Endocytosis and nuclear trafficking of adeno-associated virus type 2 are controlled by rac1 and phosphatidylinositol-3 kinase activation

Identification and Validation of Small Molecules That Enhance Recombinant Adeno-associated Virus Transduction following High-Throughput Screens

While the recent success of adeno-associated virus (AAV)-mediated gene therapy in clinical trials is promising, challenges still face the widespread applicability of recombinant AAV(rAAV). A major goal is to enhance the transduction efficiency of vectors in order to achieve therapeutic levels of gene expression at a vector dose that is below the immunological response threshold. In an attempt to identify novel compounds that enhance rAAV transduction, we performed two high-throughput screens comprising 2,396 compounds. We identified 13 compounds that were capable of enhancing transduction, of which 12 demonstrated vector-specific effects and 1 could also enhance vector-independent transgene expression. Many of these compounds had similar properties and could be categorized into five groups: epipodophyllotoxins (group 1), inducers of DNA damage (group 2), effectors of epigenetic modification (group 3), anthracyclines (group 4), and proteasome inhibitors (group 5). We optimized dosing for the identified compounds in several immortalized human cell lines as well as normal diploid cells. We found that the group 1 epipodophyllotoxins (teniposide and etoposide) consistently produced the greatest transduction enhancement. We also explored transduction enhancement among single-stranded, self-complementary, and fragment vectors and found that the compounds could impact fragmented rAAV2 transduction to an even greater extent than single-stranded vectors. In vivo analysis of rAAV2 and all of the clinically relevant compounds revealed that, consistent with our in vitro results, teniposide exhibited the greatest level of transduction enhancement. Finally, we explored the capability of teniposide to enhance transduction of fragment vectors in vivo using an AAV8 capsid that is known to exhibit robust liver tropism. Consistent with our in vitro results, teniposide coadministration greatly enhanced fragmented rAAV8 transduction at 48 h and 8 days. This study provides a foundation based on the rAAV small-molecule screen methodology, which is ideally used for more-diverse libraries of compounds that can be tested for potentiating rAAV transduction.

IMPORTANCE

This study seeks to enhance the capability of adeno-associated viral vectors for therapeutic gene delivery applicable to the treatment of diverse diseases. To do this, a comprehensive panel of FDA-approved drugs were tested in human cells and in animal models to determine if they increased adeno-associated virus gene delivery. The results demonstrate that particular groups of drugs enhance adeno-associated virus gene delivery by unknown mechanisms. In particular, the enhancement of gene delivery was approximately 50 to 100 times better with than without teniposide, a compound that is also used as chemotherapy for cancer. Collectively, these results highlight the potential for FDA-approved drug enhancement of adeno-associated virus gene therapy, which could result in safe and effective treatments for diverse acquired or genetic diseases.

Insulin Therapy Improves Adeno-Associated Virus Transduction of Liver and Skeletal Muscle in Mice and Cultured Cells

Adeno-associated virus (AAV) gene transfer is a promising treatment for genetic abnormalities. Optimal AAV vectors are showing success in clinical trials. Gene transfer to skeletal muscle and liver is being explored as a potential therapy for some conditions, that is, α₁-antitrypsin (AAT) disorder and hemophilia B. Exploring approaches that enhance transduction of liver and skeletal muscle, using these vectors, is beneficial for gene therapy. Regulating hormones as an approach to improve AAV transduction is largely unexplored. In this study we tested whether insulin therapy improves liver and skeletal muscle gene transfer. In vitro studies demonstrated that the temporary coadministration (2, 8, and 24 hr) of insulin significantly improves AAV2-CMV-LacZ transduction of cultured liver cells and differentiated myofibers, but not of lung cells. In addition, there was a dose response related to this improved transduction. Interestingly, when insulin was not coadministered with the virus but given 24 hr afterward, there was no increase in the transgene product. Insulin receptor gene (INSR) expression levels were increased 5- to 13-fold in cultured liver cells and differentiated myofibers when compared with lung cells. Similar INSR gene expression profiles occurred in mouse tissues. Insulin therapy was performed in mice, using a subcutaneously implanted insulin pellet or a high-carbohydrate diet. Insulin treatment began just before intramuscular delivery of AAV1-CMV-schFIX or liver-directed delivery of AAV8-CMV-schFIX and continued for 28 days. Both insulin augmentation therapies improved skeletal muscle- and liver-directed gene transduction in mice as seen by a 3.0- to 4.5-fold increase in human factor IX (hFIX) levels. The improvement was observed even after the insulin therapy ended. Monitoring insulin showed that insulin levels increased during the brief period of rAAV delivery and during the entire insulin augmentation period (28 days). This study demonstrates that AAV transduction of liver or skeletal muscle can be improved by insulin therapy.

Small G Rac1 is involved in replication cycle of dengue serotype 2 virus in EAhy926 cells via the regulation of actin cytoskeleton

Bleeding is a clinical characteristic of severe dengue and may be due to increased vascular permeability. However, the pathogenesis of severe dengue remains unclear. In this study, we showed that the Rac1-microfilament signal pathway was involved in the process of DENV serotype 2 (DENV2) infection in EAhy926 cells. DENV2 infection induced dynamic changes in actin organization, and treatment with Cytochalasin D or Jasplakinolide disrupted microfilament dynamics, reduced DENV2 entry, and inhibited DENV2 assembly and maturation. Rac1 activities decreased during the early phase and gradually increased by the late phase of infection. Expression of the dominant-negative form of Rac1 promoted DENV2 entry but inhibited viral assembly, maturation and release. Our findings demonstrated that Rac1 plays an important role in the DENV2 life cycle by regulating actin reorganization in EAhy926 cells. This finding provides further insight into the pathogenesis of severe dengue.

The SUMOylation Pathway Restricts Gene Transduction by Adeno-Associated Viruses

Adeno-associated viruses are members of the genus dependoviruses of the parvoviridae family. AAV vectors are considered promising vectors for gene therapy and genetic vaccination as they can be easily produced, are highly stable and non-pathogenic. Nevertheless, transduction of cells in vitro and in vivo by AAV in the absence of a helper virus is comparatively inefficient requiring high multiplicity of infection. Several bottlenecks for AAV transduction have previously been described, including release from endosomes, nuclear transport and conversion of the single stranded DNA into a double stranded molecule. We hypothesized that the bottlenecks in AAV transduction are, in part, due to the presence of host cell restriction factors acting directly or indirectly on the AAV-mediated gene transduction. In order to identify such factors we performed a whole genome siRNA screen which identified a number of putative genes interfering with AAV gene transduction. A number of factors, yielding the highest scores, were identified as members of the SUMOylation pathway. We identified Ubc9, the E2 conjugating enzyme as well as Sae1 and Sae2, enzymes responsible for activating E1, as factors involved in restricting AAV. The restriction effect, mediated by these factors, was validated and reproduced independently. Our data indicate that SUMOylation targets entry of AAV capsids and not downstream processes of uncoating, including DNA single strand conversion or DNA damage signaling. We suggest that transiently targeting SUMOylation will enhance application of AAV in vitro and in vivo.

Particle tracking in drug and gene delivery research: State-of-the-art applications and methods

Particle tracking is a powerful microscopy technique to quantify the motion of individual particles at high spatial and temporal resolution in complex fluids and biological specimens. Particle tracking's applications and impact in drug and gene delivery research have greatly increased during the last decade. Thanks to advances in hardware and software, this technique is now more accessible than ever, and can be reliably automated to enable rapid processing of large data sets, thereby further enhancing the role that particle tracking will play in drug and gene delivery studies in the future. We begin this review by discussing particle tracking-based advances in characterizing extracellular and cellular barriers to therapeutic nanoparticles and in characterizing nanoparticle size and stability. To facilitate wider adoption of the technique, we then present a user-friendly review of state-of-the-art automated particle tracking algorithms and methods of analysis. We conclude by reviewing technological developments for next-generation particle tracking methods, and we survey future research directions in drug and gene delivery where particle tracking may be useful.

