Circovirus transport proceeds via direct interaction of the cytoplasmic dynein IC1 subunit with the viral capsid protein

Revisiting Porcine Circovirus Infection: Recent Insights and Its Significance in the Piggery Sector

Porcine circovirus (PCV), a member of the Circoviridae family within the genus Circovirus, poses a significant economic risk to the global swine industry. PCV2, which has nine identified genotypes (a-i), has emerged as the predominant genotype worldwide, particularly PCV2d. PCV2 has been commonly found in both domestic pigs and wild boars, and sporadically in non-porcine animals. The virus spreads among swine populations through horizontal and vertical transmission routes. Despite the availability of commercial vaccines for controlling porcine circovirus infections and associated diseases, the continuous genotypic shifts from a to b, and subsequently from b to d, have maintained PCV2 as a significant pathogen with substantial economic implications. This review aims to provide an updated understanding of the biology, genetic variation, distribution, and preventive strategies concerning porcine circoviruses and their associated diseases in swine.

Transcriptomic and Proteomic Analyses of Myzus persicae Carrying Brassica Yellows Virus

Viruses in the genus Polerovirus infect a wide range of crop plants and cause severe economic crop losses. BrYV belongs to the genus Polerovirus and is transmitted by Myzus persicae. However, the changes in transcriptome and proteome profiles of M. persicae during viral infection are unclear. Here, RNA-Seq and TMT-based quantitative proteomic analysis were performed to compare the differences between viruliferous and nonviruliferous aphids. In total, 1266 DEGs were identified at the level of transcription with 980 DEGs being upregulated and 286 downregulated in viruliferous aphids. At the protein level, among the 18 DEPs identified, the number of upregulated proteins in viruliferous aphids was twice that of the downregulated DEPs. Enrichment analysis indicated that these DEGs and DEPs were mainly involved in epidermal protein synthesis, phosphorylation, and various metabolic processes. Interestingly, the expressions of a number of cuticle proteins and tubulins were upregulated in viruliferous aphids. Taken together, our study revealed the complex regulatory network between BrYV and its vector M. persicae from the perspective of omics. These findings should be of great benefit to screening key factors involved in the process of virus circulation in aphids and provide new insights for BrYV prevention via vector control in the field.

Porcine Circovirus Type 2 Hijacks Host IPO5 to Sustain the Intracytoplasmic Stability of Its Capsid Protein

Nuclear entrance and stability of porcine circovirus type 2 (PCV2), the smallest virus in mammals, are crucial for its infection and replication. However, the mechanisms are not fully understood. Here, we found that the PCV2 virion maintains self-stability via the host importin 5 (IPO5) during infection. Coimmunoprecipitation combined with mass spectrometry and glutathione S-transferase pulldown assays showed that the capsid protein (Cap) of PCV2 binds directly to IPO5. Fine identification demonstrated that the N-terminal residue arginine²⁴ of Cap is the most critical to efficient binding to the proline⁷⁰⁹ residue of IPO5. Detection of replication ability further showed that IPO5 supports PCV2 replication by promoting the nuclear import of incoming PCV2 virions. Knockdown of IPO5 delayed the nuclear transport of incoming PCV2 virions and significantly decreased the intracellular levels of overexpressed PCV2 Cap, which was reversed by treatment with a proteasome inhibitor or by rescuing IPO5 expression. Cycloheximide treatment showed that IPO5 increases the stability of the PCV2 Cap protein. Taken together, our findings demonstrated that during infection, IPO5 facilitates PCV2 replication by directly binding to the nuclear localization signal of Cap to block proteasome degradation. IMPORTANCE Circovirus is the smallest virus to cause immune suppression in pigs. The capsid protein (Cap) is the only viral structural protein that is closely related to viral infection. The nuclear entry and stability of Cap are necessary for PCV2 replication. However, the molecular mechanism maintaining the stability of Cap during nuclear trafficking of PCV2 is unknown. Here, we report that IPO5 aggregates within the nuclear periphery and combines with incoming PCV2 capsids to promote their nuclear entry. Concurrently, IPO5 inhibits the degradation of newly synthesized Cap protein, which facilitates the synthesis of virus proteins and virus replication. These findings highlight a mechanism whereby IPO5 plays a dual role in PCV2 infection, which not only enriches our understanding of the virus replication cycle but also lays the foundation for the subsequent development of antiviral drugs.

Serum investigation of antibodies against porcine circovirus 4 Rep and Cap protein in Jiangxi Province, China

In 2019, a novel porcine circovirus 4 (PCV4) was first identified in Hunan Province, China. The circular PCV4 DNA was detected in both diseased and healthy pigs. Recently, PCV4 prevalence surveys have been analyzed in many provinces in both China and South Korea with low positive rates. However, no serological data has been conducted to investigate the prevalence of PCV4 in pigs from Jiangxi Province. To address this issue, an indirect anti-PCV4 antibody enzyme-linked immunosorbent assay (ELISA) based on Cap and Rep protein as a coating antigen was established and applied to study the serum epidemiology of PCV4 in Jiangxi Province. Purified PCV4-His-tagged Cap and Rep were used as the coating antigen to develop an ELISA detection kit. There was no cross-reaction of the Cap/Rep-based ELISA with antisera against PCV2, TGEV and PRRSV, indicating a high specificity of this ELISA assay. The intra-assay coefficient variations (CVs) of Cap-based were 1.239%−9.796%, Rep-based 1.288%−5.011%, and inter-assay CVs of 1.167%−4.694% and 1.621%−8.979%, respectively, indicating a good repeatability. Finally, a total number of 507 serum samples were collected from Jiangxi Province to test for antibody prevalence of PCV4, and 17 (3.35%) and 36 (7.10%) of the samples were Cap and Rep antibody positive, respectively. In summary, our established ELISA kit could be used to detect PCV4 antibodies in serum with good repeatability and high specificity. In addition, field samples detection results showed that the antibody of PCV4 was poorly distributed in intensive pig farms in Jiangxi Province, China.

