Ubiquitination of both adeno-associated virus type 2 and 5 capsid proteins affects the transduction efficiency of recombinant vectors

Doxorubicin augments rAAV-2 transduction in rat neuronal cells

Doxorubicin, an approved drug for cancer therapy, was recently found to be a potent agent to augment adeno-associated virus (AAV)-mediated transgene expression, especially in airway cells. Recombinant AAV type 2 (rAAV-2) has been shown to preferentially transduce neural tissues and is considered as the primary viral vector for the treatments of various neurodegenerative diseases including Parkinson's disease (PD). The goal of this study is to investigate whether doxorubicin can be applied to increase the efficacy of rAAV-2 transduction in the central nervous system. We co-administrated doxorubicin with AV2.luc/EGFP into the rat striatum, a preferred target site for PD gene therapy, and found that doxorubicin augmented rAAV-2 transduction dramatically without significant cytotoxicity and alteration of rAAV-2 tropism. By evaluating the effects of doxorubicin on rAAV-2 transduction in PC12 and MN9D neuronal cells, we found that doxorubicin appeared to promote the nuclear accumulation of rAAV-2, but did not affect viral binding or uptake. Our data suggested that doxorubicin might play an important role in modulating rAAV-2 intracellular trafficking in neuron-like cells. Our study also provided the initial in vivo evidence to facilitate AAV-mediated gene expression in the midbrain with the treatment of doxorubicin.

Enhancement of adeno-associated virus infection by mobilizing capsids into and out of the nucleolus

Adeno-associated virus (AAV) serotypes are being tailored for numerous therapeutic applications, but the parameters governing the subcellular fate of even the most highly characterized serotype, AAV2, remain unclear. To understand how cellular conditions control capsid trafficking, we have tracked the subcellular fate of recombinant AAV2 (rAAV2) vectors using confocal immunofluorescence, three-dimensional infection analysis, and subcellular fractionation. Here we report that a population of rAAV2 virions enters the nucleus and accumulates in the nucleolus after infection, whereas empty capsids are excluded from nuclear entry. Remarkably, after subcellular fractionation, virions accumulating in nucleoli were found to retain infectivity in secondary infections. Proteasome inhibitors known to enhance transduction were found to potentiate nucleolar accumulation. In contrast, hydroxyurea, which also increases transduction, mobilized virions into the nucleoplasm, suggesting that two separate pathways influence vector delivery in the nucleus. Using a small interfering RNA (siRNA) approach, we then evaluated whether nucleolar proteins B23/nucleophosmin and nucleolin, previously shown to interact with AAV2 capsids, affect trafficking and transduction efficiency. Similar to effects observed with proteasome inhibition, siRNA-mediated knockdown of nucleophosmin potentiated nucleolar accumulation and increased transduction 5- to 15-fold. Parallel to effects from hydroxyurea, knockdown of nucleolin mobilized capsids to the nucleoplasm and increased transduction 10- to 30-fold. Moreover, affecting both pathways simultaneously using drug and siRNA combinations was synergistic and increased transduction over 50-fold. Taken together, these results support the hypothesis that rAAV2 virions enter the nucleus intact and can be sequestered in the nucleolus in stable form. Mobilization from the nucleolus to nucleoplasmic sites likely permits uncoating and subsequent gene expression or genome degradation. In summary, with these studies we have refined our understanding of AAV2 trafficking dynamics and have identified cellular parameters that mobilize virions in the nucleus and significantly influence AAV 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.

