) harbored a VP1 phosphorylation level fivefold higher than that of VP2, and hidden Nt. Upon nuclear translocation at S phase, VP1-Nt became exposed in the heterotrimer and subsequent subviral assembly intermediates. Empty capsid subunits showed a phosphorylation level restored to VP1:VP2 stoichiometry, and the Nt concealed in their interior. However ssDNA-filled virus maturing at S/G2 lacked VP1 phosphorylation and one major VP2 phosphopeptide, and exposed VP2-Nt. Endosomal VP2-Nt cleavage resulted in VP3 subunits devoid of any phospholabel, implying that incoming viral particles specifically harbor a low phosphorylation status. Phosphorylation provides a mechanistic coupling of parvovirus nuclear assembly to the cell cycle.

Reprogramming the Activatable Peptide Display Function of Adeno-Associated Virus Nanoparticles

We harnessed an intrinsic activatable peptide display behavior shared by several parvoviruses, including the adeno-associated virus (AAV), in order to design protein-based nanodevices that can carry out an exogenous functional output in response to stimulus detection. Specifically, we generated truncated viral capsid subunits that, when combined with native capsid components into mosaic capsids, can perform robust activatable peptide display. By modulating the ratio of subunits in the mosaic capsid, properties of the activatable peptide display function can be optimized. Interestingly, the truncated subunits can form homomeric capsids not observed in nature, but at the price of losing the ability to carry out activatable peptide display. Collectively, our results demonstrate the importance of capsid mosaicism when activatable peptide display is desired and help explain why the wild-type AAV capsid exists as a mosaic of different subunits. This proof-of-concept study illustrates a strategy for reprogramming a particular conformational output behavior of AAV in pursuit of the long-term vision of creating stimulus-responsive nanodevices.

Atomic Resolution Structures of Human Bufaviruses Determined by Cryo-Electron Microscopy

Bufavirus strain 1 (BuV1), a member of the Protoparvovirus genus of the Parvoviridae, was first isolated from fecal samples of children with acute diarrhea in Burkina Faso. Since this initial discovery, BuVs have been isolated in several countries, including Finland, the Netherlands, and Bhutan, in pediatric patients exhibiting similar symptoms. Towards their characterization, the structures of virus-like particles of BuV1, BuV2, and BuV3, the current known genotypes, have been determined by cryo-electron microscopy and image reconstruction to 2.84, 3.79, and 3.25 Å, respectively. The BuVs, 65-73% identical in amino acid sequence, conserve the major viral protein, VP2, structure and general capsid surface features of parvoviruses. These include a core β-barrel (βB-βI), α-helix A, and large surface loops inserted between these elements in VP2. The capsid contains depressions at the icosahedral 2-fold and around the 5-fold axes, and has three separated protrusions surrounding the 3-fold axes. Structure comparison among the BuVs and to available parvovirus structures revealed capsid surface variations and capsid 3-fold protrusions that depart from the single pinwheel arrangement of the animal protoparvoviruses. These structures provide a platform to begin the molecular characterization of these potentially pathogenic viruses.

Human Parvoviruses

Parvovirus B19 (B19V) and human bocavirus 1 (HBoV1), members of the large Parvoviridae family, are human pathogens responsible for a variety of diseases. For B19V in particular, host features determine disease manifestations. These viruses are prevalent worldwide and are culturable in vitro, and serological and molecular assays are available but require careful interpretation of results. Additional human parvoviruses, including HBoV2 to -4, human parvovirus 4 (PARV4), and human bufavirus (BuV) are also reviewed. The full spectrum of parvovirus disease in humans has yet to be established. Candidate recombinant B19V vaccines have been developed but may not be commercially feasible. We review relevant features of the molecular and cellular biology of these viruses, and the human immune response that they elicit, which have allowed a deep understanding of pathophysiology.

