Rapid small-scale isolation of SV40 virions and SV40 DNA

Modern Techniques for the Isolation of Extracellular Vesicles and Viruses

Extracellular signaling is pivotal to maintain organismal homeostasis. A quickly emerging field of interest within extracellular signaling is the study of extracellular vesicles (EV), which act as messaging vehicles for nucleic acids, proteins, metabolites, lipids, etc. from donor cells to recipient cells. This transfer of biologically active material within a vesicular body is similar to the infection of a cell through a virus particle, which transfers genetic material from one cell to another to preserve an infection state, and viruses are known to modulate EV. Although considerable heterogeneity exists within EV and viruses, this review focuses on those that are small (< 200 nm in diameter) and of relatively low density (< 1.3 g/mL). A multitude of isolation methods for EV and virus particles exist. In this review, we present an update on methods for their isolation, purification, and phenotypic characterization. We hope that the information we provide will be of use to basic science and clinical investigators, as well as biotechnologists in this emerging field. Graphical Abstract.

Interaction of the Mouse Polyomavirus Capsid Proteins with Importins Is Required for Efficient Import of Viral DNA into the Cell Nucleus

The mechanism used by mouse polyomavirus (MPyV) to overcome the crowded cytosol to reach the nucleus has not been fully elucidated. Here, we investigated the involvement of importin α/β1 mediated transport in the delivery of MPyV genomes into the nucleus. Interactions of the virus with importin β1 were studied by co-immunoprecipitation and proximity ligation assay. For infectivity and nucleus delivery assays, the virus and its capsid proteins mutated in the nuclear localization signals (NLSs) were prepared and produced. We found that at early times post infection, virions bound importin β1 in a time dependent manner with a peak of interactions at 6 h post infection. Mutation analysis revealed that only when the NLSs of both VP1 and VP2/3 were disrupted, virus did not bind efficiently to importin β1 and its infectivity remarkably decreased (by 80%). Nuclear targeting of capsid proteins was improved when VP1 and VP2 were co-expressed. VP1 and VP2 were effectively delivered into the nucleus, even when one of the NLS, either VP1 or VP2, was disrupted. Altogether, our results showed that MPyV virions can use VP1 and/or VP2/VP3 NLSs in concert or individually to bind importins to deliver their genomes into the cell nucleus.

Cys254 and Cys49/Cys87of simian virus 40 Vp1 are essential in formation of infectious virions

The SV40 capsid is composed of pentameric capsomeres of Vp1. We have previously shown that disulfide linkages at Vp1 Cys9, Cys104, and Cys207 are essential in formation of infectious virions. Here, the role of the remaining four cysteines was explored. Single, double, and quadruple cys --> ser mutant genomes at Vp1 Cys49, Cys87, Cys254, and Cys267 codons were generated and transfected into CV-1 cells. The quadruple mutant Vp1 continued to localize to the nucleus and to bind DNA, but resulted in no plaques. SV40Vp1.Cys254 was the only single mutant with complete defect in plaque formation. The double mutant at Vp1.Cys49.Cys87 showed complete defect in plaque formation, while single mutants at the two residues resulted in plaques, suggesting a cumulative effect. All mutants defective in plaque formation continued to localize viral proteins in the nucleus. Taken together, our results suggest that Cys254 and the Cys49/Cys87 combination are essential in late stages of infectious virion formation.

The simian virus 40 minor structural protein Vp3, but not Vp2, is essential for infectious virion formation

The SV40 capsid is composed of pentameric capsomeres of the major structural protein Vp1. The two minor structural proteins, Vp2 and Vp3, interact with the capsid. Here, the roles of Vp2 and Vp3 were explored during the course of SV40 infection. Start codons of Vp2, Vp3, or both Vp2 and Vp3, were destroyed by site-directed mutagenesis, and mutant genomes were transfected into CV-1 cells. SV40DeltaVp2 produced plaques and infectious virion particles with titres indistinguishable from wild-type. SV40DeltaVp3 and SV40 DeltaVp2/Vp3 were defective in plaque formation and rendered no infectious particles. All three mutants showed normal nuclear localization of T-Ag and Vp1; they also showed packaging of SV40 DNA by nuclease digestion assays. Thus, Vp3 is essential for formation of infectious SV40 particles, whereas Vp2 is not. One critical role of full-length Vp3 appears to be in virus-cell interactions at post-packaging steps of a permissive infection.