High-efficiency transduction of the mouse retina by tyrosine-mutant AAV serotype vectors

Vectors derived from adeno-associated viruses (AAVs) have become important gene delivery tools for the treatment of many inherited ocular diseases in well-characterized animal models. Previous studies have determined that the viral capsid plays an essential role in the cellular tropism and efficiency of transgene expression. Recently, it was shown that phosphorylation of surface-exposed tyrosine residues from AAV2 capsid targets the viral particles for ubiquitination and proteasome- mediated degradation, and mutations of these tyrosine residues lead to highly efficient vector transduction in vitro and in vivo. Because the tyrosine residues are highly conserved in other AAV serotypes, in this study we evaluated the intraocular transduction characteristics of vectors containing point mutations in surface- exposed capsid tyrosine residues in AAV serotypes 2, 8, and 9. Several of these novel AAV mutants were found to display a strong and widespread transgene expression in many retinal cells after subretinal or intravitreal delivery compared with their wild-type counterparts. For the first time, we show efficient transduction of the ganglion cell layer by AAV serotype 8 or 9 mutant vectors, thus providing additional tools besides AAV2 for targeting these cells. These enhanced AAV vectors have a great potential for future therapeutic applications for retinal degenerations and ocular neovascular diseases.

Proteasome inhibitors enhance bacteriophage lambda (lambda) mediated gene transfer in mammalian cells

Bacteriophage lambda vectors can transfer their genomes into mammalian cells, resulting in expression of phage-encoded genes. However, this process is inefficient. Experiments were therefore conducted to delineate the rate limiting step(s) involved, using a phage vector that contains a mammalian luciferase reporter gene cassette. The efficiency of phage-mediated gene transfer in mammalian cells was quantitated, in the presence or absence of pharmacologic inhibitors of cell uptake and degradation pathways. Inhibitors of lysosomal proteases and proteasome inhibitors strongly enhanced phage-mediated luciferase expression, suggesting that these pathways contribute to the destruction of intracellular phage particles. In contrast, inhibition of endosome acidification had no effect on phage-mediated gene transfer, presumably because phage lambda is tolerant to extended exposure to low pH. These findings provide insights into the pathways by which phage vectors enter and transduce mammalian cells, and suggest that it may be possible to pharmacologically enhance the efficiency of phage-mediated gene transfer in mammalian cells. Finally, the data also suggest that the proteasome complex may serve as an innate defense mechanism that restricts the infection of mammalian cells by diverse viral agents.

Analysis of adeno-associated virus progenitor cell transduction in mouse lung

Although recombinant adeno-associated virus (rAAV) has been widely used in lung gene therapy approaches, it remains unclear to what extent commonly used AAV serotypes transduce adult progenitors in the lung. In this study, we evaluated the life span and proliferative capacity of rAAV1-, 2-, and 5-transduced airway cells in mouse lung, using a LacZ-CRE reporter transgenic model and Cre-expressing rAAV. In this model, the expression of CRE recombinase led to permanent genetic marking of transduced cells and their descendants with LacZ. To investigate whether the rAAV-transduced cells included airway progenitors, we injured the airways of rAAV-infected mice with Naphthalene, while simultaneously labeling with 5-bromodeoxyuridine (BrdU) to identify slow-cycling progenitor/stem cells that entered the cell cycle and retained label. Both rAAV5 and rAAV1 vectors were capable of transducing a subset of long-lived Clara cells and alveolar type II (ATII) cells that retained nucleotide label and proliferated following lung injury. Importantly, rAAV1 and 5 appeared to preferentially transduce conducting airway epithelial progenitors that had the capacity to clonally expand, both in culture and in vivo following lung injury. These studies suggest that rAAV may be a useful vector for gene targeting of airway stem/progenitor cells.

Tyrosine-phosphorylation of AAV2 vectors and its consequences on viral intracellular trafficking and transgene expression