Advances in Crosstalk between Porcine Circoviruses and Host

Porcine circoviruses (PCVs), including PCV1 to PCV4, are non-enveloped DNA viruses with a diameter of about 20 nm, belonging to the genus Circovirus in the family Circoviridae. PCV2 is an important causative agent of porcine circovirus disease or porcine circovirus-associated disease (PCVD/PCVAD), which is highly prevalent in pigs and seriously affects the swine industry globally. Furthermore, PCV2 mainly causes subclinical symptoms and immunosuppression, and PCV3 and PCV4 were detected in healthy pigs, sick pigs, and other animals. Although the pathogenicity of PCV3 and PCV4 in the field is still controversial, the infection rates of PCV3 and PCV4 in pigs are increasing. Moreover, PCV3 and PCV4 rescued from infected clones were pathogenic in vivo. It is worth noting that the interaction between virus and host is crucial to the infection and pathogenicity of the virus. This review discusses the latest research progress on the molecular mechanism of PCVs–host interaction, which may provide a scientific basis for disease prevention and control.

Baicalein self-microemulsion based on drug-phospholipid complex for the alleviation of cytokine storm

Cytokine storm is a phenomenon whereby the overreaction of the human immune system leads to the release of inflammatory cytokines, which can lead to multiple organ dysfunction syndrome. At present, the existing drugs for the treatment of cytokine storm have limited efficacy and severe adverse effects. Here, we report a lymphatic targeting self-microemulsifying drug delivery system containing baicalein to effectively inhibit cytokine storm. Baicalein self-microemulsion with phospholipid complex as an intermediate carrier (BAPC-SME) prepared in this study could be spontaneously emulsified to form 12-nm oil-in-water nanoemulsion after administration. And then BAPC-SME underwent uptake by enterocyte through endocytosis mediated by lipid valve and clathrin, and had obvious characteristics of mesenteric lymph node targeting distribution. Oral administration of BAPC-SME could significantly inhibit the increase in plasma levels of 14 cytokines: TNF-α, IL-6, IFN-γ, MCP-1, IL-17A, IL-27, IL-1α, GM-CSF, MIG, IFN-β, IL-12, MIP-3α, IL-23, and RANTES in mice experiencing systemic cytokine storm. BAPC-SME could also significantly improve the pathological injury and inflammatory cell infiltration of lung tissue in mice experiencing local cytokine storm. This study does not only provide a new lymphatic targeted drug delivery strategy for the treatment of cytokine storm but also has great practical significance for the clinical development of baicalein self-microemulsion therapies for cytokine storm.

Interaction Network of Porcine Circovirus Type 3 and 4 Capsids with Host Proteins

An extensive understanding of the interactions between host cellular and viral proteins provides clues for studying novel antiviral strategies. Porcine circovirus type 3 (PCV3) and type 4 (PCV4) have recently been identified as viruses that can potentially damage the swine industry. Herein, 401 putative PCV3 Cap-binding and 484 putative PCV4 Cap-binding proteins were characterized using co-immunoprecipitation and liquid chromatography-mass spectrometry. Both PCV3 and PCV4 Caps shared 278 identical interacting proteins, but some putative interacting proteins (123 for PCV3 Cap and 206 for PCV4 Cap) differed. A protein-protein interaction network was constructed, and according to gene ontology (GO) annotation and Kyoto Encyclopedia of Genes and Genomes (KEGG) database analyses, both PCV3 Cap- and PCV4 Cap-binding proteins participated mainly in ribosome biogenesis, nucleic acid binding, and ATP-dependent RNA helicase activities. Verification assays of eight putative interacting proteins indicated that nucleophosmin-1, nucleolin, DEAD-box RNA helicase 21, heterogeneous nuclear ribonucleoprotein A2/B1, YTH N6-methyladenosine RNA binding protein 1, and Y-box binding protein 1 bound directly to both PCV3 and PCV4 Caps, but ring finger protein 2 and signal transducer and activator of transcription 6 did not. Therefore, the interaction network provided helpful information to support further research into the underlying mechanisms of PCV3 and PCV4 infection.

Nucleolar protein NPM1 is essential for circovirus replication by binding to viral capsid

Entry of circovirus into the host cell nucleus is essential for viral replication during the early stage of infection. However, the mechanisms by which nucleolar shuttle proteins are used during viral replication is still not well understood. Here, we report a previously unidentified nucleolar localization signal in circovirus capsid protein (Cap), and that circovirus hijacks the nucleolar phosphoprotein nucleophosmin-1 (NPM1) to facilitate its replication. Colocalization analysis showed that NPM1 translocates from the nucleolus to the nucleoplasm and cytoplasm during viral infection. Coimmunoprecipitation and glutathione S-transferase pull-down assays showed that Cap interacts directly with NPM1. Binding domain mapping showed that the arginine-rich N-terminal motif ¹MTYPRRRYRRRRHRPRSHLG²⁰ of Cap, and residue serine-48 of the N-terminal oligomerization domain of NPM1, are essential for the interaction. Virus rescue experiments showed that all arginine to alanine substitution in the N-terminal arginine-rich motif of Cap resulted in diminished viral replication. Knockdown of NPM1 and substitution of serine-48 in NPM1 to glutamic acid also decreased viral replication. In addition, binding assays showed that the arginine-rich motif of Cap is a nucleolar localization signal. Taken together, our findings demonstrate that circovirus protein Cap is a nucleolus-located, and regulates viral replication by directly binding to NPM1.