Peptide presentation on primate erythroparvovirus 1 virus-like particles: In vitro assembly, stability and immunological properties

Virus-like particles (VLPs) have demonstrated to be valuable scaffolds for the display of heterologous peptides for vaccine development and other specific interactions. VLPs of primate erythroparvovirus 1, generally referred as parvovirus B19 (B19V), have already been produced in-vivo and in-vitro from the recombinant VP2 protein of this virus. In this study, chimeric forms of B19V VP2 were constructed, and their ability to assemble into VLPs was evaluated. Chimeras were composed of the VP2 protein fused, at its N-terminus, with two peptides derived from the fusion glycoprotein (F) of the respiratory syncytial virus (RSV). The chimeric proteins self-assembled into VLPs morphologically similar to B19V virions. Stability of these VLPs was analyzed under denaturation conditions with guanidinium chloride (GdnHCl). Our results indicate that the presence of the heterologous fragments increased the stability of VLPs assembled by any of the VP2 chimeras. Specific proteolysis assays shown that a fraction of the N-termini of the chimeric proteins is located on the outer surface of the VLPs. Immunogenicity of VLPs against RSV was evaluated and the results indicate that the particles can elicit a humoral immune response, although these antibodies did not cross-react with RSV in ELISA tests. These results provide novel insights into the localization of the N-termini of B19V VP2 protein after in vitro assembly into VLPs, and point them to be attractive sites to display peptides or proteins without compromise the assembly or stability of VLPs.

The Receptor-Binding Domain in the VP1u Region of Parvovirus B19

Parvovirus B19 (B19V) is known as the human pathogen causing the mild childhood disease erythema infectiosum. B19V shows an extraordinary narrow tissue tropism for erythroid progenitor cells in the bone marrow, which is determined by a highly restricted uptake. We have previously shown that the specific internalization is mediated by the interaction of the viral protein 1 unique region (VP1u) with a yet unknown cellular receptor. To locate the receptor-binding domain (RBD) within the VP1u, we analyzed the effect of truncations and mutations on the internalization capacity of the recombinant protein into UT7/Epo cells. Here we report that the N-terminal amino acids 5-80 of the VP1u are necessary and sufficient for cellular binding and internalization; thus, this N-terminal region represents the RBD required for B19V uptake. Using site-directed mutagenesis, we further identified a cluster of important amino acids playing a critical role in VP1u internalization. In silico predictions and experimental results suggest that the RBD is structured as a rigid fold of three α-helices. Finally, we found that dimerization of the VP1u leads to a considerably enhanced cellular binding and internalization. Taken together, we identified the RBD that mediates B19V uptake and mapped functional and structural motifs within this sequence. The findings reveal insights into the uptake process of B19V, which contribute to understand the pathogenesis of the infection and the neutralization of the virus by the immune system.

Structure of an enteric pathogen, bovine parvovirus

Bovine parvovirus (BPV), the causative agent of respiratory and gastrointestinal disease in cows, is the type member of the Bocaparvovirus genus of the Parvoviridae family. Toward efforts to obtain a template for the development of vaccines and small-molecule inhibitors for this pathogen, the structure of the BPV capsid, assembled from the major capsid viral protein 2 (VP2), was determined using X-ray crystallography as well as cryo-electron microscopy and three-dimensional image reconstruction (cryo-reconstruction) to 3.2- and 8.8-Å resolutions, respectively. The VP2 region ordered in the crystal structure, from residues 39 to 536, conserves the parvoviral eight-stranded jellyroll motif and an αA helix. The BPV capsid displays common parvovirus features: a channel at and depressions surrounding the 5-fold axes and protrusions surrounding the 3-fold axes. However, rather than a depression centered at the 2-fold axes, a raised surface loop divides this feature in BPV. Additional observed density in the capsid interior in the cryo-reconstructed map, compared to the crystal structure, is interpreted as 10 additional N-terminal residues, residues 29 to 38, that radially extend the channel under the 5-fold axis, as observed for human bocavirus 1 (HBoV1). Surface loops of various lengths and conformations extend from the core jellyroll motif of VP2. These loops confer the unique surface topology of the BPV capsid, making it strikingly different from HBoV1 as well as the type members of other Parvovirinae genera for which structures have been determined. For the type members, regions structurally analogous to those decorating the BPV capsid surface serve as determinants of receptor recognition, tissue and host tropism, pathogenicity, and antigenicity.