We have documented that epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK) signaling negatively affects intracellular trafficking and transduction efficiency of recombinant adeno-associated virus 2 (AAV2) vectors. Specifically, inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsid proteins, which in turn, facilitates viral nuclear transport by limiting proteasome-mediated degradation of AAV2 vectors. In the present studies, we observed that AAV capsids can indeed be phosphorylated at tyrosine residues by EGFR-PTK in in vitro phosphorylation assays and that phosphorylated AAV capsids retain their structural integrity. However, although phosphorylated AAV vectors enter cells as efficiently as their unphosphorylated counterparts, their transduction efficiency is significantly reduced. This reduction is not due to impaired viral second-strand DNA synthesis since transduction efficiency of both single-stranded AAV (ssAAV) and self-complementary AAV (scAAV) vectors is decreased by approximately 68% and approximately 74%, respectively. We also observed that intracellular trafficking of tyrosine-phosphorylated AAV vectors from cytoplasm to nucleus is significantly decreased, which results from ubiquitination of AAV capsids followed by proteasome-mediated degradation, although downstream consequences of capsid ubiquitination may also be affected by tyrosine-phosphorylation. These studies provide new insights into the role of tyrosine-phosphorylation of AAV capsids in various steps in the virus life cycle, which has implications in the optimal use of recombinant AAV vectors in human gene therapy.

Enhancing transduction of the liver by adeno-associated viral vectors

A number of distinct factors acting at different stages of the adeno-associated virus vector (AAV)-mediated gene transfer process were found to influence murine hepatocyte transduction. Foremost among these was the viral capsid protein. Self-complementary (sc) AAV pseudotyped with capsid from serotype 8 or rh.10 mediated fourfold greater hepatocyte transduction for a given vector dose when compared with vector packaged with AAV7 capsid. An almost linear relationship between vector dose and transgene expression was noted for all serotypes with vector doses as low as 1 x 10(7) vg per mouse (4 x 10(8) vg kg(-1)) mediating therapeutic levels of human FIX (hFIX) expression. Gender significantly influenced scAAV-mediated transgene expression, with twofold higher levels of expression observed in male compared with female mice. Pretreatment of mice with the proteasome inhibitor bortezomib increased scAAV-mediated hFIX expression from 4+/-0.6 to 9+/-2 microg ml(-1) in female mice, although the effect of this agent was less profound in males. Exposure of mice to adenovirus 10-20 weeks after gene transfer with AAV vectors augmented AAV transgene expression twofold by increasing the level of proviral mRNA. Hence, optimization of individual steps in the AAV gene transfer process can further enhance the potency of AAV-mediated transgene expression, thus increasing the probability of successful gene therapy.

Next generation of adeno-associated virus 2 vectors: point mutations in tyrosines lead to high-efficiency transduction at lower doses

Recombinant adeno-associated virus 2 (AAV2) vectors are in use in several Phase I/II clinical trials, but relatively large vector doses are needed to achieve therapeutic benefits. Large vector doses also trigger an immune response as a significant fraction of the vectors fails to traffic efficiently to the nucleus and is targeted for degradation by the host cell proteasome machinery. We have reported that epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK) signaling negatively affects transduction by AAV2 vectors by impairing nuclear transport of the vectors. We have also observed that EGFR-PTK can phosphorylate AAV2 capsids at tyrosine residues. Tyrosine-phosphorylated AAV2 vectors enter cells efficiently but fail to transduce effectively, in part because of ubiquitination of AAV capsids followed by proteasome-mediated degradation. We reasoned that mutations of the surface-exposed tyrosine residues might allow the vectors to evade phosphorylation and subsequent ubiquitination and, thus, prevent proteasome-mediated degradation. Here, we document that site-directed mutagenesis of surface-exposed tyrosine residues leads to production of vectors that transduce HeLa cells approximately 10-fold more efficiently in vitro and murine hepatocytes nearly 30-fold more efficiently in vivo at a log lower vector dose. Therapeutic levels of human Factor IX (F.IX) are also produced at an approximately 10-fold reduced vector dose. The increased transduction efficiency of tyrosine-mutant vectors is due to lack of capsid ubiquitination and improved intracellular trafficking to the nucleus. These studies have led to the development of AAV vectors that are capable of high-efficiency transduction at lower doses, which has important implications in their use in human gene therapy.