Computational modeling of dynein motor proteins at work

Along with various experimental methods, a combination of theoretical and computational methods is essential to explore different length-scale and time-scale processes in the biological system. The functional mechanism of a dynein, an ATP-fueled motor protein, working in a multiprotein complex, involves a wide range of length/time-scale events. It generates mechanical force from chemical energy and moves on microtubules towards the minus end direction while performing a large number of biological processes including ciliary beating, intracellular material transport, and cell division. Like in the cases of other conventional motor proteins, a combination of experimental techniques including X-crystallography, cryo-electron microscopy, and single molecular assay have provided a wealth of information about the mechanochemical cycle of a dynein. Dyneins have a large and complex structural architecture and therefore, computational modeling of different aspects of a dynein is extremely challenging. As the process of dynein movement involves varying length and timescales, it demands, like in experiments, a combination of computational methods covering such a wide range of processes for the comprehensive investigation of the mechanochemical cycle. In this review article, we will summarize how the use of state-of-the-art computational methods can provide a detailed molecular understanding of the mechanochemical cycle of the dynein. We implemented all-atom molecular dynamics simulations and hybrid quantum-mechanics/molecular-mechanics simulations to explore the ATP hydrolysis mechanisms at the primary ATPase site (AAA1) of dynein. To investigate the large-scale conformational changes we employed coarse-grained structure-based molecular dynamics simulations to capture the domain motions. Here we explored the conformational changes upon binding of ATP at AAA1, nucleotide state-dependent regulation of the mechanochemical cycle, and inter-head coordination by inter-head tension. Additionally, implementing a phenomenological theoretical model we explore the force-dependent detachment rate of a motorhead from the microtubule and the principle of multi-dynein cooperation during cargo transport.

C1QBP inhibits proliferation of porcine circovirus type 2 by restricting nuclear import of the capsid protein

Complement component 1 Q subcomponent-binding protein (C1QBP) has been shown to interact with the porcine circovirus type 2 (PCV2) Cap protein. Here, using yeast two-hybrid (Y2H) and co-immunoprecipitation assays, as well as laser confocal microscopy, the interaction between C1QBP and Cap was confirmed. Furthermore, overexpression of C1QBP in cells altered the intracellular location of Cap, which was observed using confocal microscopy and verified by detection of Cap in nuclear protein extracts in a Western blot assay. By inhibiting nuclear transport of Cap, overexpression of C1QBP downregulated PCV2 proliferation in PK-15 cells, as determined by quantitative polymerase chain reaction (qPCR). As C1QBP plays a similar role in a fusion of green fluorescent protein (GFP) with the Cap nuclear localisation signal (NLS) sequence, (CapNLS-GFP), we propose that the target site for C1QBP in Cap is possibly located in the NLS region. Considering all the results together, this study demonstrated that C1QBP interacts with the Cap NLS region, resulting in changes in the intracellular localisation of the Cap protein. We confirmed that overexpression of C1QBP inhibits the proliferation of PCV2, and this is possibly related to the function of C1QBP in controlling nuclear transport of Cap.

The serine-48 residue of nucleolar phosphoprotein nucleophosmin-1 plays critical role in subcellular localization and interaction with porcine circovirus type 3 capsid protein

The transport of circovirus capsid protein into nucleus is essential for viral replication in infected cell. However, the role of nucleolar shuttle proteins during porcine circovirus 3 capsid protein (PCV3 Cap) import is still not understood. Here, we report a previously unidentified nucleolar localization signal (NoLS) of PCV3 Cap, which hijacks the nucleolar phosphoprotein nucleophosmin-1 (NPM1) to facilitate nucleolar localization of PCV3 Cap. The NoLS of PCV3 Cap and serine-48 residue of N-terminal oligomerization domain of NPM1 are essential for PCV3 Cap/NPM1 interaction. In addition, charge property of serine-48 residue of NPM1 is critical for nucleolar localization and interaction with PCV3 Cap. Taken together, our findings demonstrate for the first time that NPM1 interacts with PCV3 Cap and is responsible for its nucleolar localization.

Epidemiological investigation of porcine circovirus type 2 and its coinfection rate in Shandong province in China from 2015 to 2018

BACKGROUND

Porcine circovirus type 2 (PCV2) is one of the crucial swine viral pathogens, caused porcine circovirus associated diseases (PCVAD). Shandong province is one of the most important pork producing areas and bears a considerable economic loss due to PCVAD. However, there is limited information on epidemiology and coinfection rate of PCV2 with other critical swine diseases in this area, such as porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), Pseudorabies virus (PRV), and porcine epidemic diarrhea virus (PEDV).

RESULTS

Overall, 89.59% serum samples and 36.98% tissue samples were positive for PCV2 specified ELISA and PCR positive for PCV2, respectively. The coinfection rates of PCV2 with PRRSV, PRV, CSFV, and PEDV were 26.73%, 18.37%, 13.06%, and 3.47%, respectively. Moreover, genetic characteristic of PCV2 were analyzed based on the cap genes showing that PCV2d is the dominant sub-genotype circulating in the province.

CONCLUSIONS

Our findings reveal that PCV2d, as the dominant strain, is prevailing in pig farms in Shandong province at high levels. There was a high frequency of coinfection of PCV2 and PRRSV.

PCV2 Induces Reactive Oxygen Species To Promote Nucleocytoplasmic Translocation of the Viral DNA Binding Protein HMGB1 To Enhance Its Replication