IMPORTANCE

Bovine parvovirus (BPV), identified in the 1960s in diarrheic calves, is the type member of the Bocaparvovirus genus of the nonenveloped, single-stranded DNA (ssDNA) Parvoviridae family. The recent isolation of human bocaparvoviruses from children with severe respiratory and gastrointestinal infections has generated interest in understanding the life cycle and pathogenesis of these emerging viruses. We have determined the high-resolution structure of the BPV capsid assembled from its predominant capsid protein VP2, known to be involved in a myriad of functions during host cell entry, pathogenesis, and antigenicity for other members of the Parvovirinae. Our results show the conservation of the core secondary structural elements and the location of the N-terminal residues for the known bocaparvovirus capsid structures. However, surface loops with high variability in sequence and conformation give BPV a unique capsid surface topology. Similar analogous regions in other Parvovirinae type members are important as determinants of receptor recognition, tissue and host tropism, pathogenicity, and antigenicity.

Parvoviruses: Small Does Not Mean Simple

Parvoviruses are small, rugged, nonenveloped protein particles containing a linear, nonpermuted, single-stranded DNA genome of ∼5 kb. Their limited coding potential requires optimal adaptation to the environment of particular host cells, where entry is mediated by a variable program of capsid dynamics, ultimately leading to genome ejection from intact particles within the host nucleus. Genomes are amplified by a continuous unidirectional strand-displacement mechanism, a linear adaptation of rolling circle replication that relies on the repeated folding and unfolding of small hairpin telomeres to reorient the advancing fork. Progeny genomes are propelled by the viral helicase into the preformed capsid via a pore at one of its icosahedral fivefold axes. Here we explore how the fine-tuning of this unique replication system and the mechanics that regulate opening and closing of the capsid fivefold portals have evolved in different viral lineages to create a remarkably complex spectrum of phenotypes.

Structure of viruses: a short history

This review is a partially personal account of the discovery of virus structure and its implication for virus function. Although I have endeavored to cover all aspects of structural virology and to acknowledge relevant individuals, I know that I have favored taking examples from my own experience in telling this story. I am anxious to apologize to all those who I might have unintentionally offended by omitting their work. The first knowledge of virus structure was a result of Stanley's studies of tobacco mosaic virus (TMV) and the subsequent X-ray fiber diffraction analysis by Bernal and Fankuchen in the 1930s. At about the same time it became apparent that crystals of small RNA plant and animal viruses could diffract X-rays, demonstrating that viruses must have distinct and unique structures. More advances were made in the 1950s with the realization by Watson and Crick that viruses might have icosahedral symmetry. With the improvement of experimental and computational techniques in the 1970s, it became possible to determine the three-dimensional, near-atomic resolution structures of some small icosahedral plant and animal RNA viruses. It was a great surprise that the protecting capsids of the first virus structures to be determined had the same architecture. The capsid proteins of these viruses all had a 'jelly-roll' fold and, furthermore, the organization of the capsid protein in the virus were similar, suggesting a common ancestral virus from which many of today's viruses have evolved. By this time a more detailed structure of TMV had also been established, but both the architecture and capsid protein fold were quite different to that of the icosahedral viruses. The small icosahedral RNA virus structures were also informative of how and where cellular receptors, anti-viral compounds, and neutralizing antibodies bound to these viruses. However, larger lipid membrane enveloped viruses did not form sufficiently ordered crystals to obtain good X-ray diffraction. Starting in the 1990s, these enveloped viruses were studied by combining cryo-electron microscopy of the whole virus with X-ray crystallography of their protein components. These structures gave information on virus assembly, virus neutralization by antibodies, and virus fusion with and entry into the host cell. The same techniques were also employed in the study of complex bacteriophages that were too large to crystallize. Nevertheless, there still remained many pleomorphic, highly pathogenic viruses that lacked the icosahedral symmetry and homogeneity that had made the earlier structural investigations possible. Currently some of these viruses are starting to be studied by combining X-ray crystallography with cryo-electron tomography.