A new improved method for the concentration of HIV-1 infective particles

Improvement of the sensitivity of detection systems for human immunodeficiency virus-1 (HIV-1) has been carried out. One approach to improve the sensitivity is purification and/or concentration of the virus from a specimen. In this study, a method for concentrating HIV-1 using polyethylene glycol (PEG) has been re-evaluated and the optimal protocol for concentrating the virus from low-titer specimens was determined. That is, to obtain a virus pellet, a mixture of equal volumes of a specimen and 20% PEG 20,000 solution in saline is incubated at 4 degrees C for 16 h and then centrifuged at 17860 x g in a microcentrifuge for 20 min. HIV-1 in the pellet could be detectable by HIV-1 p24 antigen capture assay for viral protein, reverse transcriptase (RT) assay for viral enzyme, reverse transcriptase polymerase chain reaction (RT-PCR) assay for viral RNA and a virus infectivity assay.

The antiviral agent 5-chloro-1,3-dihydroxyacridone interferes with assembly and maturation of herpes simplex virus

Antiviral drug screening and exploratory mechanistic work identified 5-chloro-1,3-dihydroxyacridone as a lead inhibitor of herpes simplex virus (HSV) replication, one without a primary effect on either HSV DNA or late viral protein synthesis (Antivir. Res. 45 (2000) 123). In this report, drug effects on viral DNA cleavage and packaging, HSV capsid production and virion morphogenesis in infected Vero cells were studied systematically in order to better localize the sensitive stage of the replication cycle. Maturation of replicating HSV DNA and virion production at late times were inhibited in the same dose-dependent fashion, suggesting that the drug might directly inhibit the cleavage and packaging processes. Based on density centrifugation analysis however, this possibility appears unlikely because overproduction of neither A- or B-capsids occurred upon drug treatment. Interestingly, similar studies coupled with either Western immunoblot or ultrastructural analysis showed that B-capsids with apparent normal protein composition accumulated at reduced levels (maximally about two- to three-fold) in drug-treated cells. Limited attempts to isolate drug-resistant viral mutants using standard approaches proved unsuccessful. In summery, 5-chloro-1,3-dihydroxyacridone inhibits one or more steps of HSV assembly since treatment results in reduced levels of capsids (particularly B-type) and reduced levels of encapsidated DNA. The action of the acridone derivative is an unusual one, with distinctive features when compared to a recently reported class of HSV encapsidation inhibitor and to the late replication defects of relevant viral mutants.

Cys(9), Cys(104) and Cys(207) of simian virus 40 Vp1 are essential for infectious virion formation in CV-1 cells

Structural studies have implicated Cys(9), Cys(104) and Cys(207) of simian virus 40 (SV40) Vp1 in disulfide bond formation. Recently, we have shown the three cysteines to be essential for disulfide linkage of Vp1 complexes in vitro. Here, the role of the three cysteines was explored during the course of SV40 infection. Single-, double- and triple-mutant Vp1 at Cys(9), Cys(104) and Cys(207) continued to localize to the nuclei of transfected CV-1 cells and to bind DNA, but showed a range of abilities to form plaques. Only mutants containing the Cys(9)-->Ser change showed defects in plaque formation. Single mutants at Cys(9) formed small plaques; mutants at Cys(9). Cys(104), Cys(9). Cys(207) and Cys(9). Cys(104). Cys(207) formed no plaques. All three isolated revertants contained back-mutations at the Vp1 Cys(9) codon. These results further confirm the involvement of the three Vp1 cysteines in protein-protein interactions during virus assembly. Cys(9) is critical for production of wild-type infectious virions, whereas Cys(104) and Cys(207) play secondary roles.