Recombinant adeno-associated virus utilizes cell-specific infectious entry mechanisms

Understanding the entry and trafficking mechanism(s) of recombinant adeno-associated virus (rAAV) into host cells can lead to evolution in capsid and vector design and delivery methods, resulting in enhanced transduction and therapeutic gene expression. Variability of findings regarding the early entry pathway of rAAV supports the possibility that rAAV, like other viruses, can utilize more than one infectious entry pathway. We tested whether inhibition of macropinocytosis impacted rAAV transduction of HeLa cells compared to hepatocellular carcinoma cell lines. We found that macropinocytosis inhibitor cytochalasin D blocked rAAV transduction of HeLa cells (>2-fold) but enhanced (10-fold) transduction in HepG2 and Huh7 lines. Similar results were obtained with another macropinocytosis inhibitor, 5-(N-ethyl-N-isopropyl) amiloride (EIPA). The augmented transduction was due to neither viral binding nor promoter activity, affected multiple rAAV serotypes (rAAV2, rAAV2-R585E, and rAAV8), and influenced single-stranded and self-complementary virions to comparable extents. Follow-up studies using CDC42 inhibitor ML141 and p21-activated kinase 1 (PAK1) siRNA knockdown also resulted in enhanced HepG2 transduction. Microscopy revealed that macropinocytosis inhibition correlated with expedited nuclear entry of the rAAV virions into HepG2 cells. Enhancement of hepatocellular rAAV transduction extended to the mouse liver in vivo (4-fold enhancement) but inversely blocked heart tissue transduction (13-fold). This evidence of host cell-specific rAAV entry pathways confers a potent means for controlling and enhancing vector delivery and could help unify the divergent accounts of rAAV cellular entry mechanisms.

IMPORTANCE

There is a recognized need for improved rAAV vector targeting strategies that result in delivery of fewer total particles, averting untoward toxicity and/or an immune response against the vector. A critical step in rAAV transduction is entry and early trafficking through the host cellular machinery, the mechanisms of which are under continued study. However, should the early entry and trafficking mechanisms of rAAV differ across virus serotype or be dependent on host cell environment, this could expand our ability to target particular cells and tissue for selective transduction. Thus, the observation that inhibiting macropinocytosis leads to cell-specific enhancement or inhibition of rAAV transduction that extends to the organismic level exposes a new means of modulating vector targeting.

Coat as a dagger: the use of capsid proteins to perforate membranes during non-enveloped DNA viruses trafficking

To get access to the replication site, small non-enveloped DNA viruses have to cross the cell membrane using a limited number of capsid proteins, which also protect the viral genome in the extracellular environment. Most of DNA viruses have to reach the nucleus to replicate. The capsid proteins involved in transmembrane penetration are exposed or released during endosomal trafficking of the virus. Subsequently, the conserved domains of capsid proteins interact with cellular membranes and ensure their efficient permeabilization. This review summarizes our current knowledge concerning the role of capsid proteins of small non-enveloped DNA viruses in intracellular membrane perturbation in the early stages of infection.

Hepatitis B virus infection enhances susceptibility toward adeno-associated viral vector transduction in vitro and in vivo

Gene therapy has become an accepted concept for the treatment of a variety of different diseases. In contrast to preclinical models, subjects enrolled in clinical trials, including gene therapy, possess a history of infection with microbes that may influence its safety and efficacy. Especially, viruses that establish chronic infections in the liver, one of the main targets for in vivo gene therapy, raise important concerns. Among them is the hepatitis B virus (HBV), which has chronically infected more than 350 million people worldwide. Here, we investigated the effect of HBV on adeno-associated viral (AAV) vectors, the most frequently applied gene transfer vehicles for in vivo gene therapy. Unexpectedly, we found that HBV greatly improved AAV transduction in cells replicating HBV and identified HBV protein x (HBx) as a key factor. Whereas HBV-positive and -negative cells were indistinguishable with respect to cell-entry efficiency, significantly higher numbers of AAV vector genomes were successfully delivered to the nucleus in the presence of HBV. The HBV-promoting effect was abolished by inhibitors of phosphatidylinositol 3-kinase (PI3K). PI3K was required for efficient trafficking of AAV to the nucleus and was enhanced in HBV-replicating cells and upon HBx expression. Enhancement of AAV transduction was confirmed in vivo using HBV transgenic mice and could successfully be applied to inhibit HBV progeny release.

CONCLUSION

Our results demonstrate that acute, as well as chronic, infections with unrelated viruses change the intracellular milieu, thereby likely influencing gene therapy outcomes. In the case of HBV, HBx-mediated enhancement of AAV transduction is an advantage that could be exploited for development of novel treatments of HBV infection.

GLP-1-mediated gene therapy approaches for diabetes treatment

Glucagon-like peptide (GLP)-1 is an incretin hormone with several antidiabetic functions including stimulation of glucose-dependent insulin secretion, increase in insulin gene expression and beta-cell survival. Despite the initial technical difficulties and profound inefficiency of direct gene transfer into the pancreas that seriously restricted in vivo gene transfer experiments with GLP-1, recent exploitation of various routes of gene delivery and alternative means of gene transfer has permitted the detailed assessment of the therapeutic efficacy of GLP-1 in animal models of type 2 diabetes (T2DM). As a result, many clinical benefits of GLP-1 peptide/analogues observed in clinical trials involving induction of glucose tolerance, reduction of hyperglycaemia, suppression of appetite and food intake linked to weight loss have been replicated in animal models using gene therapy. Furthermore, GLP-1-centered gene therapy not only improved insulin sensitivity, but also reduced abdominal and/or hepatic fat associated with obesity-induced T2DM with drastic alterations in adipokine profiles in treated subjects. Thus, a comprehensive assessment of recent GLP-1-mediated gene therapy approaches with detailed analysis of current hurdles and resolutions, is discussed.

Enhancing gene delivery of adeno-associated viruses by cell-permeable peptides

Adeno-associated virus type 2 (AAV2) is considered a promising gene delivery vector and has been extensively applied in several disease models; however, inefficient transduction in various cells and tissues has limited its widespread application in many areas of gene therapy. In this study, we have developed a general, but efficient, strategy to enhance viral transduction, both in vitro and in vivo, by incubating viral particles with cell-permeable peptides (CPPs). We show that CPPs increase internalization of viral particles into cells by facilitating both energy-independent and energy-dependent endocytosis. Moreover, CPPs can significantly enhance the endosomal escape process of viral particles, thus enhancing viral transduction to those cells that have exhibited very low permissiveness to AAV2 infection as a result of impaired intracellular viral processing. We also demonstrated that this approach could be applicable to other AAV serotypes. Thus, the membrane-penetrating ability of CPPs enables us to generate an efficient method for enhanced gene delivery of AAV vectors, potentially facilitating its applicability to human gene therapy.

Proteomics analysis of co-purifying cellular proteins associated with rAAV vectors

Recombinant adeno-associated vectors (rAAV) are commonly purified by either chromatography or equilibrium CsCl gradient. Nevertheless, even after purification various cellular proteins often associate with rAAV vector capsids. Such co-purifying cellular proteins may raise concern about safety of gene therapy. Here we report identification and characterization of the co-purifying cellular protein in the vector preparations by using a combination of two proteomics approaches, GeLC-MS (gel electrophoresis liquid chromatography-mass spectrometry) and 2DE (two-dimensional gel electrophoresis). Most prominent bands revealed by Coomassie Blue staining were mostly similar to the AAV capsid proteins. Posttranslational modifications of capsid proteins were detected by the proteomics analysis. A total of 13 cellular proteins were identified in the rAAV vectors purified by two rounds of cesium chloride gradient centrifugation, including 9 by the GeLC-MS analysis and 4 by the 2DE analysis. Selected cellular proteins were verified by western blot. Furthermore, the cellular proteins could be consistently found associated with different AAV serotypes and carrying different transgenes. Yet, the proteins were not integral components of the viral capsis since a stringent washing procedure by column purification could remove them. These co-purified proteins in AAV vector preparations may have a role in various stages of the AAV life cycle.

