The adeno-associated virus type 2 Rep protein regulates RNA processing via interaction with the transcription template

Identification of the role of SNARE proteins in rAAV vector production through interaction with the viral MAAP

Adeno-associated virus (AAV) expresses a membrane-associated accessory protein (MAAP), a small nonstructural protein, that facilitates AAV secretion out of the plasma membrane through an association with extracellular vesicles during AAV egress. Here, we investigated the host proteins that interact with AAV2 MAAP (MAAP2) using APEX2-mediated proximity labeling. We identified two SNARE proteins, Syntaxin 7 (STX7) and synaptosome-associated protein 23 (SNAP23), a vesicle (v-)SNARE and a target (t-)SNARE, respectively, that mediate intracellular trafficking of membrane vesicles aand exhibited associations with MAAP2 in HEK293 cells. We found that MAAP2 indirectly interacted with STX7 or SNAP23, and that the knockout of STX7 or SNAP23 not only enhanced rAAV secretion into the media but also increased total vector yield during rAAV vector production in HEK293 cells. Thus, our study revealed a practical approach for producing higher yields of rAAV vectors from the media, easing downstream processes in rAAV manufacturing.

The Expression and Function of the Small Nonstructural Proteins of Adeno-Associated Viruses (AAVs)

Adeno-associated viruses (AAVs) are small, non-enveloped viruses that package a single-stranded (ss)DNA genome of 4.7 kilobases (kb) within their T = 1 icosahedral capsid. AAVs are replication-deficient viruses that require a helper virus to complete their life cycle. Recombinant (r)AAVs have been utilized as gene delivery vectors for decades in gene therapy applications. So far, six rAAV-based gene medicines have been approved by the US FDA. The 4.7 kb ssDNA genome of AAV encodes nine proteins, including three viral structural/capsid proteins, VP1, VP2, and VP3; four large nonstructural proteins (replication-related proteins), Rep78/68 and Rep52/40; and two small nonstructural proteins. The two nonstructured proteins are viral accessory proteins, namely the assembly associated protein (AAP) and membrane-associated accessory protein (MAAP). Although the accessory proteins are conserved within AAV serotypes, their functions are largely obscure. In this review, we focus on the expression strategy and functional properties of the small nonstructural proteins of AAVs.

Role of the membrane-associated accessory protein (MAAP) in adeno-associated virus (AAV) infection

Adeno-associated viruses (AAVs) package a single-stranded (ss) DNA genome of 4.7 kb in their capsid of ~20 nm in diameter. AAV replication requires co-infection of a helper virus, such as adenovirus. During the optimization of recombinant AAV production, a small viral nonstructural protein, membrane-associated accessory protein (MAAP), was identified. However, the function of the MAAP in the context of AAV infection remains unknown. Here, we investigated the expression strategy and function of the MAAP during infection of both AAV2 and AAV5 in human embryonic kidney (HEK)293 cells. We found that AAV2 MAAP2 and AAV5 MAAP5 are expressed from the capsid gene (cap)-transcribing mRNA spliced from the donor to the second splice site that encodes VP2 and VP3. Thus, this AAV cap gene transcribes a multicistronic mRNA that can be translated to four viral proteins, MAAP, VP2, AAP, and VP3 in order. In AAV2 infection, MAAP2 predominantly localized in the cytoplasm, alongside the capsid, near the nuclear and plasma membranes, but a fraction of MAAP2 exhibited nuclear localization. In AAV5 infection, MAAP5 revealed a distinct pattern, predominantly localizing within the nucleus. In the cells infected with an MAAP knockout mutant of AAV2 or AAV5, both viral DNA replication and virus replication increased, whereas virus egress decreased, and the decrease in virus egress can be restored by providing MAAP in trans. In summary, MAAP, a novel AAV nonstructural protein translated from a multicistronic viral cap mRNA, not only facilitates cellular egress of AAV but also likely negatively affects viral DNA replication during infection.

IMPORTANCE

Recombinant adeno-associated virus (rAAV) has been used as a gene delivery vector in clinical gene therapy. In current gene therapies employing rAAV, a high dose of the vector is required. Consequently, there is a high demand for efficient and high-purity vector production systems. In this study, we demonstrated that membrane-associated accessory protein (MAAP), a small viral nonstructural protein, is translated from the same viral mRNA transcript encoding VP2 and VP3. In AAV-infected cells, apart from its prevalent expression in the cytoplasm with localization near the plasma and nuclear membranes, the MAAP also exhibits notable localization within the nucleus. During AAV infection, MAAP expression increases the cellular egress of progeny virions and decreases viral DNA replication and progeny virion production. Thus, the choice of MAAP expression has pros and cons during AAV infection, which could provide a guide to rAAV production.

Molecular design of controllable recombinant adeno-associated virus (AAV) expression systems for enhanced vector production

Recombinant adeno-associated virus (rAAV) is the leading vector for the delivery of gene therapies. However, low viral genome (VG) titers are common and the proportion of "full" capsids containing the therapeutic gene payload can be highly variable. The coordinated molecular design of plasmids encoding viral components and Helper functions remains a major challenge for rAAV manufacturing. Here we present the design of improved Rep/Cap and Helper plasmids for rAAV2/8 production, (i) a Rep/Cap expression vector harboring independently controllable rep and cap genes and (ii) an improved Helper plasmid harboring E4 gene deletion variants. First, an optimized Rep/Cap vector utilized a truncated p5 promoter, a p5 cis-regulatory element at the 3' end in combination with a heterologous promoter to drive Cap expression and an additional copy of the rep52/40 gene to overexpress short Rep proteins. We demonstrate that Rep78 is essential for efficient rAAV2/8 production in HEK293 cells, and a higher ratio of short Rep to long Rep proteins enhances genome packaging. Second, we identified regulators and open reading frames within the Helper plasmid that contribute to increased rAAV2/8 production. While L4-33k/22k is integral to optimal production, the use of E4orf6-6/7 subset significantly enhanced VG titer. Together, an optimal combination of engineered Rep/Cap and Helper plasmid variants increased VG titer by 3.1-fold. This study demonstrates that configuring and controlling the expression of the different AAV genetic elements contributes toward high rAAV production and product quality (full/empty capsid ratio).

Viral RNA capping: Mechanisms and antiviral therapy

RNA capping is an essential trigger for protein translation in eukaryotic cells. Many viruses have evolved various strategies for initiating the translation of viral genes and generating progeny virions in infected cells via synthesizing cap structure or stealing the RNA cap from nascent host messenger ribonucleotide acid (mRNA). In addition to protein translation, a new understanding of the role of the RNA cap in antiviral innate immunity has advanced the field of mRNA synthesis in vitro and therapeutic applications. Recent studies on these viral RNA capping systems have revealed startlingly diverse ways and molecular machinery. A comprehensive understanding of how viruses accomplish the RNA capping in infected cells is pivotal for designing effective broad-spectrum antiviral therapies. Here we systematically review the contemporary insights into the RNA-capping mechanisms employed by viruses causing human and animal infectious diseases, while also highlighting its impact on host antiviral innate immune response. The therapeutic applications of targeting RNA capping against viral infections and the development of RNA-capping inhibitors are also summarized.

Pathogenicity and virulence of human adenovirus F41: Possible links to severe hepatitis in children

Over 100 human adenoviruses (HAdVs) have been isolated and allocated to seven species, A-G. Species F comprises two members-HAdV-F40 and HAdV-F41. As their primary site of infection is the gastrointestinal tract they have been termed, with species A, enteric adenoviruses. HAdV-F40 and HAdV-F41 are a common cause of gastroenteritis and diarrhoea in children. Partly because of difficulties in propagating the viruses in the laboratory, due to their restrictions on growth in many cell lines, our knowledge of the properties of individual viral proteins is limited. However, the structure of HAdV-F41 has recently been determined by cryo-electron microscopy. The overall structure is similar to those of HAdV-C5 and HAdV-D26 although with some differences. The sequence and arrangement of the hexon hypervariable region 1 (HVR1) and the arrangement of the C-terminal region of protein IX differ. Variations in the penton base and hexon HVR1 may play a role in facilitating infection of intestinal cells by HAdV-F41. A unique feature of HAdV-F40 and F41, among human adenoviruses, is the presence and expression of two fibre genes, giving long and short fibre proteins. This may also contribute to the tropism of these viruses. HAdV-F41 has been linked to a recent outbreak of severe acute hepatitis "of unknown origin" in young children. Further investigation has shown a very high prevalence of adeno-associated virus-2 in the liver and/or plasma of some cohorts of patients. These observations have proved controversial as HAdV-F41 had not been reported to infect the liver and AAV-2 has generally been considered harmless.