Porcine circovirus type 2 (PCV2) is an important swine pathogen that causes significant economic losses to the pig industry. PCV2 interacts with host cellular factors to regulate its replication. High-mobility-group box 1 (HMGB1) protein, a major nonhistone protein in the nucleus, was recently discovered to participate in viral infections. Here, we demonstrate that nuclear HMGB1 negatively regulated PCV2 replication as shown by overexpression of HMGB1 or blockage of its nucleocytoplasmic translocation with ethyl pyruvate. The B box domain was essential in restricting PCV2 replication. Nuclear HMGB1 restricted PCV2 replication by sequestering the viral genome via binding to the Ori region. However, PCV2 infection induced translocation of HMGB1 from cell nuclei to the cytoplasmic compartment. Elevation of reactive oxygen species (ROS) induced by PCV2 infection was closely associated with cytosolic translocation of nuclear HMGB1. Treatment of PCV2-infected cells with ethyl pyruvate or N-acetylcysteine downregulated PCV2-induced ROS production, suppressed nucleocytoplasmic HMGB1 translocation, and decreased PCV2 replication. Collectively, these findings offer new insight into the mechanism of the PCV2 evasion strategy: PCV2 manages to escape restriction of its replication by nuclear HMGB1 by inducing ROS to trigger the nuclear-to-cytoplasmic translocation of HMGB1.IMPORTANCE Porcine circovirus type 2 (PCV2) is a small DNA virus that depends heavily on host cells for its infection. This study reports the close relationship between subcellular localization of host high-mobility-group box 1 (HMGB1) protein and viral replication during PCV2 infection. Restriction of PCV2 replication by nuclear HMGB1 is the early step of host defense at the host-pathogen interface. PCV2 then upregulates host reactive oxygen species (ROS) to prevent sequestration of its genome by expelling nuclear HMGB1 into the cytosol. It will be interesting to study if a similar evasion strategy is employed by other circoviruses such as beak and feather disease virus, recently discovered PCV3, and geminiviruses in plants. This study also provides insight into the justification and pharmacological basis of antioxidants as an adjunct therapy in PCV2 infection or possibly other diseases caused by the viruses that deploy the ROS-HMGB1 interaction favoring their replication.

Protein Interactions Network of Porcine Circovirus Type 2 Capsid With Host Proteins

Virus-host interaction is a tug of war between pathogenesis and immunity, followed by either activating the host immune defense system to eliminate virus or manipulating host immune control mechanisms to survive and facilitate virus propagation. Comprehensive knowledge of interactions between host and viral proteins might provide hints for developing novel antiviral strategies. To gain a more detailed knowledge of the interactions with porcine circovirus type 2 capsid protein, we employed a coimmunoprecipitation combined with liquid chromatography mass spectrometry (LC-MS) approach and 222 putative PCV2 Cap-interacting host proteins were identified in the infected porcine kidney (PK-15) cells. Further, a protein-protein interactions (PPIs) network was plotted, and the PCV2 Cap-interacting host proteins were potentially involved in protein binding, DNA transcription, metabolism and innate immune response based on the gene ontology annotation and Kyoto Encyclopedia of Genes and Genomes database enrichment. Verification in vitro assay demonstrated that eight cellular proteins, namely heterogeneous nuclear ribonucleoprotein C, nucleophosmin-1, DEAD-box RNA helicase 21, importin β3, eukaryotic translation initiation factor 4A2, snail family transcriptional repressor 2, MX dynamin like GTPase 2, and intermediate chain 1 interacted with PCV2 Cap. Thus, this work effectively provides useful protein-related information to facilitate further investigation of the underlying mechanism of PCV2 infection and pathogenesis.

NAP1L4 inhibits porcine circovirus type 2 replication via IFN-β signaling pathway

Porcine circovirus type 2 (PCV2) capsid protein (Cap) was previously reported to interact with nucleosome assembly protein 1-like 4 (NAP1L4). The biological function of Cap-NAP1L4 interaction is unknown. Here, we demonstrated that PCV2 Cap could directly interact with NAP1L4, which the amino acid residues 124-279 of NAP1L4 were responsible for the interaction. Furthermore, over-expression of NAP1L4 down-regulated the expression of PCV2 Cap and Rep. DNA copies and virus titers were also decreased in NAP1L4 over-expressed PK15 cells. While, knockout of NAP1L4 through CRISPR/Cas9 technology in PK15 cells could up-regulate the mRNA and protein levels of PCV2 Cap and Rep. PCV2 genomic DNA copies and virus titers were also increased in NAP1L4-knockdown/-knockout PK15 cells compared with wild type PK15 cells. In addition, NAP1L4 depletion was demonstrated to facilitate cytosolic carboxypeptidase-like protein 5 (CCP5) and cytosolic carboxypeptidase 6 (CCP6) expression, which could activate cGAS to promote IFN-β production. Indeed, knockout of NAP1L4 was also confirmed to increase IFN-β expression. And IFN-β stimulation could promote PCV2 replication in PK15 cells. Taken together, our findings suggest that NAP1L4 interacts with PCV2 Cap and inhibits PCV2 replication through regulating IFN-β production. Our study provides theoretical basis for further study of PCV2.

Role of AAA3 Domain in Allosteric Communication of Dynein Motor Proteins

Cytoplasmic dynein, an AAA+ motif containing motor, generates force and movement along the microtubule to execute important biological functions including intracellular material transport and cell division by hydrolyzing ATP. Among the six AAA+ domains, AAA1 is the primary ATPase site where a single ATP hydrolysis generates a single step. Nucleotide states in AAA3 gate dynein's activity, suggesting that AAA3 acts as a regulatory switch. However, the comprehensive structural perspective of AAA3 in dynein's mechanochemical cycle remains unclear. Here, we explored the allosteric transition path of dynein involving AAA3 using a coarse-grained structure-based model. ATP binding to the AAA1 domain creates a cascade of conformational changes through the other domains of the ring, which leads to the pre-power stroke formation. The linker domain, which is the mechanical element of dynein, shifts from a straight to a bent conformation during this process. In our present study, we observe that AAA3 gates the allosteric communication from AAA1 to the microtubule binding domain (MTBD) through AAA4 and AAA5. The MTBD is linked to the AAA+ ring via a coiled-coil stalk and a buttress domain, which are extended from AAA4 and AAA5, respectively. Further analysis also uncovers the role of AAA3 in the linker movement. The free energy calculation shows that the linker prefers the straight conformation when AAA3 remains in the ATP-bound condition. As AAA3 restricts the motion of AAA4 and AAA5, the linker/AAA5 interactions get stabilized, and the linker cannot move to the pre-power stroke state that halts the complete structural transition required for the mechanochemical cycle. Therefore, we suggest that AAA3 governs dynein's mechanochemical cycle and motility by controlling the AAA4 and AAA5 domains that further regulate the linker movement and the power stroke formation.