Recombinant adeno-associated virus utilizes host cell nuclear import machinery to enter the nucleus

Recombinant adeno-associated viral (rAAV) vectors have garnered much promise in gene therapy applications. However, widespread clinical use has been limited by transduction efficiency. Previous studies suggested that the majority of rAAV accumulates in the perinuclear region of cells, presumably unable to traffic into the nucleus. rAAV nuclear translocation remains ill-defined; therefore, we performed microscopy, genetic, and biochemical analyses in vitro in order to understand this mechanism. Lectin blockade of the nuclear pore complex (NPC) resulted in inhibition of nuclear rAAV2. Visualization of fluorescently labeled particles revealed that rAAV2 localized to importin-β-dense regions of cells in late trafficking steps. Additionally, small interfering RNA (siRNA) knockdown of importin-β partially inhibited rAAV2 nuclear translocation and inhibited transduction by 50 to 70%. Furthermore, coimmunopreciptation (co-IP) analysis revealed that capsid proteins from rAAV2 could interact with importin-β and that this interaction was sensitive to the small GTPase Ran. More importantly, mutations to key basic regions in the rAAV2 capsid severely inhibited interactions with importin-β. We tested several other serotypes and found that the extent of importin-β interaction varied, suggesting that different serotypes may utilize alternative import proteins for nuclear translocation. Co-IP and siRNA analyses were used to investigate the role of other karyopherins, and the results suggested that rAAV2 may utilize multiple import proteins for nuclear entry. Taken together, our results suggest that rAAV2 interacts with importin-β alone or in complex with other karyopherins and enters the nucleus via the NPC. These results may lend insight into the design of novel AAV vectors that have an enhanced nuclear entry capability and transduction potential.

IMPORTANCE

Use of recombinant adeno-associated viral (rAAV) vectors for gene therapy applications is limited by relatively low transduction efficiency, in part due to cellular barriers that hinder successful subcellular trafficking to the nucleus, where uncoating and subsequent gene expression occur. Nuclear translocation of rAAV has been regarded as a limiting step for successful transduction but it remains ill-defined. We explored potential nuclear entry mechanisms for rAAV2 and found that rAAV2 can utilize the classical nuclear import pathway, involving the nuclear pore complex, the small GTPase Ran, and cellular karyopherins. These results could lend insight into the rational design of novel rAAV vectors that can more efficiently translocate to the nucleus, which may lead to more efficient transduction.

Gene delivery via the hybrid vector of recombinant adeno-associated virus and polyethylenimine

The aim of this study was to investigate the cellular delivery mechanism of the hybrid vector comprising the recombinant adeno-associated virus (rAAV) and polyethylenimine (PEI). The rAAV vector, rAAV-rIns1-hInsM2-ΔEGFP, was fluorescently labeled with Cy3, a cyanine dye, and complexed with PEI. The interaction of the hybrid vector with the Huh7 hepatoma cells was monitored by confocal microscopy. Complexation of rAAV with PEI enhanced the transduction efficiency, which was decreased by pretreatment of the cells with sodium chlorate, an inhibitor of glycosaminoglycan sulfation, suggesting the roles of heparan sulfate proteoglycans (HSPG) in the uptake of the hybrid vector by the cells. Examination by flow cytometry and confocal microscopy demonstrated an enhanced interaction between the cells and the virus when complexed with PEI. Pretreatment with wortmannin or cytochalasin B significantly reduced the virus uptake by the cells, suggesting the involvement of phosphatidylinositol 3-kinase (PI3K) signaling and phagocytosis in the interaction between the cells and the hybrid vectors. Treatment of cells with the antioxidants, including l-ascorbic acid, δ-tocotrienol, or N-acetylcysteine (NAC), impaired the rAAV-PEI-mediated transduction. Results obtained in this study illustrated the involvement of PI3K/Akt signaling and the ROS production in gene delivery via the rAAV-PEI hybrid vector.

Long-distance axonal transport of AAV9 is driven by dynein and kinesin-2 and is trafficked in a highly motile Rab7-positive compartment

Adeno-associated virus (AAV) vectors can move along axonal pathways after brain injection, resulting in transduction of distal brain regions. This can enhance the spread of therapeutic gene transfer and improve treatment of neurogenetic disorders that require global correction. To better understand the underlying cellular mechanisms that drive AAV trafficking in neurons, we investigated the axonal transport of dye-conjugated AAV9, utilizing microfluidic primary neuron cultures that isolate cell bodies from axon termini and permit independent analysis of retrograde and anterograde axonal transport. After entry, AAV was trafficked into nonmotile early and recycling endosomes, exocytic vesicles, and a retrograde-directed late endosome/lysosome compartment. Rab7-positive late endosomes/lysosomes that contained AAV were highly motile, exhibiting faster retrograde velocities and less pausing than Rab7-positive endosomes without virus. Inhibitor experiments indicated that the retrograde transport of AAV within these endosomes is driven by cytoplasmic dynein and requires Rab7 function, whereas anterograde transport of AAV is driven by kinesin-2 and exhibits unusually rapid velocities. Furthermore, increasing AAV9 uptake by neuraminidase treatment significantly enhanced virus transport in both directions. These findings provide novel insights into AAV trafficking within neurons, which should enhance progress toward the utilization of AAV for improved distribution of transgene delivery within the brain.

Ex vivo intracoronary gene transfer of adeno-associated virus 2 leads to superior transduction over serotypes 8 and 9 in rat heart transplants

Heart transplant gene therapy requires vectors with long-lasting gene expression, high cardiotropism, and minimal pathological effects. Here, we examined transduction properties of ex vivo intracoronary delivery of adeno-associated virus (AAV) serotype 2, 8, and 9 in rat syngenic and allogenic heart transplants. Adult Dark Agouti (DA) rat hearts were intracoronarily perfused ex vivo with AAV2, AAV8, or AAV9 encoding firefly luciferase and transplanted heterotopically into the abdomen of syngenic DA or allogenic Wistar-Furth (WF) recipients. Serial in vivo bioluminescent imaging of syngraft and allograft recipients was performed for 6 months and 4 weeks, respectively. Grafts were removed for PCR-, RT-PCR, and luminometer analysis. In vivo bioluminescent imaging of recipients showed that AAV9 induced a prominent and stable luciferase activity in the abdomen, when compared with AAV2 and AAV8. However, ex vivo analyses revealed that intracoronary perfusion with AAV2 resulted in the highest heart transplant transduction levels in syngrafts and allografts. Ex vivo intracoronary delivery of AAV2 resulted in efficient transgene expression in heart transplants, whereas intracoronary AAV9 escapes into adjacent tissues. In terms of cardiac transduction, these results suggest AAV2 as a potential vector for gene therapy in preclinical heart transplants studies, and highlight the importance of delivery route in gene transfer studies.

Roles of rho GTPases in intracellular transport and cellular transformation

Rho family GTPases belong to the Ras GTPase superfamily and transduce intracellular signals known to regulate a variety of cellular processes, including cell polarity, morphogenesis, migration, apoptosis, vesicle trafficking, viral transport and cellular transformation. The three best-characterized Rho family members are Cdc42, RhoA and Rac1. Cdc42 regulates endocytosis, the transport between the endoplasmic reticulum and Golgi apparatus, post-Golgi transport and exocytosis. Cdc42 influences trafficking through interaction with Wiskott-Aldrich syndrome protein (N-WASP) and the Arp2/3 complex, leading to changes in actin dynamics. Rac1 mediates endocytic and exocytic vesicle trafficking by interaction with its effectors, PI3kinase, synaptojanin 2, IQGAP1 and phospholipase D1. RhoA participates in the regulation of endocytosis through controlling its downstream target, Rho kinase. Interestingly, these GTPases play important roles at different stages of viral protein and genome transport in infected host cells. Importantly, dysregulation of Cdc42, Rac1 and RhoA leads to numerous disorders, including malignant transformation. In some cases, hyperactivation of Rho GTPases is required for cellular transformation. In this article, we review a number of findings related to Rho GTPase function in intracellular transport and cellular transformation.