Adeno-Associated Virus Monoinfection Induces a DNA Damage Response and DNA Repair That Contributes to Viral DNA Replication

Adeno-associated virus (AAV) belongs to the Dependoparvovirus genus of the Parvoviridae family. AAV replication relies on a helper virus, such as adenovirus (Ad). Co-infection of AAV and Ad induces a DNA damage response (DDR), although its function in AAV DNA replication remains unknown. In this study, monoinfection of AAV2 in HEK293T cells expressing a minimal set of Ad helper genes was used to investigate the role of the DDR solely induced by AAV. We found that AAV2 DNA replication, but not single stranded (ss)DNA genome accumulation and Rep expression only, induced a robust DDR in HEK293T cells. The induced DDR featured the phosphorylation of replication protein A32 (RPA32), histone variant H2AX (H2A histone family member X), and all 3 phosphatidylinositol 3-kinase-related kinases (PIKKs). We also found that the kinase ataxia telangiectasia and Rad3-related protein (ATR) plays a major role in AAV2 DNA replication and that Y family DNA repair DNA polymerases η (Pol η) and Pol κ contribute to AAV2 DNA replication both in vitro and in HEK293T cells. Knockout of Pol η and Pol κ in HEK293T cells significantly decreased wild-type AAV2 replication and recombinant AAV2 production. Thus, our study has proven that AAV2 DNA replication induces a DDR, which in turn initiates a DNA repairing process that partially contributes to the viral genome amplification in HEK293T cells. IMPORTANCE Recombinant AAV (rAAV) has emerged as one of the preferred delivery vectors for clinical gene therapy. rAAV production in HEK293 cells by transfection of a rAAV transgene plasmid, an AAV Rep and Cap expression packaging plasmid, and an Ad helper plasmid remains the popular method. Here, we demonstrated that the high fidelity Y family DNA repair DNA polymerase, Pol η, and Pol κ, plays a significant role in AAV DNA replication and rAAV production in HEK293T cells. Understanding the AAV DNA replication mechanism in HEK293T cells could provide clues to increase rAAV vector yield produced from the transfection method. We also provide evidence that the ATR-mediated DNA repair process through Pol η and Pol κ is one of the mechanisms to amplify AAV genome, which could explain AAV replication and rAAV ssDNA genome conversion in mitotic quiescent cells.

Recent Advances in Molecular Biology of Human Bocavirus 1 and Its Applications

Human bocavirus 1 (HBoV1) was discovered in human nasopharyngeal specimens in 2005. It is an autonomous human parvovirus and causes acute respiratory tract infections in young children. HBoV1 infects well differentiated or polarized human airway epithelial cells in vitro. Unique among all parvoviruses, HBoV1 expresses 6 non-structural proteins, NS1, NS1-70, NS2, NS3, NS4, and NP1, and a viral non-coding RNA (BocaSR), and three structural proteins VP1, VP2, and VP3. The BocaSR is the first identified RNA polymerase III (Pol III) transcribed viral non-coding RNA in small DNA viruses. It plays an important role in regulation of viral gene expression and a direct role in viral DNA replication in the nucleus. HBoV1 genome replication in the polarized/non-dividing airway epithelial cells depends on the DNA damage and DNA repair pathways and involves error-free Y-family DNA repair DNA polymerase (Pol) η and Pol κ. Importantly, HBoV1 is a helper virus for the replication of dependoparvovirus, adeno-associated virus (AAV), in polarized human airway epithelial cells, and HBoV1 gene products support wild-type AAV replication and recombinant AAV (rAAV) production in human embryonic kidney (HEK) 293 cells. More importantly, the HBoV1 capsid is able to pseudopackage an rAAV2 or rHBoV1 genome, producing the rAAV2/HBoV1 or rHBoV1 vector. The HBoV1 capsid based rAAV vector has a high tropism for human airway epithelia. A deeper understanding in HBoV1 replication and gene expression will help find a better way to produce the rAAV vector and to increase the efficacy of gene delivery using the rAAV2/HBoV1 or rHBoV1 vector, in particular, to human airways. This review summarizes the recent advances in gene expression and replication of HBoV1, as well as the use of HBoV1 as a parvoviral vector for gene delivery.

Binding of CCCTC-Binding Factor (CTCF) to the Minute Virus of Mice Genome Is Important for Proper Processing of Viral P4-Generated Pre-mRNAs

Specific chromatin immunoprecipitation of salt-fractionated infected cell extracts has demonstrated that the CCCTC-binding factor (CTCF), a highly conserved, 11-zinc-finger DNA-binding protein with known roles in cellular and viral genome organization and gene expression, specifically binds the genome of Minute Virus of Mice (MVM). Mutations that diminish binding of CTCF to MVM affect processing of the P4-generated pre-mRNAs. These RNAs are spliced less efficiently to generate the R1 mRNA, and definition of the NS2-specific exon upstream of the small intron is reduced, leading to relatively less R2 and the generation of a novel exon-skipped product. These results suggest a model in which CTCF is required for proper engagement of the spliceosome at the MVM small intron and for the first steps of processing of the P4-generated pre-mRNA.

Establishment of a Recombinant AAV2/HBoV1 Vector Production System in Insect Cells

We have previously developed an rAAV2/HBoV1 vector in which a recombinant adeno-associated virus 2 (rAAV2) genome is pseudopackaged into a human bocavirus 1 (HBoV1) capsid. Recently, the production of rAAV2/HBoV1 in human embryonic kidney (HEK) 293 cells has been greatly improved in the absence of any HBoV1 nonstructural proteins (NS). This NS-free production system yields over 16-fold more vectors than the original production system that necessitates NS expression. The production of rAAV with infection of baculovirus expression vector (BEV) in the suspension culture of Sf9 insect cells is highly efficient and scalable. Since the replication of the rAAV2 genome in the BEV system is well established, we aimed to develop a BEV system to produce the rAAV2/HBoV1 vector in Sf9 cells. We optimized the usage of translation initiation signals of the HBoV1 capsid proteins (Cap), and constructed a BEV Bac-AAV2Rep-HBoV1Cap, which expresses the AAV2 Rep78 and Rep52 as well as the HBoV1 VP1, VP2, and VP3 at the appropriate ratios. We found that it is sufficient as a trans helper to the production of rAAV2/HBoV1 in Sf9 cells that were co-infected with the transfer Bac-AAV2ITR-GFP-luc that carried a 5.4-kb oversized rAAV2 genome with dual reporters. Further study found that incorporation of an HBoV1 small NS, NP1, in the system maximized the viral DNA replication and thus the rAAV2/HBoV1 vector production at a level similar to that of the rAAV2 vector in Sf9 cells. However, the transduction potency of the rAAV2/HBoV1 vector produced from BEV-infected Sf9 cells was 5-7-fold lower in polarized human airway epithelia than that packaged in HEK293 cells. Transmission electron microscopy analysis found that the vector produced in Sf9 cells had a high percentage of empty capsids, suggesting the pseudopackage of the rAAV2 genome in HBoV1 capsid is not as efficient as in the capsids of AAV2. Nevertheless, our study demonstrated that the rAAV2/HBoV1 can be produced in insect cells with BEVs at a comparable yield to rAAV, and that the highly efficient expression of the HBoV1 capsid proteins warrants further optimization.