A slender tract of glycine residues is required for translocation of the VP2 protein N-terminal domain through the parvovirus MVM capsid channel to initiate infection

Viruses constitute paradigms to study conformational dynamics in biomacromolecular assemblies. Infection by the parvovirus MVM (minute virus of mice) requires a conformational rearrangement that involves the intracellular externalization through capsid channels of the 2Nt (N-terminal region of VP2). We have investigated the role in this process of conserved glycine residues in an extended glycine-rich tract located immediately after 2Nt. Based on the virus structure, residues with hydrophobic side chains of increasing volume were substituted for glycine residues 31 or 33. Mutations had no effect on capsid assembly or stability, but inhibited virus infectivity. All mutations, except those to alanine residues which had minor effects, impaired 2Nt externalization in nuclear maturing virions and in purified virions, to an extent that correlated with the side chain size. Different biochemical and biophysical analyses were consistent with this result. Importantly, all of the tested glycine residue replacements impaired the capacity of the virion to initiate infection, at ratios correlating with their restrictive effects on 2Nt externalization. Thus small residues within the evolutionarily conserved glycine-rich tract facilitate 2Nt externalization through the capsid channel, as required by this virus to initiate cell entry. The results demonstrate the exquisite dependence on geometric constraints of a biologically relevant translocation event in a biomolecular complex.

Structural insights into adeno-associated virus serotype 5

The adeno-associated viruses (AAVs) display differential cell binding, transduction, and antigenic characteristics specified by their capsid viral protein (VP) composition. Toward structure-function annotation, the crystal structure of AAV5, one of the most sequence diverse AAV serotypes, was determined to 3.45-Å resolution. The AAV5 VP and capsid conserve topological features previously described for other AAVs but uniquely differ in the surface-exposed HI loop between βH and βI of the core β-barrel motif and have pronounced conformational differences in two of the AAV surface variable regions (VRs), VR-IV and VR-VII. The HI loop is structurally conserved in other AAVs despite amino acid differences but is smaller in AAV5 due to an amino acid deletion. This HI loop is adjacent to VR-VII, which is largest in AAV5. The VR-IV, which forms the larger outermost finger-like loop contributing to the protrusions surrounding the icosahedral 3-fold axes of the AAVs, is shorter in AAV5, creating a smoother capsid surface topology. The HI loop plays a role in AAV capsid assembly and genome packaging, and VR-IV and VR-VII are associated with transduction and antigenic differences, respectively, between the AAVs. A comparison of interior capsid surface charge and volume of AAV5 to AAV2 and AAV4 showed a higher propensity of acidic residues but similar volumes, consistent with comparable DNA packaging capacities. This structure provided a three-dimensional (3D) template for functional annotation of the AAV5 capsid with respect to regions that confer assembly efficiency, dictate cellular transduction phenotypes, and control antigenicity.

Maturation-associated destabilization of hepatitis B virus nucleocapsid

The mature nucleocapsid (NC) of hepatitis B virus containing the relaxed circular (RC) DNA genome can be secreted extracellularly as virions after envelopment with the viral surface proteins or, alternatively, can be disassembled to release RC DNA (i.e., uncoating) into the host cell nucleus to form the covalently closed circular (CCC) DNA, which sustains viral replication and persistence. In contrast, immature NCs containing the viral single-stranded DNA or the pregenomic RNA are incompetent for either envelopment or uncoating. Little is currently known about how mature NCs, and not the immature ones, are specifically selected for these processes. Here, we have carried out a biochemical analysis of the different NC populations upon their separation through sucrose gradient centrifugation. We have found that the maturation of NCs is associated with their destabilization, manifested as increased protease and nuclease sensitivity, altered sedimentation during sucrose gradient centrifugation, and retarded mobility during native agarose gel electrophoresis. Also, three distinct populations of intracellular mature NCs could be differentiated based on these characteristics. Furthermore, mature NCs generated in vitro under cell-free conditions acquired similar properties. These results have thus revealed significant structural changes associated with NC maturation that likely play a role in the selective uncoating of the mature NC for CCC DNA formation and/or its preferential envelopment for virion secretion.