Recombinant adeno-associated virus transduction and integration

Recombinant adeno-associated virus (rAAV) holds promise as a gene therapy vector for a multitude of genetic disorders such as hemophilia, cystic fibrosis, and the muscular dystrophies. Given the variety of applications and tissue types toward which these vectors may be targeted, an understanding of rAAV transduction is crucial for the effective application of therapy. rAAV transduction mechanisms have been the subject of much study, resulting in a body of knowledge relating to events from virus-cell attachment through to vector genome conformation in the target cell nucleus. Instead of utilizing one mechanism in each phase of vector transduction, rAAV appears to employ multiple possible pathways toward transgene expression, in part dependent on rAAV serotype, dose, and target cell type. Once inside the nucleus, the rAAV genome exists in a predominantly episomal form; therefore, nondividing cells tend to be most stably transduced. However, rAAV has a low frequency of integration into the host cell genome, often in or near genes, and can be associated with host genome mutations. This review describes the current understanding of the mechanisms and rate-limiting steps involved in rAAV transduction.

Production of CFTR-null and CFTR-DeltaF508 heterozygous pigs by adeno-associated virus-mediated gene targeting and somatic cell nuclear transfer

Progress toward understanding the pathogenesis of cystic fibrosis (CF) and developing effective therapies has been hampered by lack of a relevant animal model. CF mice fail to develop the lung and pancreatic disease that cause most of the morbidity and mortality in patients with CF. Pigs may be better animals than mice in which to model human genetic diseases because their anatomy, biochemistry, physiology, size, and genetics are more similar to those of humans. However, to date, gene-targeted mammalian models of human genetic disease have not been reported for any species other than mice. Here we describe the first steps toward the generation of a pig model of CF. We used recombinant adeno-associated virus (rAAV) vectors to deliver genetic constructs targeting the CF transmembrane conductance receptor (CFTR) gene to pig fetal fibroblasts. We generated cells with the CFTR gene either disrupted or containing the most common CF-associated mutation (DeltaF508). These cells were used as nuclear donors for somatic cell nuclear transfer to porcine oocytes. We thereby generated heterozygote male piglets with each mutation. These pigs should be of value in producing new models of CF. In addition, because gene-modified mice often fail to replicate human diseases, this approach could be used to generate models of other human genetic diseases in species other than mice.

Viral strategies for intracellular trafficking: motors and microtubules

To overcome barriers to diffusion, many viruses utilize the microtubule-associated molecular motor cytoplasmic dynein 1 to drive transport towards the nucleus of a target cell. Cytoplasmic dynein 1 generates movement towards the minus end of microtubules located at the microtubule organizing centre (MTOC), a structure that is typically in close proximity to the nucleus. Physiological cargoes for cytoplasmic dynein include membranous organelles, protein complexes and aggregates of misfolded protein. In this review, we discuss the study of microtubule-based translocation of viruses and raise questions about the mechanisms for association with and then dissociation from cytoplasmic dynein with a goal of understanding whether viruses are seen by the intracellular trafficking machinery as functional protein complexes or misfolded protein aggregates.

Comparative biology of rAAV transduction in ferret, pig and human airway epithelia

Differences between rodent and human airway cell biology have made it difficult to translate recombinant adeno-associated virus (rAAV)-mediated gene therapies to the lung for cystic fibrosis (CF). As new ferret and pig models for CF become available, knowledge about host cell/vector interactions in these species will become increasingly important for testing potential gene therapies. To this end, we have compared the transduction biology of three rAAV serotypes (AAV1, 2 and 5) in human, ferret, pig and mouse-polarized airway epithelia. Our results indicate that apical transduction of ferret and pig airway epithelia with these rAAV serotypes closely mirrors that observed in human epithelia (rAAV1>rAAV2 congruent withrAAV5), while transduction of mouse epithelia was significantly different (rAAV1>rAAV5>>rAAV2). Similarly, ferret, pig and human epithelia also shared serotype-specific differences in the polarity (apical vs basolateral) and proteasome dependence of rAAV transduction. Despite these parallels, N-linked sialic acid receptors were required for rAAV1 and rAAV5 transduction of human and mouse airway epithelia, but not ferret or pig airway epithelia. Hence, although the airway tropisms of rAAV serotypes 1, 2 and 5 are conserved better among ferret, pig and human as compared to mouse, viral receptors/co-receptors appear to maintain considerable species diversity.