Detection of Viral RNA Splicing in Diagnostic Virology

This chapter is the first one to introduce the detection of viral RNA splicing as a new tool for clinical diagnosis of virus infections. These include various infections caused by influenza viruses, human immunodeficiency viruses (HIV), human T-cell leukemia viruses (HTLV), Torque teno viruses (TTV), parvoviruses, adenoviruses, hepatitis B virus, polyomaviruses, herpesviruses, and papillomaviruses. Detection of viral RNA splicing for active viral gene expression in a clinical sample is a nucleic acid-based detection. The interpretation of the detected viral RNA splicing results is straightforward without concern for carry-over DNA contamination, because the spliced RNA is smaller than its corresponding DNA template.

Although many methods can be used, a simple method to detect viral RNA splicing is reverse transcription-polymerase chain reaction (RT-PCR). In principle, the detection of spliced RNA transcripts by RT-PCR depends on amplicon selection and primer design. The most common approach is the amplification over the intron regions by a set of primers in flanking exons. A larger product than the predicted size of smaller, spliced RNA is in general an unspliced RNA or contaminating viral genomic DNA. A spliced mRNA always gives a smaller RT-PCR product than its unspliced RNA due to removal of intron sequences by RNA splicing. The contaminating viral DNA can be determined by a minus RT amplification (PCR). Alternatively, specific amplification of a spliced RNA can be obtained by using an exon-exon junction primer because the sequence at exon-exon junction is not present in the unspliced RNA nor in viral genomic DNA.

The Human Bocavirus 1 NP1 Protein Is a Multifunctional Regulator of Viral RNA Processing

Human bocavirus 1 (HBoV1) encodes a genus-specific protein, NP1, which regulates viral alternative pre-mRNA processing. Similar to NP1 of the related bocavirus minute virus of canine (MVC), HBoV1 NP1 suppressed cleavage and polyadenylation of RNAs at the viral internal polyadenylation site (pA)p. HBoV1 (pA)p is a complex region. It contains 5 significant cleavage and polyadenylation sites, and NP1 was found to regulate only the three of these sites that are governed by canonical AAUAAA hexamer signals. HBoV1 NP1 also facilitated splicing of the upstream intron adjacent to (pA)p. Alternative polyadenylation and splicing of the upstream intron were independent of each other, functioned efficiently within an isolated transcription unit, and were responsive independent of NP1. Characterization of HBoV1 NP1 generalizes its function within the genus Bocaparvovirus, uncovers important differences, and provides important comparisons with MVC NP1 for mechanistic and evolutionary considerations.IMPORTANCE The Parvovirinae are small nonenveloped icosahedral viruses that are important pathogens in many animal species, including humans. The NP1 protein of human bocavirus 1 (HBoV1), similar to NP1 of the bocavirus minute virus of canine (MVC), regulates viral alternative RNA processing by both suppressing polyadenylation at an internal site, (pA)p, and facilitating splicing of an upstream adjacent intron. These effects allow both extension into the capsid gene and splicing of the viral pre-mRNA that correctly registers the capsid gene open reading frame. Characterization of HBoV1 NP1 generalizes this central mode of parvovirus gene regulation to another member of the bocavirus genus and uncovers both important similarities and differences in function compared to MVC NP1 that will be important for future comparative studies.

Development of a Novel Recombinant Adeno-Associated Virus Production System Using Human Bocavirus 1 Helper Genes

Human bocavirus 1 (HBoV1), an autonomous parvovirus, is a helper virus supporting replication of wild-type adeno-associated virus 2 (AAV2). In this study, we compared the helper functions from HBoV1 with those from adenovirus (Ad) for the production of recombinant AAV (rAAV) vector in HEK293 cells. We demonstrated that triple plasmids transfection of (1) a cloned HBoV1 helper minigenome (pBocaHelper) that expresses HBoV1 genes NP1, NS2, and BocaSR, (2) pAAV transfer plasmid, and (3) pAAVRepCap supports rAAV production in HEK293 cells. Despite a production yield of 1–2 log lower than that using pAdHelper (expressing Ad genes E2A, E4, and VA), rAAV vector produced using pBocaHelper transduced cells as efficiently as that produced using pAdHelper. The low vector production is largely due to the inefficient expression of the AAV Rep52 and capsid proteins, as well as reduced rAAV genome replication. When the AAV capsid proteins and Rep52 were ectopically expressed under strong promoters, the enhanced protein expression significantly improved the rAAV production using pBocaHelper, approaching a level of 50%–70% of that produced using pAdHelper. Through further dissection of the helper functions from pAdHelper in a five-plasmid transfection system, we found that the addition of the Ad E2A gene to the above HBoV1 helper system significantly increased rAAV DNA replication, which increased the rAAV vector production to a level of 3–7 times higher than that using pAdHelper. We finally combined HBoV1 NP1 and NS2 genes with Ad helper genes to create a novel dual helper plasmid (pABHelper) for rAAV vector production in the conventional three-plasmid transfection system. The pABHelper facilitated rAAV production at a yield ∼2 times higher than that using the pAdHelper.

Human Bocavirus 1 Is a Novel Helper for Adeno-associated Virus Replication

Human bocavirus 1 (HBoV1) is an autonomous parvovirus that infects well-differentiated primary human airway epithelia (HAE) in vitro In human embryonic kidney HEK293 cells, the transfection of a duplex HBoV1 genome initiates viral DNA replication and produces progeny virions that are infectious in HAE. HBoV1 takes advantage of signaling pathways in the DNA damage response for efficient genome amplification in both well-differentiated (nondividing) HAE and dividing HEK293 cells. On the other hand, adeno-associated virus 2 (AAV2) is a helper-dependent dependoparvovirus, and productive AAV2 replication requires coinfection with a helper virus (e.g., adenovirus or herpesvirus) or treatment with genotoxic agents. Here, we report that HBoV1 is a novel helper virus for AAV2 replication. Coinfection by HBoV1 and AAV2 rescued AAV2 replication in HAE cells. The helper function of HBoV1 for AAV2 is not limited to HAE cells but also includes HEK293 and HeLa cells. Importantly, the helper function of HBoV1 for AAV2 relies on neither HBoV1 replication nor the DNA damage response. Following transfection of HEK293 cells, the minimal requirements for the replication of the AAV2 duplex DNA genome and the production of progeny virions included the HBoV1 NP1 and NS4 proteins and a newly identified viral long noncoding RNA (BocaSR). However, following infection of HEK293 and HeLa cells with AAV2 virions, HBoV1 NS2 (but not NS4), NP1, and BocaSR were required for AAV2 DNA replication and progeny virion formation. These new methods for packaging the AAV2 genome may be useful for generating recombinant AAV-packaging cell lines and the directed evolution of AAV capsids.IMPORTANCE We first report that an autonomous parvovirus, HBoV1, helps the replication of a dependoparvovirus, AAV2, in differentiated human airway epithelia. We identified the minimal sets of HBoV1 genes required to facilitate the replication of the AAV2 duplex genome and for AAV2 infection. Notably, together with the expression of the NP1 and BocaSR genes, HBoV1 NS2 is required for the productive infection of HEK293 and HeLa cells by AAV2, whereas NS4 is sufficient for viral DNA replication of an AAV2 duplex genome. The identification of HBoV1 as a helper virus for AAV2 replication has implications for the improvement of recombinant AAV production in HEK293 cells and cell types that do not express the adenovirus E1 gene as well as for the rescue of wild-type AAV genomes from tissues during directed evolution in the absence of wild-type adenovirus. A further understanding of the mechanism underlying HBoV1 helper-dependent AAV2 replication may also provide insights into its functions in HBoV1 replication.