Structure and dynamics of adeno-associated virus serotype 1 VP1-unique N-terminal domain and its role in capsid trafficking

The importance of the phospholipase A2 domain located within the unique N terminus of the capsid viral protein VP1 (VP1u) in parvovirus infection has been reported. This study used computational methods to characterize the VP1 sequence for adeno-associated virus (AAV) serotypes 1 to 12 and circular dichroism and electron microscopy to monitor conformational changes in the AAV1 capsid induced by temperature and the pHs encountered during trafficking through the endocytic pathway. Circular dichroism was also used to monitor conformational changes in AAV6 capsids assembled from VP2 and VP3 or VP1, VP2, and VP3 at pH 7.5. VP1u was predicted (computationally) and confirmed (in solution) to be structurally ordered. This VP domain was observed to undergo a reversible pH-induced unfolding/refolding process, a loss/gain of α-helical structure, which did not disrupt the capsid integrity and is likely facilitated by its difference in isoelectric point compared to the other VP sequences assembling the capsid. This study is the first to physically document conformational changes in the VP1u region that likely facilitate its externalization from the capsid interior during infection and establishes the order of events in the escape of the AAV capsid from the endosome en route to the nucleus.

Arsenic trioxide stabilizes accumulations of adeno-associated virus virions at the perinuclear region, increasing transduction in vitro and in vivo

Interactions with cellular stress pathways are central to the life cycle of many latent viruses. Here, we utilize adeno-associated virus (AAV) as a model to study these interactions, as previous studies have demonstrated that cellular stressors frequently increase transduction of recombinant AAV (rAAV) vectors and may even substitute for helper virus functions. Since several chemotherapeutic drugs are known to increase rAAV transduction, we investigated the effect of arsenic trioxide (As(2)O(3)), an FDA-approved chemotherapeutic agent with known effects on several other virus life cycles, on the transduction of rAAV. In vitro, As(2)O(3) caused a dose-dependent increase in rAAV2 transduction over a broad range of cell lines from various cell types and species (e.g., HEK-293, HeLa, HFF hTERT, C-12, and Cos-1). Mechanistically, As(2)O(3) treatment acted to prevent loss of virions from the perinuclear region, which correlated with increased cellular vector genome retention, and was distinguishable from proteasome inhibition. To extend our investigation of the cellular mechanism, we inhibited reactive oxygen species formation and determined that the As(2)O(3)-mediated increase in rAAV2 transduction was dependent upon production of reactive oxygen species. To further validate our in vitro data, we tested the effect of As(2)O(3) on rAAV transduction in vivo and determined that treatment initiated transgene expression as early as 2 days posttransduction and increased reporter expression by up to 10-fold. Moreover, the transduction of several other serotypes of rAAV was also enhanced in vivo, suggesting that As(2)O(3) affects a pathway used by several AAV serotypes. In summary, our data support a model wherein As(2)O(3) increases rAAV transduction both in vitro and in vivo and maintains perinuclear accumulations of capsids, facilitating productive nuclear trafficking.

Raft endocytosis of autocrine motility factor regulates mitochondrial dynamics via rac1 signaling and the gp78 ubiquitin ligase

Gp78 is a cell surface receptor that also functions as an E3 ubiquitin ligase in the endoplasmic reticulum-associated degradation (ERAD) pathway. The Gp78 ligand, the glycolytic enzyme phosphoglucose isomerase (also called autocrine motility factor or AMF), functions as a cytokine upon secretion by tumor cells. AMF is internalized via a PI3K- and dynamin-dependent raft endocytic pathway to the smooth endoplasmic reticulum (ER), however the relationship between AMF and Gp78 ubiquitin ligase activity remains unclear. AMF uptake to the smooth ER is inhibited by the dynamin inhibitor, dynasore, reduced in Gp78 knockdown cells and induces the dynamin-dependent downregulation of its cell surface receptor. AMF uptake is Rac1-dependent, inhibited by expression of dominant-negative Rac1 and the Rac1 inhibitor NSC23766, and therefore distinct from Cdc42 and RhoA-dependent raft endocytic pathways. AMF stimulates Rac1 activation, that is reduced by dynasore treatment and absent in Gp78-knockdown cells and therefore requires Gp78-mediated endocytosis. AMF also prevents Gp78-induced degradation of the mitochondrial fusion proteins, Mitofusin 1 and 2 in a dynamin, Rac1 and PI3K-dependent manner. Gp78 induces mitochondrial clustering and fission in a ubiquitin ligase-dependent manner that is also reversed by AMF. The raft-dependent endocytosis of AMF therefore promotes Rac1/PI3K signaling that feeds back to promote AMF endocytosis and also inhibits the ability of Gp78 to target the mitofusins for degradation, thereby preventing Gp78-dependent mitochondrial fission. Through regulation of an ER-localized ubiquitin ligase, the raft-dependent endocytosis of AMF represents an extracellular regulator of mitochondrial fusion and dynamics.

Insulin gene therapy from design to beta cell generation

Despite the fact that insulin injection can protect diabetic patients from developing diabetes-related complications, recent meta-analyses indicate that rapid and long-acting insulin analogues only provide a limited benefit compared with conventional insulin regarding glycemic control. As insulin deficiency is the main sequel of type-1 diabetes (T1D), transfer of the insulin gene-by-gene therapy is becoming an attractive treatment modality even though T1D is not caused by a single genetic defect. In contrast to human insulin and insulin analogues, insulin gene therapy targets to supplement patients not only with insulin but also with C-peptide. So far, insulin gene therapy has had limited success because of delayed and/or transient gene expression. Sustained insulin gene expression is now feasible using current gene-therapy vectors providing patients with basal insulin coverage, but management of postprandial hyperglycaemia is still difficult to accomplish because of the inability to properly control insulin secretion. Enteroendocrine cells of the gastrointestinal track (K cells and L cells) may be ideal targets for insulin gene therapy, but cell-targeting difficulties have limited practical implementation of insulin gene therapy for diabetes treatment. Therefore, recent gene transfer technologies developed to generate authentic beta cells through transdifferentiation are also highlighted in this review.

Cytoplasmic trafficking, endosomal escape, and perinuclear accumulation of adeno-associated virus type 2 particles are facilitated by microtubule network

Understanding adeno-associated virus (AAV) trafficking is critical to advance our knowledge of AAV biology and exploit novel aspects of vector development. Similar to the case for most DNA viruses, after receptor binding and entry, AAV traverses the cytoplasm and deposits the viral genome in the cell nucleus. In this study, we examined the role of the microtubule (MT) network in productive AAV infection. Using pharmacological reagents (e.g., nocodazole), live-cell imaging, and flow cytometry analysis, we demonstrated that AAV type 2 (AAV2) transduction was reduced by at least 2-fold in the absence of the MT network. Cell surface attachment and viral internalization were not dependent on an intact MT network. In treated cells at 2 h postinfection, quantitative three-dimensional (3D) microscopy determined a reproducible difference in number of intracellular particles associated with the nuclear membrane or the nucleus compared to that for controls (6 to 7% versus 26 to 30%, respectively). Confocal microscopy analysis demonstrated a direct association of virions with MTs, further supporting a critical role in AAV infection. To investigate the underling mechanisms, we employed single-particle tracking (SPT) to monitor the viral movement in real time. Surprisingly, unlike other DNA viruses (e.g., adenovirus [Ad] and herpes simplex virus [HSV]) that display bidirectional motion on MTs, AAV2 displays only unidirectional movement on MTs toward the nuclei, with peak instantaneous velocities at 1.5 to 3.5 μm/s. This rapid and unidirectional motion on MTs lasts for about 5 to 10 s and results in AAV particles migrating more than 10 μm in the cytoplasm reaching the nucleus very efficiently. Furthermore, electron microscopy analysis determined that, unlike Ad and HSV, AAV2 particles were transported on MTs within membranous compartments, and surprisingly, the acidification of AAV2-containing endosomes was delayed by the disruption of MTs. These findings together suggest an as-yet-undescribed model in which after internalization, AAV2 exploits MTs for rapid cytoplasmic trafficking in endosomal compartments unidirectionally toward the perinuclear region, where most acidification events for viral escape take place.