Parvovirus B19 Achievements and Challenges

Parvovirus B19 is a widespread human pathogenic virus, member of the Erythrovirus genus in the Parvoviridae family. Infection can be associated with an ample range of pathologies and clinical manifestations, whose characteristics and outcomes depend on the interplay between the pathogenetic potential of the virus, its adaptation to different cellular environments, and the physiological and immune status of the infected individuals. The scope of this review is the advances in knowledge on the biological characteristics of the virus and of virus-host relationships; in particular, the interactions of the virus with different cellular environments in terms of tropism and ability to achieve a productive replicative cycle, or, on the contrary, to establish persistence; the consequences of infection in terms of interference with the cell physiology; the process of recognition of the virus by the innate or adaptive immune system, hence the role of the immune system in controlling the infection or in the development of clinical manifestations. Linked to these issues is the continuous effort to develop better diagnostic algorithms and methods and the need for development of prophylactic and therapeutic options for B19V infections.

Structural characterization of H-1 parvovirus: comparison of infectious virions to empty capsids

The structure of single-stranded DNA (ssDNA) packaging H-1 parvovirus (H-1PV), which is being developed as an antitumor gene delivery vector, has been determined for wild-type (wt) virions and noninfectious (empty) capsids to 2.7- and 3.2-Å resolution, respectively, using X-ray crystallography. The capsid viral protein (VP) structure consists of an α-helix and an eight-stranded anti-parallel β-barrel with large loop regions between the strands. The β-barrel and loops form the capsid core and surface, respectively. In the wt structure, 600 nucleotides are ordered in an interior DNA binding pocket of the capsid. This accounts for ∼12% of the H-1PV genome. The wt structure is identical to the empty capsid structure, except for side chain conformation variations at the nucleotide binding pocket. Comparison of the H-1PV nucleotides to those observed in canine parvovirus and minute virus of mice, two members of the genus Parvovirus, showed both similarity in structure and analogous interactions. This observation suggests a functional role, such as in capsid stability and/or ssDNA genome recognition for encapsulation. The VP structure differs from those of other parvoviruses in surface loop regions that control receptor binding, tissue tropism, pathogenicity, and antibody recognition, including VP sequences reported to determine tumor cell tropism for oncotropic rodent parvoviruses. These structures of H-1PV provide insight into structural features that dictate capsid stabilization following genome packaging and three-dimensional information applicable for rational design of tumor-targeted recombinant gene delivery vectors.

Human parvovirus B19 DNA replication induces a DNA damage response that is dispensable for cell cycle arrest at phase G2/M

Human parvovirus B19 (B19V) infection is highly restricted to human erythroid progenitor cells, in which it induces a DNA damage response (DDR). The DDR signaling is mainly mediated by the ATR (ataxia telangiectasia-mutated and Rad3-related) pathway, which promotes replication of the viral genome; however, the exact mechanisms employed by B19V to take advantage of the DDR for virus replication remain unclear. In this study, we focused on the initiators of the DDR and the role of the DDR in cell cycle arrest during B19V infection. We examined the role of individual viral proteins, which were delivered by lentiviruses, in triggering a DDR in ex vivo-expanded primary human erythroid progenitor cells and the role of DNA replication of the B19V double-stranded DNA (dsDNA) genome in a human megakaryoblastoid cell line, UT7/Epo-S1 (S1). All the cells were cultured under hypoxic conditions. The results showed that none of the viral proteins induced phosphorylation of H2AX or replication protein A32 (RPA32), both hallmarks of a DDR. However, replication of the B19V dsDNA genome was capable of inducing the DDR. Moreover, the DDR per se did not arrest the cell cycle at the G(2)/M phase in cells with replicating B19V dsDNA genomes. Instead, the B19V nonstructural 1 (NS1) protein was the key factor in disrupting the cell cycle via a putative transactivation domain operating through a p53-independent pathway. Taken together, the results suggest that the replication of the B19V genome is largely responsible for triggering a DDR, which does not perturb cell cycle progression at G(2)/M significantly, during B19V infection.