Minute Virus of Canines NP1 Protein Governs the Expression of a Subset of Essential Nonstructural Proteins via Its Role in RNA Processing

Parvoviruses use a variety of means to control the expression of their compact genomes. The bocaparvovirus minute virus of canines (MVC) encodes a small, genus-specific protein, NP1, which governs access to the viral capsid gene via its role in alternative polyadenylation and alternative splicing of the single MVC pre-mRNA. In addition to NP1, MVC encodes five additional nonstructural proteins (NS) that share an initiation codon at the left end of the genome and which are individually encoded by alternative multiply spliced mRNAs. We found that three of these proteins were encoded by mRNAs that excise the NP1-regulated MVC intron immediately upstream of the internal polyadenylation site, (pA)p, and that generation of these proteins was thus regulated by NP1. Splicing of their progenitor mRNAs joined the amino termini of these proteins to the NP1 open reading frame, and splice site mutations that prevented their expression inhibited virus replication in a host cell-dependent manner. Thus, in addition to controlling capsid gene access, NP1 also controls the expression of three of the five identified NS proteins via its role in governing MVC pre-mRNA splicing.IMPORTANCE The Parvovirinae are small nonenveloped icosahedral viruses that are important pathogens in many animal species, including humans. Minute virus of canine (MVC) is an autonomous parvovirus in the genus Bocaparvovirus It has a single promoter that generates a single pre-mRNA. NP1, a small genus-specific MVC protein, participates in the processing of this pre-mRNA and so controls capsid gene access via its role in alternative internal polyadenylation and splicing. We show that NP1 also controls the expression of three of the five identified NS proteins via its role in governing MVC pre-mRNA splicing. These NS proteins together are required for virus replication in a host cell-dependent manner.

A chimeric human APOBEC3A protein with a three amino acid insertion confers differential HIV-1 and adeno-associated virus restriction

Old World monkey (OWM) and hominid APOBEC3Aproteins exhibit differential restriction activities against lentiviruses and DNA viruses. Human APOBEC3A(hA3A)has weak restriction activity against HIV-1Δvifbut is efficiently restricted by an artificially generated chimeric from mandrills (mndA3A/G). We show that a chimeric hA3Acontaining the "WVS" insertion (hA3A[(27)WVS(29)]) conferred potent HIV-1restriction activity. Analysis of each amino acid of the "WVS" motif show that the length and not necessarily the charge or hydrophobicity of the amino acids accounted for restriction activity. Our results suggest that hA3A[(27)WVS(29)]restricts HIV-1at the level of reverse transcription in target cells. Finally, our results suggest that insertion of "WVS" into hA3Amodestly reduces restriction of adeno-associated virus 2(AAV-2)while insertion of the AC Loop1region of the mndA3A/G into hA3A abolished AAV-2 restriction, strengthening the role of this molecular interface in the functional evolution of primate A3A.

A Regulatory Element Near the 3' End of the Adeno-Associated Virus rep Gene Inhibits Adenovirus Replication in cis by Means of p40 Promoter-Associated Short Transcripts

Adeno-associated virus (AAV) has long been known to inhibit helper adenovirus (Ad) replication independently of AAV Rep protein expression. More recently, replication of Ad serotype 5 (Ad5)/AAV serotype 2 (AAV-2) hybrid vectors was shown to be inhibited incisby a sequence near the 3' end of AAVrep, termed the Rep inhibition sequence for adenoviral replication (RIS-Ad). RIS-Ad functions independently of Rep protein expression. Here we demonstrate that inhibition of adenoviral replication by RIS-Ad requires an active AAV p40 promoter and the 5' half of the intron. In addition, Ad inhibition is critically dependent on the integrity of the p40 transcription start site (TSS) leading to short p40-associated transcripts. These do not give rise to effector molecules capable of inhibiting adenoviral replication intrans, like small polypeptides or microRNAs. Our data point to an inhibitory mechanism in which RNA polymerase II (Pol II) pauses directly downstream of the p40 promoter, leading to interference of the stalled Pol II transcription complex with the adenoviral replication machinery. Whereas inhibition by RIS-Ad is mediated exclusively incis, it can be overcome by providing a replication-competent adenoviral genome intrans Moreover, the inhibitory effect of RIS-Ad is not limited to AAV-2 but could also be shown for the corresponding regions of other AAV serotypes, including AAV-5. These findings have important implications for the future generation of Ad5/AAV hybrid vectors.

IMPORTANCE

Insertion of sequences from the 3' part of therepgene of adeno-associated virus (AAV) into the genome of its helper adenovirus strongly reduces adenoviral genome replication. We could show that this inhibition is mediated exclusively inciswithout the involvement oftrans-acting regulatory RNAs or polypeptides but nevertheless requires an active AAV-2 p40 promoter and p40-associated short transcripts. Our results suggest a novel inhibitory mechanism that has so far not been described for AAV and that involves stalled RNA polymerase II complexes and their interference with adenoviral DNA replication. Such a mechanism would have important implications both for the generation of adenoviral vectors expressing the AAVrepandcapgenes and for the regulation of AAV gene expression in the absence and presence of helper virus.

Identification and Functional Analysis of Novel Nonstructural Proteins of Human Bocavirus 1

Human bocavirus 1 (HBoV1) is a single-stranded DNA parvovirus that causes lower respiratory tract infections in young children worldwide. In this study, we identified novel splice acceptor and donor sites, namely, A1' and D1', in the large nonstructural protein (NS1)-encoding region of the HBoV1 precursor mRNA. The novel small NS proteins (NS2, NS3, and NS4) were confirmed to be expressed following transfection of an HBoV1 infectious proviral plasmid and viral infection of polarized human airway epithelium cultured at an air-liquid interface (HAE-ALI). We constructed mutant pIHBoV1 infectious plasmids which harbor silent mutations (sm) smA1' and smD1' at the A1' and D1' splice sites, respectively. The mutant infectious plasmids maintained production of HBoV1 progeny virions at levels less than five times lower than that of the wild-type plasmid. Importantly, the smA1' mutant virus that does not express NS3 and NS4 replicated in HAE-ALI as effectively as the wild-type virus; however, the smD1' mutant virus that does not express NS2 and NS4 underwent an abortive infection in HAE-ALI. Thus, our study identified three novel NS proteins, NS2, NS3, and NS4, and suggests an important function of the NS2 protein in HBoV1 replication in HAE-ALI.

IMPORTANCE

Human bocavirus 1 infection causes respiratory diseases, including acute wheezing in infants, of which life-threatening cases have been reported. In vitro, human bocavirus 1 infects polarized human bronchial airway epithelium cultured at an air-liquid interface that mimics the environment of human lower respiratory airways. Viral nonstructural proteins are often important for virus replication and pathogenesis in infected tissues or cells. In this report, we identified three new nonstructural proteins of human bocavirus 1 that are expressed during infection of polarized human bronchial airway epithelium. Among them, we proved that one nonstructural protein is critical to the replication of the virus in polarized human bronchial airway epithelium. The creation of nonreplicating infectious HBoV1 mutants may have particular utility in vaccine development for this virus.

Productive life cycle of adeno-associated virus serotype 2 in the complete absence of a conventional polyadenylation signal

We showed that WT adeno-associated virus serotype 2 (AAV2) genome devoid of a conventional polyadenylation [poly(A)] signal underwent complete genome replication, encapsidation and progeny virion production in the presence of adenovirus. The infectivity of the progeny virion was also retained. Using recombinant AAV2 vectors devoid of a human growth hormone poly(A) signal, we also demonstrated that a subset of mRNA transcripts contained the inverted terminal repeat (ITR) sequence at the 3' end, which we designated ITR in RNA (ITRR). Furthermore, AAV replication (Rep) proteins were able to interact with the ITRR. Taken together, our studies suggest a new function of the AAV2 ITR as an RNA element to mediate transgene expression from poly(A)-deleted mRNA.

Viral and Cellular Components of AAV2 Replication Compartments

Adeno-associated virus 2 (AAV2) is a helpervirus-dependent parvovirus with a bi-phasic life cycle comprising latency in absence and lytic replication in presence of a helpervirus, such as adenovirus (Ad) or herpes simplex virus type 1 (HSV-1). Helpervirus-supported AAV2 replication takes place in replication compartments (RCs) in the cell nucleus where virus DNA replication and transcription occur. RCs consist of a defined set of helper virus-, AAV2-, and cellular proteins. Here we compare the profile of cellular proteins recruited into AAV2 RCs or identified in Rep78-associated complexes when either Ad or HSV-1 is the helpervirus, and we discuss the potential roles of some of these proteins in AAV2 and helpervirus infection.