Adeno-associated viral delivery of a metabolically regulated insulin transgene to hepatocytes

Transduction with a liver specific, metabolically responsive insulin transgene produces near-normal blood sugars in STZ-diabetic rats. To overcome the limited duration of hepatic transgene expression induced by E1A-deleted adenoviral vectors, we evaluated recombinant adeno-associated virus (rAAV2) for cell type specificity and glucose responsiveness in vitro. Co-infection of AAV2 containing the glucose responsive, liver-specific (GlRE)(3)BP-1 promoter with an empty adenovirus enhanced transduction efficiency, and shortened the duration of transgene expression in HepG2 hepatoma cells, but not primary hepatocytes. However, in the context of rAAV2, (GlRE)(3)BP-1 promoter activity remained confined to cells of hepatocyte lineage, and retained glucose responsiveness. While isolated infection with an insulin expressing rAAV2 failed to attenuate blood sugars in diabetic mice, adenoviral co-administration with the same rAAV2 induced transient, near-normal random blood sugars in a diabetic animal. We conclude that rAAV2 can induce metabolically responsive insulin secretion from hepatocytes in vitro and in vivo. However, alternative AAV serotypes will likely be required to efficiently deliver therapeutic genes to the liver for the treatment of diabetes mellitus.

Separate basic region motifs within the adeno-associated virus capsid proteins are essential for infectivity and assembly

Adeno-associated virus (AAV) is gaining momentum as a gene therapy vector for human applications. However, there remain impediments to the development of this virus as a vector. One of these is the incomplete understanding of the biology of the virus, including nuclear targeting of the incoming virion during initial infection, as well as assembly of progeny virions from structural components in the nucleus. Toward this end, we have identified four basic regions (BR) on the AAV2 capsid that represent possible nuclear localization sequence (NLS) motifs. Mutagenesis of BR1 ((120)QAKKRVL(126)) and BR2 ((140)PGKKRPV(146)) had minor effects on viral infectivity ( approximately 4- and approximately 10-fold, respectively), whereas BR3 ((166)PARKRLN(172)) and BR4 ((307)RPKRLN(312)) were found to be essential for infectivity and virion assembly, respectively. Mutagenesis of BR3, which is located in Vp1 and Vp2 capsid proteins, does not interfere with viral production or trafficking of intact AAV capsids to the nuclear periphery but does inhibit transfer of encapsidated DNA into the nucleus. Substitution of the canine parvovirus NLS rescued the BR3 mutant to wild-type (wt) levels, supporting the role of an AAV NLS motif. In addition, rAAV2 containing a mutant form of BR3 in Vp1 and a wt BR3 in Vp2 was found to be infectious, suggesting that the function of BR3 is redundant between Vp1 and Vp2 and that Vp2 may play a role in infectivity. Mutagenesis of BR4 was found to inhibit virion assembly in the nucleus of transfected cells. This affect was not completely due to the inefficient nuclear import of capsid subunits based on Western blot analysis. In fact, aberrant capsid foci were observed in the cytoplasm of transfected cells, compared to the wild type, suggesting a defect in early viral assembly or trafficking. Using three-dimensional structural analysis, the lysine- and arginine-to-asparagine change disrupts hydrogen bonding between these basic residues and adjacent beta strand glutamine residues that may prevent assembly of intact virions. Taken together, these data support that the BR4 domain is essential for virion assembly. Each BR was also found to be conserved in serotypes 1 to 11, suggesting that these regions are significant and function similarly in each serotype. This study establishes the importance of two BR motifs on the AAV2 capsid that are essential for infectivity and virion assembly.