Conformational Changes and Nuclear Entry of Porcine Circovirus without Disassembly

A relatively stable and flexible capsid is critical to the viral life cycle. However, the capsid dynamics and cytosol trafficking of porcine circovirus type 2 (PCV2) during its infectious cycle are poorly understood. Here, we report the structural stability and conformation flexibility of PCV2 virions by genome labeling and the use of three monoclonal antibodies (MAbs) against the native capsid of PCV2. Genome labeling showed that the infectivity of the PCV2 virion was not affected by conjugation with deoxy-5-ethynylcytidine (EdC). Heat stability experiments indicated that PCV2 capsids started to disassemble at 65°C, causing binding incompetence for all antibodies, and the viral genome was released without capsid disassembly upon heating at 60°C. Antibody binding experiments with PCV2 showed that residues 186 to 192 were concealed in the early endosomes of epithelial PK-15 and monocytic 3D4/31 cells with or without chloroquine treatment and then exposed in PK-15 cytosol and the 3D4/31 nucleus. Viral propagation and localization experiments showed that PCV2 replication and cytosol trafficking were not significantly affected by microtubule depolymerization in monocytic 3D4/31 cells treated with nocodazole. These findings demonstrated that nuclear targeting of viral capsids involved conformational changes, the PCV2 genome was released from the assembled capsid, and the transit of PCV2 particles was independent of microtubules in 3D4/31 cells.IMPORTANCE Circovirus is the smallest virus known to replicate autonomously. Knowledge of viral genome release may provide understanding of viral replication and a method to artificially inactivate viral particles. Currently, little is known about the release model of porcine circovirus type 2 (PCV2). Here, we report the release of the PCV2 genome from assembled capsid and the intracellular trafficking of infectious PCV2 by alterations in the capsid conformation. Knowledge of PCV2 capsid stability and dynamics is essential to understanding its infectious cycle and lays the foundation for discovering powerful targets for therapeutic and prophylactic intervention.

SUMO1 Modification Facilitates Avibirnavirus Replication by Stabilizing Polymerase VP1

SUMOylation is a posttranslational modification that has crucial roles in diverse cellular biological pathways and in various viral life cycles. In this study, we found that the VP1 protein, the RNA-dependent RNA polymerase of avibirnavirus infectious bursal disease virus (IBDV), regulates virus replication by SUMOylation during infection. Our data demonstrated that the polymerase VP1 is efficiently modified by small ubiquitin-like modifier 1 (SUMO1) in avibirnavirus-infected cell lines. Mutation analysis showed that residues ⁴⁰⁴I and ⁴⁰⁶I within SUMO interaction motif 3 of VP1 constitute the critical site for SUMO1 modification. Protein stability assays showed that SUMO1 modification enhanced significantly the stability of polymerase VP1 by inhibiting K48-linked ubiquitination. A reverse genetic approach showed that only IBDV with I⁴⁰⁴C/T and I⁴⁰⁶C/F mutations of VP1 could be rescued successfully with decreased replication ability. Our data demonstrated that SUMO1 modification is essential to sustain the stability of polymerase VP1 during IBDV replication and provides a potential target for designing antiviral drugs targeting IBDV.IMPORTANCE SUMOylation is an extensively discussed posttranslational modification in diverse cellular biological pathways. However, there is limited understanding about SUMOylation of viral proteins of IBDV during infection. In the present study, we revealed a SUMO1 modification of VP1 protein, the RNA-dependent RNA polymerase of avibirnavirus infectious bursal disease virus (IBDV). The required site of VP1 SUMOylation comprised residues ⁴⁰⁴I and ⁴⁰⁶I of SUMO interaction motif 3, which was essential for maintaining its stability by inhibiting K48-linked ubiquitination. We also showed that IBDV with SUMOylation-deficient VP1 had decreased replication ability. These data demonstrated that the SUMOylation of IBDV VP1 played an important role in maintaining IBDV replication.

Host proteins involved in microtubule-dependent HIV-1 intracellular transport and uncoating

Microtubules and microtubule-associated proteins are critical for cargo transport throughout the cell. Many viruses are able to usurp these transport systems for their own replication and spread. HIV-1 utilizes these proteins for many of its early events postentry, including transport, uncoating and reverse transcription. The molecular motor proteins dynein and kinesin-1 are the primary drivers of cargo transport, and HIV-1 utilizes these proteins for infection. In this Review, we highlight recent developments in the understanding of how HIV-1 hijacks motor transport, the key cellular and viral proteins involved, and the ways that transport influences other steps in the HIV-1 lifecycle.

Ivermectin treatment inhibits the replication of Porcine circovirus 2 (PCV2) in vitro and mitigates the impact of viral infection in piglets

Porcine circovirus 2 (PCV2) capsid protein (Cap) has a nuclear localization signal (NLS) and can enter the nucleus. In this study, ivermectin, a small-molecule nuclear import inhibitor of proteins was used to determine the role of nuclear localization of Cap on PCV2 replication. Observation by fluorescence microscopy of the intracellular localization of Cap and Cap NLS in cells cultured with ivermectin (50 μg/mL) determined that Cap and Cap NLS were located in the cytoplasm; in contrast, for cells cultured without ivermectin, they accumulated in the cell nucleus. Ivermectin treatment also reduced nuclear transport of Cap derived from PCV2 infection as well as PCV2 replication in PK-15 cells. In addition, lower levels of PCV2 in tissues and sera of piglets treated with ivermectin were detected by qPCR. These results established for the first time that ivermectin has potent antiviral activity towards PCV2 both in vitro and vivo.

The cytoplasmic dynein transport machinery and its many cargoes

Cytoplasmic dynein 1 is an important microtubule-based motor in many eukaryotic cells. Dynein has critical roles both in interphase and during cell division. Here, we focus on interphase cargoes of dynein, which include membrane-bound organelles, RNAs, protein complexes and viruses. A central challenge in the field is to understand how a single motor can transport such a diverse array of cargoes and how this process is regulated. The molecular basis by which each cargo is linked to dynein and its cofactor dynactin has started to emerge. Of particular importance for this process is a set of coiled-coil proteins - activating adaptors - that both recruit dynein-dynactin to their cargoes and activate dynein motility.