Characterization of the nonstructural proteins of the bocavirus minute virus of canines

We present a detailed characterization of a single-cycle infection of the bocavirus minute virus of canines (MVC) in canine WRD cells. This has allowed identification of an additional smaller NS protein that derives from an mRNA spliced within the NS gene that had not been previously reported. In addition, we have identified a role for the viral NP1 protein during infection. NP1 is required for read-through of the MVC internal polyadenylation site and, thus, access of the capsid gene by MVC mRNAs. Although the mechanism of NP1's action has not yet been fully elucidated, it represents the first parvovirus protein to be implicated directly in viral RNA processing.

The adeno-associated virus type 5 small rep proteins expressed via internal translation initiation are functional

Although precluded from using splicing to produce multiple small Rep proteins, adeno-associated virus type 5 (AAV5) generates a Rep40-like protein by alternative translation initiation at an internal AUG. A defined region upstream of the internal AUG was both required and sufficient to program internal initiation within AAV5 and may act similarly in heterologous contexts. The internally initiated AAV5 Rep40-like protein was functional and had helicase activity similar to that of AAV2 Rep40. Surprisingly, both the AAV5 Rep40-like protein and Rep52 were able to be translated from the AAV5 upstream P7-generated RNAs; however, the relative level of small to large Rep proteins was reduced compared to that of the wild type. A P19 mutant AAV5 infectious clone generated near-wild-type levels of the double-stranded monomer replicative form (mRF) replicative intermediate but reduced levels of virus, consistent with the previously defined role of Rep40-like proteins in genome encapsidation. Levels of mutant virus were dramatically reduced upon amplification.

Capsid protein expression and adeno-associated virus like particles assembly in Saccharomyces cerevisiae

Background

The budding yeast Saccharomyces cerevisiae supports replication of many different RNA or DNA viruses (e.g. Tombusviruses or Papillomaviruses) and has provided means for up-scalable, cost- and time-effective production of various virus-like particles (e.g. Human Parvovirus B19 or Rotavirus). We have recently demonstrated that S. cerevisiae can form single stranded DNA AAV2 genomes starting from a circular plasmid. In this work, we have investigated the possibility to assemble AAV capsids in yeast.

Results

To do this, at least two out of three AAV structural proteins, VP1 and VP3, have to be simultaneously expressed in yeast cells and their intracellular stoichiometry has to resemble the one found in the particles derived from mammalian or insect cells. This was achieved by stable co-transformation of yeast cells with two plasmids, one expressing VP3 from its natural p40 promoter and the other one primarily expressing VP1 from a modified AAV2 Cap gene under the control of the inducible yeast promoter Gal1. Among various induction strategies we tested, the best one to yield the appropriate VP1:VP3 ratio was 4.5 hour induction in the medium containing 0.5% glucose and 5% galactose. Following such induction, AAV virus like particles (VLPs) were isolated from yeast by two step ultracentrifugation procedure. The transmission electron microscopy analysis revealed that their morphology is similar to the empty capsids produced in human cells.

Conclusions

Taken together, the results show for the first time that yeast can be used to assemble AAV capsid and, therefore, as a genetic system to identify novel cellular factors involved in AAV biology.

Establishment of a reverse genetics system for studying human bocavirus in human airway epithelia

Human bocavirus 1 (HBoV1) has been identified as one of the etiological agents of wheezing in young children with acute respiratory-tract infections. In this study, we have obtained the sequence of a full-length HBoV1 genome (including both termini) using viral DNA extracted from a nasopharyngeal aspirate of an infected patient, cloned the full-length HBoV1 genome, and demonstrated DNA replication, encapsidation of the ssDNA genome, and release of the HBoV1 virions from human embryonic kidney 293 cells. The HBoV1 virions generated from this cell line-based production system exhibits a typical icosahedral structure of approximately 26 nm in diameter, and is capable of productively infecting polarized primary human airway epithelia (HAE) from the apical surface. Infected HAE showed hallmarks of lung airway-tract injury, including disruption of the tight junction barrier, loss of cilia and epithelial cell hypertrophy. Notably, polarized HAE cultured from an immortalized airway epithelial cell line, CuFi-8 (originally derived from a cystic fibrosis patient), also supported productive infection of HBoV1. Thus, we have established a reverse genetics system and generated the first cell line-based culture system for the study of HBoV1 infection, which will significantly advance the study of HBoV1 replication and pathogenesis.

Human bocavirus 1 (HBoV1) has been identified as one of the etiological agents of wheezing in young children with acute respiratory-tract infections. HBoV1 productively infects polarized primary human airway epithelia. However, no cell lines permissive to HBoV1 infection have yet been established. More importantly, the sequences at both ends of the HBoV1 genome have remained unknown. We have resolved both of these issues in this study. We have sequenced a full-length HBoV1 genome and cloned it into a plasmid. We further demonstrated that this HBoV1 plasmid replicated and produced viruses in human embryonic kidney 293 cells. Infection of these HBoV1 progeny virions produced obvious cytopathogenic effects in polarized human airway epithelia, which were represented by disruption of the epithelial barrier. Moreover, we identified an airway epithelial cell line supporting HBoV1 infection, when it was polarized. This is the first study to obtain the full-length HBoV1 genome, to demonstrate pathogenesis of HBoV1 infection in human airway epithelia, and to identify the first cell line to support productive HBoV1 infection.

Internal polyadenylation of parvoviral precursor mRNA limits progeny virus production

Aleutian Mink Disease Virus (AMDV) is the only virus in the genus Amdovirus of family Parvoviridae. In adult mink, AMDV causes a persistent infection associated with severe dysfunction of the immune system. Cleavage of AMDV capsid proteins has been previously shown to play a role in regulating progeny virus production (Fang Cheng et al., J. Virol. 84:2687-2696, 2010). The present study shows that AMDV has evolved a second strategy to limit expression of capsid proteins by preventing processing of the full-length capsid protein-encoding mRNA transcripts. Characterization of the cis-elements of the proximal polyadenylation site [(pA)p] in the infectious clone of AMDV revealed that polyadenylation at the (pA)p site is controlled by an upstream element (USE) of 200 nts in length, the AAUAAA signal, and a downstream element (DSE) of 40 nts. A decrease in polyadenylation at the (pA)p site, either by mutating the AAUAAA signal or the DSE, which does not affect the encoding of amino acids in the infectious clone, increased the expression of capsid protein VP1/VP2 and thereby increased progeny virus production approximately 2-3-fold. This increase was accompanied by enhanced replication of the AMDV genome. Thus, this study reveals correlations among internal polyadenylation, capsid production, viral DNA replication and progeny virus production of AMDV, indicating that internal polyadenylation is a limiting step for parvovirus replication and progeny virus production.

Adeno-associated virus biology

Adeno-associated virus (AAV) was first discovered as a contaminant of adenovirus stocks in the 1960s. The development of recombinant AAV vectors (rAAV) was facilitated by early studies that generated infectious molecular clones, determined the sequence of the genome, and defined the genetic elements of the virus. The refinement of methods and protocols for the production and application of rAAV vectors has come from years of studies that explored the basic biology of this virus and its interaction with host cells. Interest in improving vector performance has in turn driven studies that have provided tremendous insights into the basic biology of the AAV lifecycle. In this chapter, we review the background on AAV biology and its exploitation for vectors and gene delivery.

Differential virus restriction patterns of rhesus macaque and human APOBEC3A: implications for lentivirus evolution

The human apolipoprotein B mRNA editing enzyme catalytic peptide-like 3 (APOBEC3; A3) family of proteins (A3A-H) are known to restrict various retroviruses and retroelements, but the full complement of rhesus macaque A3 proteins remains unclear. We report the isolation and characterization of the hA3A homologue from rhesus macaques (rhA3A) and show that the rhesus macaque and human A3 genes are orthologous. RhA3A is expressed at high levels in activated CD4+ T cells, is widely expressed in macaque tissues, and is degraded in the presence of the human immunodeficiency virus (HIV-1) and simian-human immunodeficiency virus (SHIV) genomes. Our results indicate that rhA3A is a potent inhibitor of SHIVΔvif and to a lesser extent HIV-1Δvif. Unlike hA3A, rhA3A did not inhibit adeno-associated virus 2 (AAV-2) replication and L1 retrotransposition. These data suggest an evolutionary switch in primate A3A virus specificity and provide the first evidence that a primate A3A can inhibit lentivirus replication.