Dynamin2 S-nitrosylation regulates adenovirus type 5 infection of epithelial cells

Dynamin2 is a large GTPase that regulates vesicle trafficking, and the GTPase activity of dynamin2 is required for the multistep process of adenovirus infection. Activity of dynamin2 may be regulated by post-translational phosphorylation and S-nitrosylation modifications. In this study, we demonstrate a role for dynamin2 S-nitrosylation in adenovirus infection of epithelial cells. We show that adenovirus serotype 5 (Ad5) infection augments production of nitric oxide (NO) in epithelial cells and causes the S-nitrosylation of dynamin2, mainly on cysteine 86 (C86) and 607 (C607) residues. Forced overexpression of dynamin2 bearing C86A and/or C607A mutations decreases Ad5 infection. Diminishing NO synthesis by RNAi-induced knockdown of endogenous endothelial NO synthase (eNOS) expression attenuates virus infection of target cells. Ad5 infection promotes the kinetically dynamic S-nitrosylation of dynamin2 and eNOS: there is a rapid decrease in eNOS S-nitrosylation and a concomitant increase in the dynamin2 S-nitrosylation. These results support the hypothesis that dynamin2 S-nitrosylation following eNOS activation facilitates adenovirus infection of host epithelial cells.

Impact of VP1-specific protein sequence motifs on adeno-associated virus type 2 intracellular trafficking and nuclear entry

Adeno-associated virus type 2 (AAV2) has gained much interest as a gene delivery vector. A hallmark of AAV2-mediated gene transfer is an intracellular conformational change of the virus capsid, leading to the exposure of infection-relevant protein domains. These protein domains, which are located on the N-terminal portion of the structural proteins VP1 and VP2, include a catalytic phospholipase A(2) domain and three clusters of basic amino acids. We have identified additional protein sequence motifs located on the VP1/2 N terminus that also proved to be obligatory for virus infectivity. These motifs include signals that are known to be involved in protein interaction, endosomal sorting and signal transduction in eukaryotic cells. Among different AAV serotypes they are highly conserved and mutation of critical amino acids of the respective motifs led to a severe infection-deficient phenotype. In particular, mutation of a YXXQ-sequence motif significantly reduced accumulation of virus capsids around the nucleus in comparison to wild-type AAV2. Interestingly, intracellular trafficking of AAV2 was shown to be independent of PLA(2) activity. Moreover, mutation of three PDZ-binding motifs, which are located consecutively at the very tip of the VP1 N terminus, revealed a nuclear transport-defective phenotype, suggesting a role in nuclear uptake of the virus through an as-yet-unknown mechanism.

Intracellular transport of recombinant adeno-associated virus vectors

Recombinant adeno-associated viral vectors (rAAVs) have been widely used for gene delivery in animal models, and are currently evaluated for human gene therapy after successful clinical trials in the treatment of inherited, degenerative or acquired diseases, such as Leber congenital amaurosis, Parkinson disease or heart failure. However, limitations in vector tropism, such as limited tissue specificity and insufficient transduction efficiencies of particular tissues and cell types, still preclude therapeutic applications in certain tissues. Wild-type adeno-associated viruses (AAVs) are defective viruses that require the presence of a helper virus to complete their life cycle. On the one hand, this unique property makes AAV vectors one of the safest available viral vectors for gene delivery. On the other, it also represents a potential obstacle because rAAV vectors have to overcome several biological barriers in the absence of a helper virus to transduce successfully a cell. Consequently, a better understanding of the cellular roadblocks that limit rAAV gene delivery is crucial and, during the last 15 years, numerous studies resulted in an expanding body of knowledge of the intracellular trafficking pathways of rAAV vectors. This review describes our current understanding of the mechanisms involved in rAAV attachment to target cells, endocytosis, intracellular trafficking, capsid processing, nuclear import and genome release with an emphasis on the most recent discoveries in the field and the emerging strategies used to improve the efficiency of AAV-derived vectors.

Adeno-associated virus 2 infection requires endocytosis through the CLIC/GEEC pathway

Adeno-associated viruses (AAVs) are nonpathogenic, nonenveloped, single-stranded DNA viruses in development as gene therapy vectors. AAV internalization was postulated to proceed via a dynamin-dependent endocytic mechanism. Revisiting this, we find that infectious endocytosis of the prototypical AAV, AAV2, is independent of clathrin, caveolin, and dynamin. AAV2 infection is sensitive to EIPA, a fluid-phase uptake inhibitor, but is unaffected by Rac1 mutants or other macropinocytosis inhibitors. In contrast, AAV2 infection requires actin cytoskeleton remodeling and membrane cholesterol and is sensitive to inhibition of Cdc42, Arf1, and GRAF1, factors known to be involved in the formation of clathrin-independent carriers (CLIC). AAV2 virions are internalized in the detergent-resistant GPI-anchored-protein-enriched endosomal compartment (GEEC) and translocated to the Golgi apparatus, similarly to the CLIC/GEEC marker cholera toxin B. Our results indicate that-unlike the viral entry mechanisms described so far-AAV2 uses the pleiomorphic CLIC/GEEC pathway as its major endocytic infection route.

Endocytic processing of adeno-associated virus type 8 vectors for transduction of target cells

We investigated the transduction of HEK293T cells permissive to adeno-associated virus serotype 8 (AAV8) to understand the mechanisms underlying its endocytic processing. Results showed that AAV8 enters cells through clathrin-mediated endocytosis followed by trafficking through various endosomal compartments. Interestingly, compared to the relatively well-characterized AAV2, a distinct involvement of late endosomes was observed for AAV8 trafficking within the target cell. AAV8 particles were also shown to exploit the cytoskeleton network to facilitate their transport within cells. Moreover, the cellular factors involved during endosomal escape were examined by an in vitro membrane permeabilization assay. Our data demonstrated that an acidic endosomal environment was required for AAV2 penetration through endosomal membranes and that the cellular endoprotease furin could promote AAV2 escape from the early endosomes. In contrast, these factors were not sufficient for AAV8 penetration through endosomal membranes. We further found that the ubiquitin-proteasome system is likely involved in the intracellular transport of AAV8 to nucleus. Taken together, our data have shed some light on the intracellular trafficking pathways of AAV8, which, in turn, could provide insight for potentializing AAV-mediated gene delivery.

Modification and labeling of AAV vector particles

Adeno-associated virus (AAV) has become a versatile vector platform. In recent years, powerful -techniques for the generation of tropism-modified vectors (rAAV-targeting vectors) and for investigation of virus-cell interaction were developed. The following chapter describes strategies for insertion of peptide ligands into the viral capsid and the subsequent characterization of capsid mutants, for producing mosaic capsids and for labeling the viral capsid chemically or genetically.

Parvoviruses: structure and infection

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

Directing integrin-linked endocytosis of recombinant AAV enhances productive FAK-dependent transduction

Recombinant adeno-associated virus (rAAV) is a widely used gene therapy vector. Although a wide range of rAAV serotypes can effectively enter most cell types, their transduction efficiencies (i.e., transgene expression) can vary widely depending on the target cell type. Integrins play important roles as coreceptors for rAAV infection, however, it remains unclear how integrin-dependent and -independent mechanisms of rAAV endocytosis influence the efficiency of intracellular virus processing and ultimately transgene expression. In this study, we examined the contribution of integrin-mediated endocytosis to transduction of fibroblasts by rAAV2. Mn(++)-induced integrin activation significantly enhanced (~17-fold) the efficiency of rAAV2 transduction, without altering viral binding or endocytosis. rAAV2 subcellular localization studies demonstrated that Mn(++) promotes increased clustering of rAAV2 on integrins and recruitment of intracellular vinculin (an integrin effector) to sites of rAAV2 binding at the cell surface. Focal adhesion kinase (FAK), a downstream effector of integrin signals, was essential for rAAV2/integrin complex internalization and transduction. These findings support a model whereby integrin activation at the cell surface can redirect rAAV2 toward a FAK-dependent entry pathway that is more productive for cellular transduction. This pathway appears to be conserved for other rAAV serotypes that contain a capsid integrin-binding domain (AAV1 and AAV6).