A two-hybrid screen identifies cathepsins B and L as uncoating factors for adeno-associated virus 2 and 8

Vectors based on different serotypes of adeno-associated virus hold great promise for human gene therapy, based on their unique tissue tropisms and distinct immunological profiles. A particularly interesting candidate is AAV8, which can efficiently and rapidly transduce a wide range of tissues in vivo. To further unravel the mechanisms behind AAV8 transduction, we used yeast two-hybrid analyses to screen a mouse liver complementary DNA library for cellular proteins capable of interacting with the viral capsid proteins. In total, we recovered approximately 700 clones, comprising over 300 independent genes. Sequence analyses revealed multiple hits for over 100 genes, including two encoding the endosomal cysteine proteases cathepsins B and L. Notably, these two proteases also physically interacted with the corresponding portion of the AAV2 capsid in yeast, but not with AAV5. We demonstrate that cathepsins B and L are essential for efficient AAV2- and AAV8-mediated transduction of mammalian cells, and document the ability of purified cathepsin B and L proteins to bind and cleave intact AAV2 and AAV8 particles in vitro. These data suggest that cathepsin-mediated cleavage could prime AAV capsids for subsequent nuclear uncoating, and indicate that analysis of additional genes recovered in our screen may help to further elucidate the mechanisms behind transduction by AAV8 and related serotypes.

Novel Therapies for the Treatment of Cystic Fibrosis: New Developments in Gene and Stem Cell Therapy

Cystic fibrosis (CF) was one of the first target diseases for lung gene therapy. Studies of lung gene transfer for CF have provided many insights into the necessary components of successful gene therapy for lung diseases. Many advancements have been achieved with promising results in vitro and in small animal models. However, studies in primate models and patients have been discouraging despite a large number of clinical trials. This reflects a number of obstacles to successful, sustained, and repeatable gene transfer in the lung. Cell-based therapy with embryonic stem cells and adult stem cells (bone marrow or cord blood), have been investigated recently and may provide a viable therapeutic approach in the future. In this article, the authors review CF pathophysiology with a focus on specific targets in the lung epithelium for gene transfer and summarize the current status and future directions of gene- and cell-based therapies.

Surface-exposed adeno-associated virus Vp1-NLS capsid fusion protein rescues infectivity of noninfectious wild-type Vp2/Vp3 and Vp3-only capsids but not that of fivefold pore mutant virions

Over the past 2 decades, significant effort has been dedicated to the development of adeno-associated virus (AAV) as a vector for human gene therapy. However, understanding of the virus with respect to the functional domains of the capsid remains incomplete. In this study, the goal was to further examine the role of the unique Vp1 N terminus, the N terminus plus the recently identified nuclear localization signal (NLS) (J. C. Grieger, S. Snowdy, and R. J. Samulski, J. Virol 80:5199-5210, 2006), and the virion pore at the fivefold axis in infection. We generated two Vp1 fusion proteins (Vp1 and Vp1NLS) linked to the 8-kDa chemokine domain of rat fractalkine (FKN) for the purpose of surface exposure upon assembly of the virion, as previously described (K. H. Warrington, Jr., O. S. Gorbatyuk, J. K. Harrison, S. R. Opie, S. Zolotukhin, and N. Muzyczka, J. Virol 78:6595-6609, 2004). The unique Vp1 N termini were found to be exposed on the surfaces of these capsids and maintained their phospholipase A2 (PLA2) activity, as determined by native dot blot Western and PLA2 assays, respectively. Incorporation of the fusions into AAV type 2 capsids lacking a wild-type Vp1, i.e., Vp2/Vp3 and Vp3 capsid only, increased infectivity by 3- to 5-fold (Vp1FKN) and 10- to 100-fold (Vp1NLSFKN), respectively. However, the surface-exposed fusions did not restore infectivity to AAV virions containing mutations at a conserved leucine (Leu336Ala, Leu336Cys, or Leu336Trp) located at the base of the fivefold pore. EM analyses suggest that Leu336 may play a role in global structural changes to the virion directly impacting downstream conformational changes essential for infectivity and not only have local effects within the pore, as previously suggested.