The RNA profile of porcine parvovirus 4, a boca-like virus, is unique among the parvoviruses

PPV4 transcribes its genome from a single promoter, and the RNAs are generated via alternate splicing coupled with alternate polyadenylation, a strategy similar to that of the bocaviruses; however, several differences were detected. The PPV4 ORF1 codes for four NS proteins, while the bocavirus ORF1 codes for 1-3 NS proteins. Whereas the VP1/VP2 capsid proteins of bocavirus are encoded by a single RNA, VP1 and VP2 of PPV4 are encoded by two separate RNAs. While ORF3 of PPV4 encodes two NP proteins, ORF3 of bocavirus codes for only one NP polypeptide. Taken together, PPV4 is unique among the parvoviruses.

Chipmunk parvovirus is distinct from members in the genus Erythrovirus of the family Parvoviridae

The transcription profile of chipmunk parvovirus (ChpPV), a tentative member of the genus Erythrovirus in the subfamily Parvovirinae of the family Parvoviridae, was characterized by transfecting a nearly full-length genome. We found that it is unique from the profiles of human parvovirus B19 and simian parvovirus, the members in the genus Erythrovirus so far characterized, in that the small RNA transcripts were not processed for encoding small non-structural proteins. However, like the large non-structural protein NS1 of the human parvovirus B19, the ChpPV NS1 is a potent inducer of apoptosis. Further phylogenetic analysis of ChpPV with other parvoviruses in the subfamily Parvovirinae indicates that ChpPV is distinct from the members in genus Erythrovirus. Thus, we conclude that ChpPV may represent a new genus in the family Parvoviridae.

Characterization of the gene expression profile of human bocavirus

We have generated a quantitative transcription profile of human bocavirus type 1 (HBoV1) by transfecting a nearly full-length clone in human lung epithelial A549 cells as well as in a replication competent system in 293 cells. The overall transcription profile of HBoV1 is similar to that of two other members of genus Bocavirus, minute virus of canines and bovine parvovirus 1. In particular, a spliced NS1-transcript that was not recognized previously expressed the large non-structural protein NS1 at approximately 100kDa; and the NP1-encoding transcripts were expressed abundantly. In addition, the protein expression profile of human bocavirus type 2 (HBoV2) was examined in parallel by transfection of a nearly full-length clone in A549 cells, which is similar to that of HBoV1. Moreover, our results showed that, unlike human parvovirus B19 infection, expression of the HBoV1 proteins only does not induce cell cycle arrest and apoptosis of A549 cells.

Novel parvovirus sublineage in the family of Parvoviridae

Parvoviridae, which are classified into two subfamilies Parvovirinae and Densovirinae, can infect both vertebrate and insects and are related to a wide range of diseases in insects, animals, and humans. In this report, several new parvoviruses were identified in swine sera collected in southeastern China. The sequence analyses showed that the parvoviruses detected in southeastern China formed a distinct sublineage within the subfamily Parvovirinae. Based on these results, we propose a novel parvovirus sublineage, Cnvirus, to describe these parvoviruses.

Identification of rep-associated factors in herpes simplex virus type 1-induced adeno-associated virus type 2 replication compartments

Adeno-associated virus (AAV) is a human parvovirus that replicates only in cells coinfected with a helper virus, such as adenovirus or herpes simplex virus type 1 (HSV-1). We previously showed that nine HSV-1 factors are able to support AAV rep gene expression and genome replication. To elucidate the strategy of AAV replication in the presence of HSV-1, we undertook a proteomic analysis of cellular and HSV-1 factors associated with Rep proteins and thus potentially recruited within AAV replication compartments (AAV RCs). This study resulted in the identification of approximately 60 cellular proteins, among which factors involved in DNA and RNA metabolism represented the largest functional categories. Validation analyses indicated that the cellular DNA replication enzymes RPA, RFC, and PCNA were recruited within HSV-1-induced AAV RCs. Polymerase delta was not identified but subsequently was shown to colocalize with Rep within AAV RCs even in the presence of the HSV-1 polymerase complex. In addition, we found that AAV replication is associated with the recruitment of components of the Mre11/Rad50/Nbs1 complex, Ku70 and -86, and the mismatch repair proteins MSH2, -3, and -6. Finally, several HSV-1 factors were also found to be associated with Rep, including UL12. We demonstrated for the first time that this protein plays a role during AAV replication by enhancing the resolution of AAV replicative forms and AAV particle production. Altogether, these analyses provide the basis to understand how AAV adapts its replication strategy to the nuclear environment induced by the helper virus.

The small 11 kDa nonstructural protein of human parvovirus B19 plays a key role in inducing apoptosis during B19 virus infection of primary erythroid progenitor cells

Human parvovirus B19 (B19V) infection shows a strong erythroid tropism and drastically destroys erythroid progenitor cells, thus leading to most of the disease outcomes associated with B19V infection. In this study, we systematically examined the 3 B19V nonstructural proteins, 7.5 kDa, 11 kDa, and NS1, for their function in inducing apoptosis in transfection of primary ex vivo-expanded erythroid progenitor cells, in comparison with apoptosis induced during B19V infection. Our results show that 11 kDa is a more significant inducer of apoptosis than NS1, whereas 7.5 kDa does not induce apoptosis. Furthermore, we determined that caspase-10, an initiator caspase in death receptor signaling, is the most active caspase in apoptotic erythroid progenitors induced by 11 kDa and NS1 as well as during B19V infection. More importantly, cytoplasm-localized 11 kDa is expressed at least 100 times more than nucleus-localized NS1 at the protein level in primary erythroid progenitor cells infected with B19V; and inhibition of 11 kDa expression using antisense oligos targeting specifically to the 11 kDa-encoding mRNAs reduces apoptosis significantly during B19V infection of erythroid progenitor cells. Taken together, these results demonstrate that the 11 kDa protein contributes to erythroid progenitor cell death during B19V infection.

Transcriptional analysis of the adeno-associated virus integration site

The nonpathogenic human adeno-associated virus type 2 (AAV-2) has adopted a unique mechanism to site-specifically integrate its genome into the human MBS85 gene, which is embedded in AAVS1 on chromosome 19. The fact that AAV has evolved to integrate into this ubiquitously transcribed region and that the chromosomal motifs required for integration are located a few nucleotides upstream of the translation initiation start codon of MBS85 suggests that the transcriptional activity of MBS85 might influence site-specific integration and thus might be involved in the evolution of this mechanism. In order to begin addressing this question, we initiated the characterization of the human MBS85 promoter region and compared its transcriptional activity to that of the AAV-2 p5 promoter. Our results clearly indicate that AAVS1 is defined by a complex transcriptional environment and that the MBS85 promoter shares key regulatory elements with the viral p5 promoter. Furthermore, we provide evidence for bidirectional MBS85 promoter activity and demonstrate that the minimal motifs required for AAV site-specific integration are present in the 5' untranslated region of the gene and play a posttranscriptional role in the regulation of MBS85 expression. These findings should provide a framework to further elucidate the complex interactions between the virus and its cellular host in this unique pathway to latency.

Molecular characterization of human parvovirus B19 genotypes 2 and 3

We have characterized the transcription profiles of parvovirus B19 (B19V) genotype-2 A6 and genotype-3 V9 variants. The A6 RNA profile differs from that of the prototype B19V in both B19V non-permissive and permissive cells, whereas the overall profile of the V9 RNA in these cells is similar to that of the prototype. A unique feature we have identified is that the genotype-2 A6 variant used only one splice acceptor to remove the first intron. We also demonstrated that the inverted terminal repeats (ITRs) of the prototype B19V support replication of the V9 genome, which produces infectious virus, but not that of the A6 genome, in B19V-permissive cells. Similar to the proapoptotic nature of the prototype B19V large non-structural protein (NS1), the A6 and V9 NS1 proteins also are potent inducers of apoptosis in B19V-permissive cells.