AAV capsid structure and cell interactions

The Adeno-associated viruses (AAVs) are not associated with any diseases, and their ability to package non-genomic DNA and to transduce different cell/tissue populations has generated significant interest in understanding their basic biology in efforts to improve their utilization for corrective gene delivery. This includes their capsid structure, cellular tropism and interactions for entry, uncoating, replication, DNA packaging, capsid assembly, and antibody neutralization. The human and nonhuman primate AAVs are clustered into serologically distinct genetic clade and serotype groups, which have distinct cellular/tissue tropisms and transduction efficiencies. These properties are highly dependent upon the AAV capsid amino acid sequence, their capsid structure, and their interactions with host cell factors, including cell surface receptors, co-receptors, signaling molecules, proteins involved in host DNA replication, and host-derived antibodies. This chapter reviews the current structural information on AAV capsids and the capsid viral protein regions playing a role in the cellular interactions conferring an infective phenotype, which are then used to annotate the functional regions of the capsid. Based on the current data, the indication is that the AAVs, like other members of the Parvoviridae and other ssDNA viruses that form a T = 1 capsid, have evolved a multifunctional capsid with conserved core regions as is required for efficient capsid trafficking, capsid assembly, and genome packaging. Disparate surface loop structures confer differential receptor recognition and are involved in antibody recognition. The role of structural regions in capsid uncoating remains to be elucidated.

Preclinical study design for rAAV

The process of moving a novel drug such as an adeno-associated viral vector from the bench top to bedside is an arduous process requiring coordination and skill from multiple laboratories and regulatory agencies. Proceeding to a phase I safety trial in humans after most of the proof-of-concept data have been acquired may take several years. During this time, agencies including the FDA, NIH Office of Biotechnology Activities (OBA), and Recombinant DNA Advisory Committee (RAC) along with the investigator's team will develop a series of preclinical toxicology and biodistribution studies in order to develop a safety profile for the intended novel drug. In this chapter, key features of the pharm-tox study design and conduct will be discussed. Highlighted features include choosing a sufficient animal number and species to use in testing, dose determination, typical toxicological assays performed, the use of Standard Operating Procedures in respect to good laboratory practices compliancy, and role of the Quality Assurance Unit.

Development of optimized AAV3 serotype vectors: mechanism of high-efficiency transduction of human liver cancer cells

Our recent studies have revealed that among the 10 different commonly used adeno-associated virus (AAV) serotypes, AAV3 vectors transduce human liver cancer cells extremely efficiently because these cells express high levels of human hepatocyte growth factor receptor (hHGFR), and AAV3 utilizes hHGFR as a cellular co-receptor for viral entry. In this report, we provide further evidence that both extracellular as well as intracellular kinase domains of hHGFR are involved in AAV3 vector entry and AAV3-mediated transgene expression. We also document that AAV3 vectors are targeted for degradation by the host cell proteasome machinery, and that site-directed mutagenesis of surface-exposed tyrosine (Y) to phenylalanine (F) residues on AAV3 capsids significantly improves the transduction efficiency of Y701F, Y705F and Y731F mutant AAV3 vectors. The transduction efficiency of the Y705+731F double-mutant vector is significantly higher than each of the single mutants in liver cancer cells in vitro. In immunodeficient mouse xenograft models, direct intratumoral injection of AAV3 vectors also led to high-efficiency transduction of human liver tumor cells in vivo. We also document here that the optimized tyrosine-mutant AAV3 vectors lead to increased transduction efficiency following both intratumoral and tail-vein injections in vivo. The optimized tyrosine-mutant AAV3 serotype vectors containing proapoptotic genes should prove useful for the potential gene therapy of human liver cancers.

Unique characteristics of AAV1, 2, and 5 viral entry, intracellular trafficking, and nuclear import define transduction efficiency in HeLa cells

Biological differences between recombinant adeno-associated virus (rAAV) serotypes define their efficiencies in expressing a transgene in a particular target cell. Few studies have directly compared how differences in viral entry, intracellular trafficking, and nuclear import of rAAV serotypes influence the effectiveness of transduction in the same cell type. We evaluated these characteristics for three rAAV serotypes in HeLa cells, using biochemical techniques and fluorescence-based detection of multiple serotypes in the same cell. Although rAAV2 exhibited the slowest entry, intracellular trafficking, and nuclear import among the three serotypes, it elicited the highest levels of transduction. Conversely, rAAV1 exhibited more rapid entry and nuclear import than the other serotypes, yet was ineffective at transducing HeLa cells due to impaired capsid disassembly in the nucleus. rAAV5, which entered the cell less rapidly than rAAV1, was imported efficiently into the nucleus, but then rapidly degraded, resulting in poor transduction of HeLa cells. We conclude that rAAV1, 2, and 5 utilize distinct mechanisms for intracellular trafficking, and that post-nuclear events play an important role in determining the efficiency of HeLa cell transduction by these serotypes. Thus, overcoming post-nuclear barriers that limit uncoating and/or promote virion degradation may enhance the efficiency of certain AAV serotypes.

Enhancers of adeno-associated virus AAV2 transduction via high throughput siRNA screening

Intracellular barriers to adeno-associated virus (AAV) transduction may limit gene delivery. We screened a short interfering RNA (siRNA) library targeting 5,520 genes to help identify pathways that modulate AAV transduction of human endothelium. In replicate screening, 50 pools (three siRNAs per gene) resulted in greater than eightfold reporter gene expression enhancement. Single siRNA confirmation tests demonstrated that at least one siRNA from each of the top 10 pools provided greater than twofold enhancement. Several siRNAs when used together resulted in additive effects and two of the most potent siRNA sequences were enhancers in cultured airway epithelium. However, enhanced transduction was not correlated with mRNA knockdown by quantitative real time PCR, indicating an off-target mechanism. In fact, four of the five most potent siRNAs contained a consensus hexamer region 5'-UGUUUC-3' at positions 2-7 of the antisense strand. The point mutation U4A within this region (but not mutations at positions 1 or 14) disrupted transduction enhancement, indicating a microRNA (miRNA)-like mechanism. Transcription profiling indicated that the hexamer also resulted in perturbation of the interferon pathway via reduced interferon-induced protein 44-like (IFI44L), interferon-inducible myxovirus resistance 1 (MX1), and interferon-induced protein with tetratricopeptide repeats (IFIT5) mRNAs. Direct interferon (α, β, and ω) receptor 2 (IFNAR2) knockdown resulted in greater than twofold transduction enhancement. In addition to providing insight into AAV biology and enhanced transduction, the results demonstrate certain beneficial siRNA off-target effects.

Enhanced real-time monitoring of adeno-associated virus trafficking by virus-quantum dot conjugates

The unique spectral properties of semiconductor quantum dots (QDs) enable long-term live-cell imaging and ultrasensitive detection of viral particles, which in turn can potentially provide a practical means for detailed analysis of the underlying molecular mechanisms of virus entry. In this study, we report a general method of labeling adeno-associated virus serotype 2 (AAV2) with QDs for enhanced visualization of the intracellular behavior of viruses in living target cells. It was found that the mild conditions required for this QD conjugation reaction allowed for the retention of viral infectivity of AAV2. Furthermore, quantitative analysis of viral motility in living cells suggested that QD-labeling had no significant effect on the intracellular transport properties of AAV2 particles compared to those of conventional organic dye-labeled AAV2. Our imaging study demonstrated that QD-AAV2 was internalized mainly through a clathrin-dependent pathway and then trafficked through various endosomes. It was also observed that QD-AAV2 particles exploit the cytoskeleton network to facilitate their transport within cells, and the labeling study provided evidence that the ubiquitin-proteasome system was likely involved in the intracellular trafficking of AAV2, at least at the level of nuclear transport. Taken together, our findings reveal the potential of this QD-labeling method for monitoring the intracellular dynamics of virus-host cell interactions and interrogating the molecular mechanisms of viral infection in greater detail.