Caspase-Dependent Apoptosis Induction via Viral Protein ORF4 of Porcine Circovirus 2 Binding to Mitochondrial Adenine Nucleotide Translocase 3

Apoptosis is an essential strategy of host defense responses and is used by viruses to maintain their life cycles. However, the apoptotic signals involved in virus replication are poorly known. In the present study, we report the molecular mechanism of apoptotic induction by the viral protein ORF4, a newly identified viral protein of porcine circovirus type 2 (PCV2). Apoptosis detection revealed not only that the activity of caspase-3 and -9 is increased in PCV2-infected and ORF4-transfected cells but also that cytochrome c release from the mitochondria to the cytosol is upregulated. Subsequently, ORF4 protein colocalization with adenine nucleotide translocase 3 (ANT3) was observed using structured illumination microscopy. Moreover, coimmunoprecipitation and pulldown analyses confirmed that the ORF4 protein interacts directly with mitochondrial ANT3 (mtANT3). Binding domain analysis further confirmed that N-terminal residues 1 to 30 of the ORF4 protein, comprising a mitochondrial targeting signal, are essential for the interaction with ANT3. Knockdown of ANT3 markedly inhibited the apoptotic induction of both ORF4 protein and PCV2, indicating that ANT3 plays an important role in ORF4 protein-induced apoptosis during PCV2 infection. Taken together, these data indicate that the ORF4 protein is a mitochondrial targeting protein that induces apoptosis by interacting with ANT3 through the mitochondrial pathway.IMPORTANCE The porcine circovirus type 2 (PCV2) protein ORF4 is a newly identified viral protein; however, little is known about its functions. Apoptosis is an essential strategy of the host defense response and is used by viruses to maintain their life cycles. In the present study, we report the molecular mechanism of the apoptosis induced by the ORF4 protein. The ORF4 protein contains a mitochondrial targeting signal and is an unstable protein that is degraded by the proteasome-dependent pathway. Viral protein ORF4 triggers caspase-3- and -9-dependent cellular apoptosis in mitochondria by directly binding to ANT3. We conclude that the ORF4 protein is a mitochondrial targeting protein and reveal a mechanism whereby circovirus recruits ANT3 to induce apoptosis.

Impacts of virus-mediated manipulation of host Dynein

In general viruses' modus operandi to propagate is achieved by the co-opting host cell components, membranes, proteins, and machineries to their advantage. This is true for virtually every aspect of a virus' replication cycle from virus entry to the budding or release of progeny virus particles. In this chapter, we will discuss new information on the impacts of virus-mediated manipulation of Dynein motor complexes and associated machineries and factors. We will highlight how these host cell components impact on pathogenicity and immune responses, as many of the virus-mediated hijacked components also play pivotal roles in immune responses to pathogen insult. There are several comprehensive reviews that define virus–Dynein interactions including the first edition of this book that describes how viruses manipulate the host cell machineries their advantage. An updated table is included to summarize these virus–host interactions. Notably, barriers to intracellular translocation represent major hurdles to viral components during de novo infection and during active replication and the generation of progeny virus particles. Clearly, the subversion of host cell molecular motor protein activities takes advantage of constitutive and regulated membrane trafficking events and will target virus components to intracytoplasmic locales and membrane assembly. Broadening our understanding of the interplay between viruses, Dynein and the cytoskeleton will likely inform on new types of therapies. Continual enhancement of the breadth of new information on how viruses manipulate host cell biology will inevitably aid in the identification of new targets that can be poisoned to block old, new, and emerging viruses alike in their tracks.

Cytoplasmic transport and nuclear import of plasmid DNA

Productive transfection and gene transfer require not simply the entry of DNA into cells and subsequent transcription from an appropriate promoter, but also a number of intracellular events that allow the DNA to move from the extracellular surface of the cell into and through the cytoplasm, and ultimately across the nuclear envelope and into the nucleus before any transcription can initiate. Immediately upon entry into the cytoplasm, naked DNA, either delivered through physical techniques or after disassembly of DNA-carrier complexes, associates with a large number of cellular proteins that mediate subsequent interactions with the microtubule network for movement toward the microtubule organizing center and the nuclear envelope. Plasmids then enter the nucleus either upon the mitotic disassembly of the nuclear envelope or through nuclear pore complexes in the absence of cell division, using a different set of proteins. This review will discuss our current understanding of these pathways used by naked DNA during the transfection process. While much has been elucidated on these processes, much remains to be discerned, but with the development of a number of model systems and approaches, great progress is being made.

Gene expression of benthic amphipods (genus: Diporeia) in relation to a circular ssDNA virus across two Laurentian Great Lakes

Circular rep-encoding ssDNA (CRESS-DNA) viruses are common constituents of invertebrate viral consortia. Despite their ubiquity and sequence diversity, the effects of CRESS-DNA viruses on invertebrate biology and ecology remain largely unknown. This study assessed the relationship between the transcriptional profile of benthic amphipods of genus Diporeia and the presence of the CRESS-DNA virus, LM29173, in the Laurentian Great Lakes to provide potential insight into the influence of these viruses on invertebrate gene expression. Twelve transcriptomes derived from Diporeia were compared, representing organisms from two amphipod haplotype clades (Great Lakes Michigan and Superior, defined by COI barcode sequencing) with varying viral loads (up to 3 × 10⁶ genome copies organism⁻¹). Read recruitment to de novo assembled transcripts revealed 2,208 significantly over or underexpressed contigs in transcriptomes with above average LM29173 load. Of these contigs, 31.5% were assigned a putative function. The greatest proportion of annotated, differentially expressed transcripts were associated with functions including: (1) replication, recombination, and repair, (2) cell structure/biogenesis, and (3) post-translational modification, protein turnover, and chaperones. Contigs putatively associated with innate immunity displayed no consistent pattern of expression, though several transcripts were significantly overexpressed in amphipods with high viral load. Quantitation (RT-qPCR) of target transcripts, non-muscular myosin heavy chain, β-actin, and ubiquitin-conjugating enzyme E2, corroborated transcriptome analysis and indicated that Lake Michigan and Lake Superior amphipods with high LM29173 load exhibit lake-specific trends in gene expression. While this investigation provides the first comparative survey of the transcriptional profile of invertebrates of variable CRESS-DNA viral load, additional inquiry is required to define the scope of host-specific responses to potential infection.