Improved splicing of adeno-associated viral (AAV) capsid protein-supplying pre-mRNAs leads to increased recombinant AAV vector production

Adeno-associated viral (AAV) capsid proteins, thought to be a rate-limiting step in the production of recombinant AAV (rAAV), are translated from spliced mRNAs. Improvement of the native AAV nonconsensus donor sequence increases splicing yet leaves the relative levels of VP1- and VP2/3-encoding mRNAs unchanged, and thus provides a means to increase delivery of correct ratios of AAV capsid proteins. This effect is independent of the AAV serotype used, and occurs whether the rep and cap genes supplied in trans are on the same or separate expression vectors. In the split-vector system, replacement of the more traditionally used cytomegalovirus promoter with that of the AAV5 P41 promoter allowed for even greater levels of splicing, and together with an improved intron donor, led to a 10- to 15-fold increase in the levels of splicing, rAAV production, and transduction compared with levels achieved by traditional cotransfection methods. Thus, the enhancement of splicing presents a useful method to enhance rAAV production via transient transfection.

The genome of human parvovirus b19 can replicate in nonpermissive cells with the help of adenovirus genes and produces infectious virus

Human parvovirus B19 (B19V) is a member of the genus Erythrovirus in the family Parvoviridae. In vitro, autonomous B19V replication is limited to human erythroid progenitor cells and in a small number of erythropoietin-dependent human megakaryoblastoid and erythroid leukemic cell lines. Here we report that the failure of B19V DNA replication in nonpermissive 293 cells can be overcome by adenovirus infection. More specifically, the replication of B19V DNA in the 293 cells and the production of infectious progeny virus were made possible by the presence of the adenovirus E2a, E4orf6, and VA RNA genes that emerged during the transfection of the pHelper plasmid. Using this replication system, we identified the terminal resolution site and the nonstructural protein 1 (NS1) binding site on the right terminal palindrome of the viral genome, which is composed of a minimal origin of replication spanning 67 nucleotides. Plasmids or DNA fragments containing an NS1 expression cassette and this minimal origin were able to replicate in both pHelper-transfected 293 cells and B19V-semipermissive UT7/Epo-S1 cells. Our results have important implications for our understanding of native B19V infection.

Splicing of the large intron present in the nonstructural gene of minute virus of mice is governed by TIA-1/TIAR binding downstream of the nonconsensus donor

The essential proteins NS1 and NS2 of minute virus of mice are encoded by mRNAs R1 and R2, respectively. R2 is derived from R1 by excision of a large intron and thus splicing governs the relative ratios of NS1 and NS2. Excision of the large intron utilizes a nonconsensus 5' donor site. We identified a U-rich and A-rich intronic sequence immediately downstream of the nonconsensus 5' donor site that functions as an intronic splicing enhancer (ISE) required for efficient large-intron excision. The ISE binds the cellular RNA-processing proteins TIA-1 and TIAR, which enhance usage of the nonconsensus donor.

Identification of cellular proteins that interact with the adeno-associated virus rep protein

Adeno-associated virus (AAV) codes for four related nonstructural Rep proteins. AAV both replicates and assembles in the nucleus and requires coinfection with a helper virus, either adenovirus (Ad) or herpesvirus, for a productive infection. Like other more complex DNA viruses, it is believed that AAV interacts or modifies host cell proteins to carry out its infection cycle. To date, relatively little is known about the host proteins that interact with the viral Rep proteins, which are known to be directly involved in DNA replication, control of viral and cellular transcription, splicing, and protein translation. In this study, we used affinity-tagged Rep protein to purify cellular protein complexes that were associated with Rep in cells that had been infected with Ad and AAV. In all, we identified 188 cellular proteins from 16 functional categories, including 14 transcription factors, 6 translation factors, 15 potential splicing proteins, 5 proteins involved in protein degradation, and 13 proteins involved in DNA replication or repair. This dramatically increases the number of potential interactions over the current number of approximately 26. Twelve of the novel proteins found were further tested by coimmunoprecipitation or colocalization using confocal immunomicroscopy. Of these, 10 were confirmed as proteins that formed complexes with Rep, including proteins of the MCM complex (DNA replication), RCN1 (membrane transport), SMC2 (chromatin dynamics), EDD1 (ubiquitin ligase), IRS4 (signal transduction), and FUS (splicing). Computer analysis suggested that 45 and 28 of the 188 proteins could be placed in a pathway of interacting proteins involved in DNA replication and protein synthesis, respectively. Of the proteins involved in DNA replication, all of the previously identified proteins involved in AAV DNA replication were found, except Ad DBP. The only Ad protein found to interact with Rep was the E1b55K protein. In addition, we confirmed that Rep interacts with Ku70/80 helicase. In vitro DNA synthesis assays demonstrated that although Ku helicase activity could substitute for MCM to promote strand displacement synthesis, its presence was not essential. Our study suggests that the interaction of AAV with cellular proteins is much more complex than previously suspected and provides a resource for further studies of the AAV life cycle.

Block to the production of full-length B19 virus transcripts by internal polyadenylation is overcome by replication of the viral genome

The pre-mRNA processing strategy of the B19 virus is unique among parvoviruses. B19 virus-generated pre-mRNAs are transcribed from a single promoter and are extensively processed by alternative splicing and alternative polyadenylation to generate 12 transcripts. Blockage of the production of full-length B19 virus transcripts at the internal polyadenylation site [(pA)p] was previously reported to be a limiting step in B19 virus permissiveness. We show here that in the absence of genome replication, internal polyadenylation of B19 virus RNAs at (pA)p is favored in cells which are both permissive and nonpermissive for B19 viral replication. Replication of the B19 virus genome, however, introduced either by viral infection or by transfection of an infectious clone into permissive cells or forced by heterologous replication systems in nonpermissive cells, enhanced readthrough of (pA)p and the polyadenylation of B19 virus transcripts at the distal site [(pA)d]. Therefore, replication of the genome facilitates the generation of sufficient full-length transcripts that encode the viral capsid proteins and the essential 11-kDa nonstructural protein. Furthermore, we show that polyadenylation of B19 viral RNA at (pA)p likely competes with splicing at the second intron. Thus, we conclude that replication of the B19 virus genome is the primary limiting step governing B19 virus tropism.

The transcription strategy of bovine adeno-associated virus (B-AAV) combines features of both adeno-associated virus type 2 (AAV2) and type 5 (AAV5)

The parvoviruses bovine adeno-associated virus (B-AAV) and adeno-associated virus type 5 (AAV5) have similar transcription maps. However, while the AAV5 capsid gene promoter P41 possesses a high basal level in 293 cells, and is further activated only poorly by Rep during adenovirus type 5 (Ad5) infection, the B-AAV P41 promoter has a low basal activity within RepCap constructs in these cells and can be strongly activated by its Rep protein in the presence of Ad5 when a Rep-binding element (RBE) is included in cis at either end of the molecule. These differences are not due to differences in the intrinsic activating capability of the individual Rep proteins. Both viral promoters contain AP1 and CRE elements that contribute to their basal activity; however, the nature of the B-AAV P41 promoter itself and the surrounding sequences contribute to its relatively lower basal activity. In addition, the B-AAV upstream transcription units themselves also are activated in the presence of Ad5 and Rep. Thus, although the transcription map of B-AAV is much more closely related to AAV5, activation of its promoters is functionally more like the prototype AAV2.

Adeno-associated viruses can induce phosphorylation of eIF2alpha via PKR activation, which can be overcome by helper adenovirus type 5 virus-associated RNA

Mutants of adenovirus type 5 (Ad5) virus-associated RNA I deficient in inhibiting the activation and subsequent phosphorylation of protein kinase R (PKR) could neither function as helpers for adeno-associated virus type 5 (AAV5) replication nor enhance AAV5 protein accumulation in either the presence or absence of Ad5 E4Orf6 and E2a. Furthermore, a short region of the AAV5 capsid gene RNA leader sequence surrounding the AUG of VP1 could induce the phosphorylation of eIF2alpha. Both short interfering RNA directed against PKR and the addition of the herpes simplex virus ICP34.5 protein enhanced the accumulation of AAV5 capsid protein in the presence of the AAV5 capsid gene PKR-inducing element, suggesting that VA RNA acted to overcome direct AAV5-induced activation of PKR that led to the phosphorylation of eIF2alpha. The expression of both the closely related goat-derived AAV and the prototype AAV2 capsid gene transcription units also induced the phosphorylation of eIF2alpha, suggesting that the induction of the PKR/eIF2alpha cellular response may be a previously unrecognized general feature of at least the Dependovirus genus of the Parvovirinae.