A dual role of EGFR protein tyrosine kinase signaling in ubiquitination of AAV2 capsids and viral second-strand DNA synthesis

A 52 kd cellular protein, FK506-binding protein (FKBP52), phosphorylated at tyrosine residues by epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK), inhibits adeno-associated virus 2 (AAV2) second-strand DNA synthesis and transgene expression. FKBP52 is dephosphorylated at tyrosine residues by T-cell protein tyrosine phosphatase (TC-PTP), and TC-PTP over-expression leads to improved viral second-strand DNA synthesis and improved transgene expression. In these studies, we observed that perturbation of EGFR-PTK signaling by a specific inhibitor, Tyrphostin 23 (Tyr23), augmented the transduction efficiency of the single-stranded AAV (ssAAV) vector as well as the self-complementary AAV (scAAV) vector. Similarly, tyrosine-dephosphorylation of FKBP52 by TC-PTP resulted in increased transduction by both vectors. These data suggested that EGFR-PTK signaling also affects aspects of AAV transduction other than viral second-strand DNA synthesis. We document that inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsids which, in turn, facilitates nuclear transport by limiting proteasome-mediated degradation of AAV vectors. We also document that Tyr23-mediated increase in AAV2 transduction efficiency is not further enhanced by a specific proteasome inhibitor, MG132. Thus, EGFR-PTK signaling modulates ubiquitin (Ub)/proteasome pathway-mediated intracellular trafficking as well as FKBP52-mediated second-strand DNA synthesis of AAV2 vectors. This has implications in the optimal use of AAV vectors in gene therapy.

A dual role of EGFR protein tyrosine kinase signaling in ubiquitination of AAV2 capsids and viral second-strand DNA synthesis

A 52 kd cellular protein, FK506-binding protein (FKBP52), phosphorylated at tyrosine residues by epidermal growth factor receptor protein tyrosine kinase (EGFR-PTK), inhibits adeno-associated virus 2 (AAV2) second-strand DNA synthesis and transgene expression. FKBP52 is dephosphorylated at tyrosine residues by T-cell protein tyrosine phosphatase (TC-PTP), and TC-PTP over-expression leads to improved viral second-strand DNA synthesis and improved transgene expression. In these studies, we observed that perturbation of EGFR-PTK signaling by a specific inhibitor, Tyrphostin 23 (Tyr23), augmented the transduction efficiency of the single-stranded AAV (ssAAV) vector as well as the self-complementary AAV (scAAV) vector. Similarly, tyrosine-dephosphorylation of FKBP52 by TC-PTP resulted in increased transduction by both vectors. These data suggested that EGFR-PTK signaling also affects aspects of AAV transduction other than viral second-strand DNA synthesis. We document that inhibition of EGFR-PTK signaling leads to decreased ubiquitination of AAV2 capsids which, in turn, facilitates nuclear transport by limiting proteasome-mediated degradation of AAV vectors. We also document that Tyr23-mediated increase in AAV2 transduction efficiency is not further enhanced by a specific proteasome inhibitor, MG132. Thus, EGFR-PTK signaling modulates ubiquitin (Ub)/proteasome pathway-mediated intracellular trafficking as well as FKBP52-mediated second-strand DNA synthesis of AAV2 vectors. This has implications in the optimal use of AAV vectors in gene therapy.

A tractable method for simultaneous modifications to the head and tail of bacteriophage lambda and its application to enhancing phage-mediated gene delivery

There is considerable interest in the use of bacteriophage vectors for mammalian cell gene transfer applications, due to their stability, excellent safety profile and inexpensive mass production. However, to date, phage vectors have been plagued by mediocre performance as gene transfer agents. This may reflect the complexity of the viral infection process in mammalian cells and the need to refine each step of this process in order to arrive at an optimal, phage-based gene transfer system. Therefore, a flexible system was designed that alowed for the introduction of multiple modifications on the surface of bacteriophage lambda. Using this novel method, multiple peptides were displayed simultaneously from both the phage head and tail. Surface head display of an ubiquitinylation motif greatly increased the efficiency of phage-mediated gene transfer in a murine macrophage cell line. Gene transfer was further increased when this peptide was displayed in combination with a tail-displayed CD40-binding motif. Overall, this work provides a novel system that can be used to rationally improve bacteriophage gene transfer vectors and shows it may be possible to enhance the efficiency of phage-mediated gene transfer by targeting and optimizing multiple steps within the viral infection pathway.