Essential role of the unordered VP2 n-terminal domain of the parvovirus MVM capsid in nuclear assembly and endosomal enlargement of the virion fivefold channel for cell entry

The unordered N-termini of parvovirus capsid proteins (Nt) are translocated through a channel at the icosahedral five-fold axis to serve for virus traffick. Heterologous peptides were genetically inserted at the Nt of MVM to study their functional tolerance to manipulations. Insertion of a 5T4-single-chain antibody at VP2-Nt (2Nt) yielded chimeric capsid subunits failing to enter the nucleus. The VEGFR2-binding peptide (V1) inserted at both 2Nt and VP1-Nt efficiently assembled in virions, but V1 disrupted VP1 and VP2 entry functions. The VP2 defect correlated with restricted externalization of V1-2Nt out of the coat. The specific infectivity of MVM and wtVP-pseudotyped mosaic MVM-V1 virions, upon heating and/or partial 2Nt cleavage, demonstrated that some 2Nt domains become intracellularly translocated out of the virus shell and cleaved to initiate entry. The V1 insertion defines a VP2-driven endosomal enlargement of the channel as an essential structural rearrangement performed by the MVM virion to infect.

Parvoviruses: structure and infection

Parvoviruses package a ssDNA genome. Both nonpathogenic and pathogenic members exist, including those that cause fetal infections, encompassing the entire spectrum of virus phenotypes. Their small genomes and simple coding strategy has enabled functional annotation of many steps in the infectious life cycle. They assemble a multifunctional capsid responsible for cell recognition and the transport of the packaged genome to the nucleus for replication and progeny virus production. It is also the target of the host immune response. Understanding how the capsid structure relates to the function of parvoviruses provides a platform for recombinant engineering of viral gene delivery vectors for the treatment of clinical diseases, and is fundamental for dissecting the viral determinants of pathogenicity. This review focuses on our current understanding of parvovirus capsid structure and function with respect to the infectious life cycle.

Diagnosis, management and possibilities to prevent parvovirus B19 infection in pregnancy

Human parvovirus B19 (B19V) infection in pregnancy can cause severe fetal anemia and nonimmune hydrops fetalis, which may be associated with spontaneous abortion and fetal death. Approximately 30–40% of women of child-bearing age are not immune to B19V infection. The risk to fetal life is particularly high if maternal infection occurs during the first 20 weeks of gestation. In this article we intend to give an overview on the molecular biology, epidemiology and management of B19V infection during pregnancy. These data will be combined with an assessment of the clinical situation of the infected fetus and the possibilities for avoiding and/or preventing B19V infection in pregnant women. Currently B19V infection is the causative agent of one of the most frequently occurring infectious complications in pregnancy that endangers fetal life, and so the necessity to develop a preventive vaccine is discussed.

A theoretical model for the dynamic structure of hepatitis B nucleocapsid

The genomic material of hepatitis B virus (HBV) is confined within a fenestrated nucleocapsid consisting of 240 identical copies of the capsid protein, which has a rigid core and a positively charged and highly flexible C-terminal domain (CTD). Although previous mutagenesis studies have demonstrated the importance of the CTD in viral RNA packaging and reverse transcription, the microscopic structure of the CTD and its interaction with encapsidated nucleic acids at various stages of viral maturation remain poorly understood. Here, we present a theoretical analysis of the radial distributions of the CTD chains and nucleic acids in the hepatitis B virus nucleocapsid at the beginning and final stages of viral reverse transcription based on classical density functional theory and a coarse-gained model for the pertinent biomolecules. We find that a significant portion of the CTD is exposed at the surface of the RNA-containing immature nucleocapsid and that the CTD is mostly confined within the DNA-containing mature nucleocapsid. Large accumulation of cations is predicted inside both immature and mature nucleocapsids. The theoretical results provide new insights into the molecular mechanism of CTD regulation of viral reverse transcription and nucleocapsid trafficking during various stages of the viral replication processes.