Expression profiles of bovine adeno-associated virus and avian adeno-associated virus display significant similarity to that of adeno-associated virus type 5

We present the first detailed expression profiles of nonprimate-derived adeno-associated viruses, namely, bovine adeno-associated virus (B-AAV) and avian adeno-associated virus (A-AAV), which were obtained after the infection of cell lines derived from their natural hosts. In general, the profiles of B-AAV and A-AAV were quite similar to that of AAV5; however, both exhibited features found for AAV2 as well. Like adeno-associated virus type 5 (AAV5), B-AAV and A-AAV utilized an internal polyadenylation site [(pA)p]; however, it was used to greater relative levels by B-AAV than by A-AAV. Similar to AAV5, >99% of B-AAV RNAs generated from upstream promoters were polyadenylated at (pA)p and hence not spliced. In contrast, ca. 50% of the A-AAV RNAs generated from upstream promoters read through (pA)p, as seen for AAV2. However, A-AAV generated lower levels of spliced P5 and P19 products than does AAV2, suggesting that A-AAV generates lower relative levels of Rep 68 and Rep 40. An additional difference in the expression profile of these viruses was that B-AAV generated a greater level of ITR-initiated RNAs than did A-AAV or AAV5. In addition, we demonstrate that, like AAV2, transactivation of transcription of the capsid-gene promoter of B-AAV required both adenovirus and targeting of its Rep protein to the transcription template; however, expression of the capsid-gene promoter of A-AAV was, like AAV5, largely independent of both adenovirus and its Rep proteins.

Distance-dependent processing of adeno-associated virus type 5 RNA is controlled by 5' exon definition

Adeno-associated virus type 5 (AAV5) is unique among human AAV serotypes in that it uses a polyadenylation site [(pA)p] within the single small intron in the center of the genome. We previously reported that inhibition of polyadenylation at (pA)p, necessary for read-through of P41-generated capsid gene pre-mRNAs which are subsequently spliced, requires binding of U1 snRNP to the upstream donor. Inhibition was reduced as the distance between the cap site and the donor was increased (increasing the size of the 5' exon). Here, we have demonstrated that U1-70K is a key component of U1 snRNP that mediates inhibition of polyadenylation at (pA)p. Furthermore, introduction of a U-rich stretch, predicted to target TIA-1 and thus increase the affinity of U1 snRNP binding to the intervening donor site, significantly augmented inhibition of (pA)p, while depletion of TIA-1 by siRNA increased (pA)p read-through. Finally, artificially tethering the cap binding complex (CBC) components CBP80 and CBP20 upstream of the intron donor increased inhibition of polyadenylation at (pA)p. Our results suggest that interaction with the CBC strengthens U1 snRNP binding to the downstream intron donor in a manner inversely proportional to the size of the 5' exon, thus governing the competition between intron splicing and polyadenylation at (pA)p. This competition must be optimized to program both the levels of polyadenylation of P7- and P19-generated RNA at (pA)p required to produce proper levels of the essential Rep proteins and the splicing of P41-generated RNAs to produce the proper ratio of capsid proteins during AAV5 infection.

Upstream AP1- and CREB-binding sites confer high basal activity on the adeno-associated virus type 5 capsid gene promoter

In contrast to the prototype adeno-associated virus type 2 (AAV2), the capsid gene P41 promoter of AAV5, within viral constructs that lack inverted terminal repeat sequences, displays a high basal level of expression in 293 cells in the absence of coinfecting adenovirus. Here we demonstrate that this was due to differences in the relative strengths of the core promoter elements and to the presence of active binding sites for the transcription factors CREB and AP1 within the upstream region of P41 that are absent from the AAV2 capsid gene promoter P40. These differences also governed the relative basal activity of the AAV capsid gene promoters within near-full-length viral genomes.

Molecular characterization of caprine adeno-associated virus (AAV-Go.1) reveals striking similarity to human AAV5

The complete genome of the caprine adeno-associated virus (AAV-Go.1) was sequenced, and the expression profile of AAV-Go.1 was determined following virus infection in primary lamb kidney cells. A remarkable similarity between the Rep coding region, the ITR sequence and the central intron of AAV-Go.1, and the human derived AAV5 was observed. The transcription profile of AAV-Go.1 was also quite similar to that of AAV5. AAV-Go.1 was able to efficiently infect human cell lines, following co-infection of human adenovirus, and in reciprocal experiments, AAV5 also was able to efficiently infect primary lamb cells and bovine cell lines. Recombinant AAV5 and AAV-Go.1 expressing the beta-gal gene flanked by AAV5 ITRs, showed similar transduction efficiency in various human and animal cells. Studies of AAV-Go.1 may expand our understanding of the evolutionary relationship between the AAV-5-like group of AAVs, which, in addition to human derived AAV5, includes AAV viruses from bovine, caprine, and avian species.

Efficient expression of the adeno-associated virus type 5 p41 capsid gene promoter in 293 cells does not require Rep

Efficient expression of the adeno-associated virus type 5 (AAV5) P41 capsid gene promoter required adenovirus E1A and/or E1B; however, in contrast to what was observed for expression of the AAV2 capsid gene promoter (P40), neither adenovirus infection nor the large Rep protein was required. Although both the AAV2 and the AAV5 large Rep proteins efficiently bound the (GAGY)(3) Rep-binding element, the AAV5 large Rep protein transactivated transcription of the inducible AAV2 P40 promoter much less well than AAV2 large Rep. Differences in their activation potentials were mapped to the amino-terminal region of the proteins, and the poorly transactivating AAV5 Rep protein could competitively inhibit AAV2 Rep transactivation.

Recombinant Avian Adeno-Associated Virus: Transgene ExpressionIn Vivoand Enhancement of ExpressionIn Vitro

Recombinant avian adeno-associated viruses coding for the LacZ gene were used to inoculate embryonating chicken eggs, to assess the usefulness of the system for the expression of a transgene in vivo. The results obtained indicate significantly higher levels of expression of the reporter gene at various time intervals in the embryos inoculated with the recombinant virus in comparison with the mock-inoculated controls. At the embryo level, significant differences were evident at 120 hr postinoculation; hatched chicks showed transgene expression up to 14 days of age. In a second experiment, different cell-line cultures were transfected with plasmids encoding for a reporter gene flanked by the avian adeno-associated virus inverted terminal repeats (ITR), either alone or in the presence of the major nonstructural proteins of the virus (Rep 78/68) to assess the ability of these proteins and DNA elements to enhance gene expression. Results indicate that the inclusion of the viral ITR alone or during coexpression of the Rep proteins significantly enhances the expression of the transgene in all cell lines tested, as evidenced by the detection of the beta-galacrosidase protein through chemiluminescence reactions and staining of transfected monolayers.

The transcription profile of Aleutian mink disease virus in CRFK cells is generated by alternative processing of pre-mRNAs produced from a single promoter

A reevaluation of the transcription profile of Aleutian mink disease parvovirus (AMDV)-infected CRFK cells at either 32 degrees C or 37 degrees C has determined that strain AMDV-G encodes six species of mRNAs produced by alternative splicing and alternative polyadenylation of a pre-mRNA generated by a single promoter at the left end of the genome. Three different splicing patterns are used, and each type is found polyadenylated at either the 3' end of the genome (the distal site) or at a site in the center of the genome (the proximal site). All spliced species accumulate similarly over the course of infection, with the R2 RNA predominant throughout. The R2 RNA, which contains and can express the NS2 coding region, encodes the viral capsid proteins VP1 and VP2.