Interactions between AAV-2 and HSV-1: implications for hybrid vector design

Herpes simplex virus type 1 (HSV-1)-based amplicon vectors have a transgene capacity of up to 150 kbp and can efficiently transduce many different cell types in culture and in vivo without causing cytopathic effects. However, these vectors do not support long-term transgene expression. Adeno-associated virus type 2 (AAV-2) has the capacity to integrate its genome into a specific site on human chromosome 19, but AAV-2-derived gene therapy vectors have a transgene capacity of only 4.5 kb. To combine the large transgene capacity of HSV-1 with the potential for site-specific genomic integration and long-term transgene expression of AAV-2, HSV/AAV hybrid vectors have been developed. This review describes the design, applications and limitations of these hybrid vectors. However, as HSV-1 is a full helper virus for AAV-2 replication, the main focus is the analysis of the molecular mechanisms of interaction between the two viruses. The knowledge of these interactions will have direct implications on the design of novel HSV/AAV hybrid vectors.

Intrinsic phospholipase A2 activity of adeno-associated virus is involved in endosomal escape of incoming particles

The unique region of the VP1 capsid protein of adeno-associated viruses (AAV) in common with autonomously replicating parvoviruses comprises a secreted phospholipase A2 (sPLA2) homology domain. While the sPLA2 domain of Minute Virus of Mice has recently been shown to mediate endosomal escape by lipolytic pore formation, experimental evidence for a similar function in AAV infection is still lacking. Here, we explored the function of the sPLA2 domain of AAV by making use of the serotype 2 mutant (76)HD/AN. The sPLA2 defect in (76)HD/AN, which severely impairs AAV's infectivity, could be complemented in trans by co-infection with wild-type AAV2. Furthermore, co-infection with endosomolytically active, but not with inactive adenoviral variants partially rescued (76)HD/AN, providing the first evidence for a function of this domain in endosomal escape of incoming AAV particles.

A gammaherpesvirus complement regulatory protein promotes initiation of infection by activation of protein kinase Akt/PKB

Background

Viruses have evolved to evade the host's complement system. The open reading frames 4 (ORF4) of gammaherpesviruses encode homologs of regulators of complement activation (RCA) proteins, which inhibit complement activation at the level of C3 and C4 deposition. Besides complement regulation, these proteins are involved in heparan sulfate and glycosaminoglycan binding, and in case of MHV-68, also in viral DNA synthesis in macrophages.

Methodology/Principal Findings

Here, we made use of MHV-68 to study the role of ORF4 during infection of fibroblasts. While attachment and penetration of virions lacking the RCA protein were not affected, we observed a delayed delivery of the viral genome to the nucleus of infected cells. Analysis of the phosphorylation status of a variety of kinases revealed a significant reduction in phosphorylation of the protein kinase Akt in cells infected with ORF4 mutant virus, when compared to cells infected with wt virus. Consistent with a role of Akt activation in initial stages of infection, inhibition of Akt signaling in wt virus infected cells resulted in a phenotype resembling the phenotype of the ORF4 mutant virus, and activation of Akt by addition of insulin partially reversed the phenotype of the ORF4 mutant virus. Importantly, the homologous ORF4 of KSHV was able to rescue the phenotype of the MHV-68 ORF4 mutant, indicating that ORF4 is functionally conserved and that ORF4 of KSHV might have a similar function in infection initiation.

Conclusions/Significance

In summary, our studies demonstrate that ORF4 contributes to efficient infection by activation of the protein kinase Akt and thus reveal a novel function of a gammaherpesvirus RCA protein.

Mutagenesis of adeno-associated virus type 2 capsid protein VP1 uncovers new roles for basic amino acids in trafficking and cell-specific transduction

The N termini of the capsid proteins VP1 and VP2 of adeno-associated virus (AAV) play important roles in subcellular steps of infection and contain motifs that are highly homologous to a phospholipase A(2) (PLA(2)) domain and nuclear localization signals (NLSs). To more clearly understand how virion components influence infection, we have generated mutations in these regions and examined their effects on subcellular trafficking, capsid stability, transduction, and sensitivity to pharmacological enhancement. All mutants tested assembled into capsids; retained the correct ratio of VP1, VP2, and VP3; packaged DNA similarly to recombinant AAV2 (rAAV2); and displayed similar stability profiles when heat denatured. Confocal microscopy demonstrated that these mutants trafficked through a perinuclear region in the vicinity of the Golgi apparatus, with a subset of mutants displaying more-diffuse localization consistent with an NLS-deficient phenotype. When tested for viral transduction, two mutant classes emerged. Class I (BR1(-), BR2(-), and BR2+K) displayed partial transduction, whereas class II (VP3 only, (75)HD/AN, BR3(-), and BR3+K) were severely defective. Surprisingly, one class II mutant (BR3+K) trafficked identically to rAAV2 and accumulated in the nucleolus, a step recently described by our laboratory that occurs with wild-type infection. The BR3+K mutant, containing an alanine-to-lysine substitution in the third basic region of VP1, was 10- to 100-fold-less infectious than rAAV2 in transformed cell lines (such as HEK-293, HeLa, and CV1-T cells), but in contrast, it was indistinguishable from rAAV2 in several nontransformed cell lines, as well as in tissues (liver, brain, and muscle) in vivo. Complementation studies with pharmacological adjuvants or adenovirus coinfection suggested that additional positive charges in NLS regions restrict mobilization in the nucleus and limit transduction in a transformed-cell-specific fashion. Remarkably, besides displaying cell-type-specific transduction, this is the first description of a capsid mutant indicating that nuclear entry is not sufficient for AAV-mediated transduction and suggests that additional steps (i.e., subnuclear mobilization or uncoating) limit successful AAV infection.

Optimized adeno-associated virus (AAV)-protein phosphatase-5 helper viruses for efficient liver transduction by single-stranded AAV vectors: therapeutic expression of factor IX at reduced vector doses

Abstract Our studies have shown that coinjection of conventional single-stranded adeno-associated virus 2 (ssAAV2) vectors carrying the enhanced green fluorescent protein (EGFP) gene with self-complementary (sc) AAV2-T cell protein tyrosine phosphatase (TC-PTP) and scAAV2-protein phosphatase-5 (PP5) vectors resulted in an approximately 16-fold increase in EGFP expression in primary murine hepatocytes in vivo [Jayandharan, G.R., Zhong, L., Li, B., Kachniarz, B., and Srivastava, A. (2008). Gene Ther. 15, 1287-1293]. In the present studies, this strategy was further optimized to achieve transgene expression at reduced vector/helper virus doses. These included the use of scAAV helper viruses containing (1) hepatocyte-specific promoters, (2) tyrosine-mutant AAV2 capsids, and (3) additional AAV serotype vectors known to efficiently transduce hepatocytes. The hepatocyte-specific transthyretin (TTR) promoter was approximately 6- to 7-fold more efficient than the Rous sarcoma virus (RSV) promoter; tyrosine-mutant AAV2 capsids were approximately 6- to 11-fold more efficient than the wild-type AAV2 capsids; and the AAV8 serotype helper virus was approximately 16-fold more efficient than AAV2 serotype helper virus. With these modifications, the vector dose of the helper virus could be further reduced by approximately 50-fold. Last, coadministration of scAAV8-PP5 helper virus increased coagulation factor IX expression from an ssAAV2 vector by approximately 7- to 10-fold, thereby achieving therapeutic levels at lower vector doses. No adverse effect on hepatocytes was observed under any of these experimental conditions. The strategy presented here should be adaptable to any ssAAV transgene cassette and, specifically, liver-directed applications of ssAAV2 vectors containing larger genes that cannot be encapsidated in scAAV vectors.