[Parvovirus B19 infection after kidney transplantation]

Prevalence for human parvovirus B19 infection is estimated to be between 2% and 30% in renal transplant recipients. In post-transplant settings, parvovirus B19 infection may occur either as a primary infection or a reactivation. Parvovirus transmission most commonly occurs through respiratory tract but may also result from graft or blood packs contamination. Co-infections with HHV-6 and CMV viruses are frequent. The hallmark symptom is anemia, more rarely pancytopenia and hemophagocytic syndrome. In respect to renal involvement, parvovirus B19 infection has been associated with graft dysfunction in 10% of cases. Both thrombotic microangiopathies and collapsing glomerulopathies have been reported concomitantly with parvovirus B19 infection but the causal link remains unclear. Other complications are seldomly reported, including hepatitis, encephalitis, and myocarditis. Diagnosis is based on pre and post-transplant serological status. In addition, the management of parvovirus B19 infection in immunocompromised patients requires quantitative assessment of blood viral load by PCR. The treatment relies primarily on reduction of immunosuppression combined with intravenous immunoglobulin infusions. Relapses occur in 30% of cases.

False-negative serology in patients with acute parvovirus B19 infection

BACKGROUND

Acute parvovirus B19 (B19V) infection is characterized by high-level viremia. Antibodies against the capsid proteins VP1 and VP2 may complex with B19V-particles thereby becoming undetectable in diagnostic tests.

OBJECTIVES

We intended to obtain data on the frequency of false-negative serology in acute B19V-infection.

STUDY DESIGN

129 plasma or serum samples of healthy blood donors and of patients with suspected B19V-infection were analyzed for B19V-DNA by qPCR and VP1/VP2-specific IgG and IgM by ELISA. Eleven of these samples were derived from four pregnant women with previous contact to B19V-infected individuals. Using acidic conditions virus/antibody-complexes were disrupted and detected by WesternLine and ELISA.

RESULTS

83/118 samples were derived from acutely infected individuals displaying viremia (10(3)-10(12)geq/mL). In 24/83 viremic samples (28.9%) VP1/VP2-specific IgM and IgG were undetectable in ELISA, but could be demonstrated to be complexed with B19V-particles. Each 7/83 (8.4%) was IgM-positive/IgG-negative and IgM-negative/IgG-positive, in 45/83 samples (54.2%) IgG and IgM could be detected. 35 samples did not contain B19V-DNA; five of these were from seronegative persons. Analyzing consecutive sera derived from four pregnant women, B19V-DNA was demonstrated in 10/11 samples, B19V-specific IgG- and IgM-antibodies were detectable in 10/11 and 4/11 samples, respectively. In 2/4 women seroconversion was observed, but IgM was not detected in 50% of the samples. B19V-specific IgG but not IgM was detectable in 2/4 women.

CONCLUSION

Acute B19V-infection cannot be diagnosed by exclusive analysis of B19V-specific antibodies. Only the combination of assays for detection of B19V-DNA and antibodies enables correct serodiagnosis.

Conformational changes in adeno-associated virus type 1 induced by genome packaging

Adeno-associated virus (AAV) is frequently used as a vector for gene therapy. The viral capsid consists of three structural proteins (VP1, VP2, and VP3) that have a common C-terminal core (VP3), with N-terminal extensions of increasing length in VP2 and VP1. The capsid encloses a single-stranded genome of up to 4.7 kb, which is packaged into empty capsids. The N-terminal extension of VP1 carries a phospholipase domain that becomes accessible during infection in the endosomal pathway. We have used cryo-electron microscopy and image reconstruction to determine subnanometer-resolution structures of recombinant AAV1 that has packaged different amounts of a 3.6-kb recombinant genome. The maps show that the AAV1 capsid undergoes continuous conformational changes upon packaging of the genome. The rearrangements occur at the inner capsid surface and lead to constrictions of the pores at the 5-fold symmetry axes and to subtle movements of the β-sheet regions of the capsid proteins. In fully packaged particles, the genome forms stem-like features that contact the inner capsid surface at the 3-fold symmetry axes. We think that the reorganization of the inner surface has an impact on the viral life cycle during infection, preparing the externalization of phospholipase domains through the pores at the 5-fold symmetry axes and possibly genome release.