Rabies Virus Infection Induces Microtubule Depolymerization to Facilitate Viral RNA Synthesis by Upregulating HDAC6

Rabies virus (RABV) is the cause of rabies, and is associated with severe neurological symptoms, high mortality rate, and a serious threat to human health. Although cellular tubulin has recently been identified to be incorporated into RABV particles, the effects of RABV infection on the microtubule cytoskeleton remain poorly understood. In this study, we show that RABV infection induces microtubule depolymerization as observed by confocal microscopy, which is closely associated with the formation of the filamentous network of the RABV M protein. Depolymerization of microtubules significantly increases viral RNA synthesis, while the polymerization of microtubules notably inhibits viral RNA synthesis and prevents the viral M protein from inducing the formation of the filamentous network. Furthermore, the histone deacetylase 6 (HDAC6) expression level progressively increases during RABV infection, and the inhibition of HDAC6 deacetylase activity significantly decreases viral RNA synthesis. In addition, the expression of viral M protein alone was found to significantly upregulate HDAC6 expression, leading to a substantial reduction in its substrate, acetylated α-tubulin, eventually resulting in microtubule depolymerization. These results demonstrate that HDAC6 plays a positive role in viral transcription and replication by inducing microtubule depolymerization during RABV infection.

Cellular defence or viral assist: the dilemma of HDAC6

Histone deacetylase 6 (HDAC6) is a unique cytoplasmic deacetylase that regulates various important biological processes by preventing protein aggregation and deacetylating different non-histone substrates including tubulin, heat shock protein 90, cortactin, retinoic acid inducible gene I and β-catenin. Growing evidence has indicated a dual role for HDAC6 in viral infection and pathogenesis: HDAC6 may represent a host defence mechanism against viral infection by modulating microtubule acetylation, triggering antiviral immune response and stimulating protective autophagy, or it may be hijacked by the virus to enhance proinflammatory response. In this review, we will highlight current data illustrating the complexity and importance of HDAC6 in viral pathogenesis. We will summarize the structure and functional specificity of HDAC6, and its deacetylase- and ubiquitin-dependent activity in key cellular events in response to virus infection. We will also discuss how HDAC6 exerts its direct or indirect histone modification ability in viral lytic-latency switch.

Cellular proteomic analysis of porcine circovirus type 2 and classical swine fever virus coinfection in porcine kidney-15 cells using isobaric tags for relative and absolute quantitation-coupled LC-MS/MS

Viral coinfection or superinfection in host has caused public health concern and huge economic losses of farming industry. The influence of viral coinfection on cellular protein abundance is essential for viral pathogenesis. Based on a coinfection model for porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV) developed previously by our laboratory, isobaric tags for relative and absolute quantitation (iTRAQ)-coupled LC-MS/MS proteomic profiling was performed to explore the host cell responses to PCV2-CSFV coinfection. Totally, 3932 proteins were identified in three independent mass spectrometry analyses. Compared with uninfected cells, 304 proteins increased (fold change >1.2) and 198 decreased (fold change <0.833) their abundance in PCV2-infected cells (p < 0.05), 60 and 61 were more and less abundant in CSFV-infected cells, and 196 and 158 were more and less abundant, respectively in cells coinfected with PCV2 and CSFV. Representative differentially abundant proteins were validated by quantitative real-time PCR, Western blotting and confocal laser scanning microscopy. Bioinformatic analyses confirmed the dominant role of PCV2, and indicated that mitochondrial dysfunction, nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated oxidative stress response and apoptosis signaling pathways might be the specifical targets during PCV2-CSFV coinfection.

Dynein light chain DYNLL1 subunit facilitates porcine circovirus type 2 intracellular transports along microtubules

Microtubule (MT) and dynein motor proteins facilitate intracytoplasmic transport of cellular proteins. Various viruses utilize microtubules and dynein for their movement from the cell periphery to the nucleus. The aim of this study was to investigate the intracellular transport of porcine circovirus type 2 (PCV2) via 8 kDa dynein light chain (DYNLL1, LC8) subunit along the MTs. At 20 μM, vinblastine sulfate inhibited tubulin polymerization resulting in disorganized morphology. In PCV2-infected PK-15 cells, double immunofluorescent labeling showed that the viral particles appeared at the cell periphery and gradually moved to the microtubule organization center (MTOC) at 0-12 hour post inoculation (hpi) while at 20-24 hpi they accumulated in the nucleus. Co-localization between DYNLL1 and PCV2 particles was observed clearly at 8-12 hpi. At 20-24 hpi, most aggregated tubulin had a paracrystalline appearance at the MTOC around the nucleus in vinblastine-treated, PCV2-infected PK-15 cells. Between 12 and 24 hpi, PCV2 particles were still bound to DYNLL1 before they were translocated to the nucleus in both treatments, indicating that vinblastine sulfate had no effect on the protein-protein co-localization. The DYNLL1 binding motif, LRLQT, was found near the C-terminus of PCV2 capsid protein (Cap). Molecular docking analysis confirmed the specific interaction between these residues and the cargo binding site on DYNLL1. Our study clearly demonstrated that dynein, in particular DYNLL1, mediated PCV2 intracellular trafficking. The results could explain, at least in part, the viral transport mechanism by DYNLL1 via MT during PCV2 infection.