Gene transfer into human cord blood-derived CD34(+) cells by adeno-associated viral vectors

OBJECTIVE

Bone marrow-derived CD34(+) cells are currently used in clinical trials in patients with ischemic heart disease. An option to enhance activity of injected progenitors may be offered by genetic engineering of progenitor cells with angiogenic growth factors. Recombinant adeno-associated viral vectors (rAAV) have emerged as a leading gene transfer systems. In contrast to other vector systems in use for genetic engineering of CD34(+) cells, rAAV-mediated gene expression does not depend on vector integration. This is relevant for application in regenerative medicine of ischemic tissues, where transient transgene expression is likely sufficient to achieve therapeutic benefits.

MATERIALS AND METHODS

We compared three different human AAV serotypes, packaged as pseudotypes by a helper virus-free production method, for their transduction efficiency in human cord blood-derived CD34(+) cells. We further assessed the impact of vector genome conformation, of alpha(v)beta(5) and alpha(5)beta(1) integrin availability and of the transcription-modulating drugs retinoic acid and Trichostatin A on rAAV-mediated human CD34(+) cell transduction.

RESULTS

We provide, for the first time, evidence that hCD34(+) cells can be reproducibly transduced with high efficiency by self-complementary rAAV2 without inducing cytotoxicity or interfering with their differentiation potential. We further show the involvement of alpha(5)beta(1) integrin as a crucial AAV2 internalization receptor and a function for transcription-modulating drugs in enhancing rAAV-mediated transgene expression.

CONCLUSION

This study represents a first step toward translation of a combined cellular/rAAV-based therapy of ischemic disease.

Pharmacological activation of guanine nucleotide exchange factors for the small GTPase Rap1 recruits high-affinity beta1 integrins as coreceptors for parvovirus B19: improved ex vivo gene transfer to human erythroid progenitor cells

Parvovirus B19 has potential as a gene therapy vector because of its restricted tropism for human erythroid progenitor cells in the bone marrow. B19 binds to the cell surface through P antigen and we identified activated beta(1) integrins as coreceptors for internalization. Because differentiation with phorbol ester induces beta(1) integrin coreceptor activity, but cell differentiation is not desirable in gene transfer to human progenitor cells and one of the downstream effectors of phorbol esters is the small GTPase Rap1, the role of Rap1 in the recruitment of beta(1) integrins on hematopoietic cells was examined. Expression of a constitutively active Rap1 (63E) was sufficient to recruit beta(1) integrin coreceptors in erythroleukemic K562 cells by inducing high-affinity integrin conformation. A crucial role of actin polymerization in Rap1-mediated beta(1) integrin recruitment was documented by complete inhibition of the 63E Rap1 effect with low-dose cytochalasin D and by the ability of a constitutively active mutant of the actin cytoskeleton regulator Rac1 to sensitize K562 cells to the pharmacological activation of endogenous Rap1, using the Rap1 exchange factor-specific 8-pCPT-2'-O-Me-cAMP [8-(4-chlorophenylthio)-2'-O-methyladenosine-3',5'-cyclic monophosphate]. Interestingly, in primary human erythroid progenitor cells, 8-pCPT-2'-O-Me-cAMP was sufficient to significantly increase B19-mediated gene transfer, suggesting that these cells possess the cytoskeleton organization capacity required for efficient recruitment of beta(1) integrins by brief pharmacological stimulation of Rap1 GTP loading. Because 8-pCPT-2'-O-Me-cAMP has been implicated in enhanced homing of progenitor cells, these results identify a novel tool with which to optimize ex vivo B19-mediated gene transfer and potentially improve homing of transduced cells by Rap1-beta(1) integrin activation with 8-pCPT-2'-O-Me-cAMP.

Adeno-associated viral vectors and their redirection to cell-type specific receptors

Efficient and specific delivery of genes to the cell type of interest is a crucial issue in gene therapy. Adeno-associated virus (AAV) has gained particular interest as gene vector recently and is therefore the focus of this chapter. Its low frequency of random integration into the genome and the moderate immune response make AAV an attractive platform for vector design. Like in most other vector systems, the tropism of AAV vectors limits their utility for certain tissues especially upon systemic application. This may in part be circumvented by using AAV serotypes with an in vivo gene transduction pattern most closely fitting the needs of the application. Also, the tropism of AAV capsids may be changed by combining parts of the natural serotype diversity. In addition, peptides mediating binding to the cell type of interest can be identified by random phage display library screening and subsequently be introduced into an AAV capsid region critical for receptor binding. Such peptide insertions can abrogate the natural tropism of AAV capsids and result in detargeting from the liver in vivo. In a novel approach, cell type-directed vector capsids can be selected from random peptide libraries displayed on viral capsids or serotype-shuffling libraries in vitro and in vivo for optimized transduction of the cell type or tissue of interest.

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

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

Adeno-associated virus infection of murine fibroblasts with help provided by mouse adenovirus

Adeno-associated virus (AAV-2) replicates to high titers when host cells are coinfected with a helper virus. Here we analyzed the coinfection of AAV-2 and mouse adenovirus (MAV-1) in murine fibroblasts. We observed that AAV-2/MAV-1 coinfected NIH 3T3 cells produced approximately 10-40-fold less AAV-2 DNAse resistant particles than Hela cells. Levels of AAV-2 DNA replication were approximately 30-fold less in 3T3 cells as compared to Hela cells coinfected with human adenovirus (Ad-5). A study of these lower levels of infection in 3T3 cells compared to Hela cells revealed that receptor binding and internalization of AAV-2 in 3T3 and Hela cells was comparable. However, AAV-2 did not enter into the nucleus of mouse cells as efficiently as it does in human cells. Furthermore, viral DNA replication levels of AAV-2 DNA were found to be lower in mouse cells than human cells, indicating limitations in the murine nucleus for viral replication.

Densovirus infectious pathway requires clathrin-mediated endocytosis followed by trafficking to the nucleus

Junonia coenia densovirus (JcDNV) is an ambisense insect parvovirus highly pathogenic for lepidopteran pests at larval stages. The potential use of DNVs as biological control agents prompted us to reinvestigate the host range and cellular mechanisms of infection. In order to understand the early events of infection, we set up a functional infection assay in a cell line of the pest Lymantria dispar to determine the intracellular pathway undertaken by JcDNV to infect a permissive lepidopteran cell line. Our results show that JcDNV particles are rapidly internalized into clathrin-coated vesicles and slowly traffic within early and late endocytic compartments. Blocking late-endocytic trafficking or neutralizing the pH with drugs inhibited infection. During internalization, disruption of the cytoskeleton, and inhibition of phosphatidylinositol 3-kinase blocked the movement of vesicles containing the virus to the nucleus and impaired infection. In summary, our results define for the first time the early endocytic steps required for a productive DNV infection.

Detection of intact rAAV particles up to 6 years after successful gene transfer in the retina of dogs and primates

Gene transfer to the retina using recombinant adeno-associated viral (rAAV) vectors has proven to be an effective option for the treatment of retinal degenerative diseases in several animal models and has recently advanced into clinical trials in humans. To date, intracellular trafficking of AAV vectors and subsequent capsid degradation has been studied only in vitro, but the fate of AAV particles in transduced cells following subretinal injection has yet to be elucidated. Using electron microscopy and western blot, we analyzed retinas of one primate and four dogs that had been subretinally injected with AAV2/4, -2/5, or -2/2 serotypes and that displayed efficient gene transfer over several years. We show that intact AAV particles are still present in retinal cells, for up to 6 years after successful gene transfer in these large animals. The persistence of intact vector particles in the target organ, several years postadministration, is totally unexpected and, therefore, represents a new and unanticipated safety issue to consider at a time when gene therapy clinical trials raise new immunological concerns.