Human bocavirus capsid structure: insights into the structural repertoire of the parvoviridae

Human bocavirus (HBoV) was recently discovered and classified in the Bocavirus genus (family Parvoviridae, subfamily Parvovirinae) on the basis of genomic similarity to bovine parvovirus and canine minute virus. HBoV has been implicated in respiratory tract infections and gastroenteric disease in children worldwide, yet despite numerous epidemiological reports, there has been limited biochemical and molecular characterization of the virus. Reported here is the three-dimensional structure of recombinant HBoV capsids, assembled from viral protein 2 (VP2), at 7.9-A resolution as determined by cryo-electron microscopy and image reconstruction. A pseudo-atomic model of HBoV VP2 was derived from sequence alignment analysis and knowledge of the crystal structure of human parvovirus B19 (genus Erythrovirus). Comparison of the HBoV capsid structure to that of parvoviruses from five separate genera demonstrates strong conservation of a beta-barrel core domain and an alpha-helix, from which emanate several loops of various lengths and conformations, yielding a unique surface topology that differs from the three already described for this family. The highly conserved core is consistent with observations for other single-stranded DNA viruses, and variable surface loops have been shown to confer the host-specific tropism and the diverse antigenic properties of this family.

Structures and functions of parvovirus capsids and the process of cell infection

To infect a cell, the parvovirus or adeno-associated virus (AAV) genome must be delivered from outside the plasma membrane to the nucleus, and in the process, the capsid must follow a series of binding and trafficking steps and also undergo necessary changes that result in exposure or release the ssDNA genome at the appropriate time and place within the cell. The 25 nm parvovirus capsid is comprised of two or three forms of a single protein, and although it is robust and stable, it is still sufficiently flexible to allow the exposure of several internal components at appropriate times during cell infection. The capsid can also accommodate insertion of peptides into surface loops, and capsid proteins from different viral serotypes can be shuffled to create novel functional variants. The capsids of the different viruses bind to one or more cell receptors, and for at least some viruses, the insertion of additional or alternative receptor binding sequences or structures into the capsid can expand or redirect its tropism. The infection process after cell binding involves receptor-mediated endocytosis followed by viral trafficking through the endosomal systems. That endosomal trafficking may be complex and prolonged for hours or be relatively brief. Generally only a small proportion of the particles taken up enter the cytoplasm after altering the endosomal membrane through the activity of a VP1-encoded phospholipase A2 domain that becomes released to the outside of the viral particle. Modifications to the capsid that can occur within the endosome or cytoplasm include structural changes to expose internal components, ubiquination and proteosomal processing, and possible trafficking of particles on molecular motors. It is still not clear how the genomes enter the nucleus, but nuclear pore-dependent entry of particles or permeabilization of nuclear membranes have been proposed. Those processes control the infection, pathogenesis, and host ranges of the autonomous viruses and determine the effectiveness of gene therapy using AAV capsids.

Adeno-associated virus-2 and its primary cellular receptor--Cryo-EM structure of a heparin complex

Adeno-associated virus serotype 2 (AAV-2) is a leading candidate vector for gene therapy. Cell entry starts with attachment to a primary receptor, Heparan Sulfate Proteoglycan (HSPG) before binding to a co-receptor. Here, cryo-electron microscopy provides direct visualization of the virus-HSPG interactions. Single particle analysis was performed on AAV-2 complexed with a 17 kDa heparin fragment at 8.3 A resolution. Heparin density covers the shoulder of spikes surrounding viral 3-fold symmetry axes. Previously implicated, positively charged residues R(448/585), R(451/588) and R(350/487) from another subunit cluster at the center of the heparin footprint. The footprint is much more extensive than apparent through mutagenesis, including R(347/484), K(395/532) and K(390/527) that are more conserved, but whose roles have been controversial. It also includes much of a region proposed as a co-receptor site, because prior studies had not revealed heparin interactions. Heparin density bridges over the viral 3-fold axes, indicating multi-valent attachment to symmetry-related binding sites.