Structural insights into the assembly and regulation of distinct viral capsid complexes

The assembly and regulation of viral capsid proteins into highly ordered macromolecular complexes is essential for viral replication. Here, we utilize crystal structures of the capsid protein from the smallest and simplest known viruses capable of autonomously replicating in animal cells, circoviruses, to establish structural and mechanistic insights into capsid morphogenesis and regulation. The beak and feather disease virus, like many circoviruses, encode only two genes: a capsid protein and a replication initiation protein. The capsid protein forms distinct macromolecular assemblies during replication and here we elucidate these structures at high resolution, showing that these complexes reverse the exposure of the N-terminal arginine rich domain responsible for DNA binding and nuclear localization. We show that assembly of these complexes is regulated by single-stranded DNA (ssDNA), and provide a structural basis of capsid assembly around single-stranded DNA, highlighting novel binding interfaces distinct from the highly positively charged N-terminal ARM domain.

Circoviruses are the simplest viruses known to autonomously replicate in vertebrates. Here the authors present three structures for distinct macromolecular assemblies of the capsid protein from the beak and feather disease virus that provides insights into the regulation of viral capsid assembly.

Molecular Basis for the Protein Recognition Specificity of the Dynein Light Chain DYNLT1/Tctex1: CHARACTERIZATION OF THE INTERACTION WITH ACTIVIN RECEPTOR IIB

It has been suggested that DYNLT1, a dynein light chain known to bind to various cellular and viral proteins, can function both as a molecular clamp and as a microtubule-cargo adapter. Recent data have shown that the DYNLT1 homodimer binds to two dynein intermediate chains to subsequently link cargo proteins such as the guanine nucleotide exchange factor Lfc or the small GTPases RagA and Rab3D. Although over 20 DYNLT1-interacting proteins have been reported, the exact sequence requirements that enable their association to the canonical binding groove or to the secondary site within the DYNLT1 surface are unknown. We describe herein the sequence recognition properties of the hydrophobic groove of DYNLT1 known to accommodate dynein intermediate chain. Using a pepscan approach, we have substituted each amino acid within the interacting peptide for all 20 natural amino acids and identified novel binding sequences. Our data led us to propose activin receptor IIB as a novel DYNLT1 ligand and suggest that DYNLT1 functions as a molecular dimerization engine bringing together two receptor monomers in the cytoplasmic side of the membrane. In addition, we provide evidence regarding a dual binding mode adopted by certain interacting partners such as Lfc or the parathyroid hormone receptor. Finally, we have used NMR spectroscopy to obtain the solution structure of human DYNLT1 forming a complex with dynein intermediate chain of ∼74 kDa; it is the first mammalian structure available.

The co-chaperone Cdc37 regulates the rabies virus phosphoprotein stability by targeting to Hsp90AA1 machinery

Cdc37, as a kinase-specific co-chaperone of the chaperone Hsp90AA1 (Hsp90), actively aids with the maturation, stabilization and activation of the cellular or viral kinase/kinase-like targets. Phosphoprotein (P) of rabies virus (RABV) is a multifunctional, non-kinase protein involved in interferon antagonism, viral transcription and replication. Here, we demonstrated that the RABV non-kinase P is chaperoned by Cdc37 and Hsp90 during infection. We found that Cdc37 and Hsp90 affect the RABV life cycle directly. Activity inhibition and knockdown of Cdc37 and Hsp90 increased the instability of the viral P protein. Overexpression of Cdc37 and Hsp90 maintained P's stability but did not increase the yield of infectious RABV virions. We further demonstrated that the non-enzymatic polymerase cofactor P protein of all the genotypes of lyssaviruses is a target of the Cdc37/Hsp90 complex. Cdc37, phosphorylated or unphosphorylated on Ser13, aids the P protein to load onto the Hsp90 machinery, with or without Cdc37 binding to Hsp90. However, the interaction between Cdc37 and Hsp90 appears to have additional allosteric regulation of the conformational switch of Hsp90. Our study highlighted a novel mechanism in which Cdc37/Hsp90 chaperones a non-kinase target, which has significant implications for designing therapeutic targets against Rabies.

Exploring the mechanochemical cycle of dynein motor proteins: structural evidence of crucial intermediates

Exploration of the biologically relevant pathways of dynein's mechanochemical cycle using structure based models.

In Vitro Coinfection and Replication of Classical Swine Fever Virus and Porcine Circovirus Type 2 in PK15 Cells

Increasing clinical lines of evidence have shown the coinfection/superinfection of porcine circovirus type 2 (PCV2) and classical swine fever virus (CSFV). Here, we investigated whether PCV2 and CSFV could infect the same cell productively by constructing an in vitro coinfection model. Our results indicated that PCV2-free PK15 cells but not ST cells were more sensitive to PCV2, and the PK15 cell line could stably harbor replicating CSFV (PK15-CSFV cells) with a high infection rate. Confocal and super-resolution microscopic analysis showed that PCV2 and CSFV colocalized in the same PK15-CSFV cell, and the CSFV E2 protein translocated from the cytoplasm to the nucleus in PK15-CSFV cells infected with PCV2. Moreover, PCV2-CSFV dual-positive cells increased gradually in PK15-CSFV cells in a PCV2 dose-dependent manner. In PK15-CSFV cells, PCV2 replicated well, and the production of PCV2 progeny was not influenced by CSFV infection. However, CSFV reproduction decreased in a PCV2 dose-dependent manner. In addition, cellular apoptosis was not strengthened in PK15-CSFV cells infected with PCV2 in comparison with PCV2-infected PK15 cells. Moreover, using this coinfection model we further demonstrated PCV2-induced apoptosis might contribute to the impairment of CSFV HCLV strain replication in coinfected cells. Taken together, our results demonstrate for the first time the coinfection/superinfection of PCV2 and CSFV within the same cell, providing an in vitro model to facilitate further investigation of the underlying mechanism of CSFV and PCV2 coinfection.