Expression of a truncated cystic fibrosis transmembrane conductance regulator with an AAV5-pseudotyped vector in primates

Gene therapy using recombinant adeno-associated virus (rAAV2) vectors for cystic fibrosis has shown gene transfer and remarkable safety, yet indeterminate expression. A new construct has been characterized with a powerful exogenous promoter, the cytomegalovirus enhancer/chicken beta-actin promoter, driving a truncated CF transmembrane conductance regulator (CFTR), pseudotyped in an AAV5 viral coat. Our goal is to demonstrate that airway delivery of a pseudotyped rAAV5 vector results in gene transfer as well as expression in non-human primates. Aerosolized pseudotyped rAAV5-DeltaCFTR or rAAV5-GFP (green fluorescent protein) genes were delivered to four and six lungs, respectively. The pseudotyped rAAV5 vector did result in GFP gene transfer (1.005x10(6) copies/mug DNA on average) and quantifiable gene expression. Microscopy confirmed protein expression in airway epithelium. Similarly, the vector also resulted in vector-specific CFTR DNA (1.24x10(5) copies/microg) and mRNA expression. Immunoprecipitation and (32)P phosphoimaging were used to demonstrate CFTR protein expression, as qualitatively enhanced beyond the barely detectable endogenous expression in untreated animals. Based on these promising studies, this CFTR minigene construct is a therapeutic candidate.

Viral Vector–mediated and Cell-based Therapies for Treatment of Cystic Fibrosis

Gene and cell-based therapies are considered to be potentially powerful new approaches for the management of cystic fibrosis (CF) lung disease. Despite tremendous efforts that have been made, especially in studies to understand the obstacles to gene delivery, major challenges to the application of these approaches remain to be solved. This article will review the advancements made and challenges remaining in the development of viral vector-mediated and cell-based approaches to treat patients with CF.

Positive and negative effects of adenovirus type 5 helper functions on adeno-associated virus type 5 (AAV5) protein accumulation govern AAV5 virus production

Full replication of adeno-associated virus type 5 (AAV5) is sustained by adenovirus type 5 (Ad5) helper functions E1a, E1b, E2a, E4Orf6, and virus-associated (VA) RNA; however, their combined net enhancement of AAV5 replication was comprised of both positive and negative individual effects. Although Ad5 E4Orf6 was required for AAV5 genomic DNA replication, it also functioned together with E1b to degrade de novo-expressed, preassembled AAV5 capsid proteins and Rep52 in a proteosome-dependent manner. VA RNA enhanced accumulation of AAV5 protein, overcoming the degradative effects of E4Orf6, and was thus required to restore adequate amounts of AAV5 proteins necessary to achieve efficient virus production.

Unique biologic properties of recombinant AAV1 transduction in polarized human airway epithelia

The choice of adeno-associated virus serotypes for clinical applications is influenced by the animal model and model system used to evaluate various serotypes. In the present study, we sought to compare the biologic properties of rAAV2/1, rAAV2/2, and rAAV2/5 transduction in polarized human airway epithelia using viruses purified by a newly developed common column chromatography method. Results demonstrated that apical transduction of human airway epithelia with rAAV2/1 was 100-fold more efficient than rAAV2/2 and rAAV2/5. This transduction profile in human airway epithelia (rAAV2/1 >> rAAV2/2 = rAAV2/5) was significantly different from that seen following nasal administration of these vectors to mouse lung (rAAV2/5 > rAAV2/1 >> rAAV2/2), emphasizing differences in transduction of these serotypes between these two species. In stark contrast to rAAV2/2 and rAAV2/5, rAAV2/1 transduced both the apical and basolateral membrane of human airway epithelia with similar efficiency. However, the overall level of transduction across serotypes did not correlate with vector internalization. We hypothesized that differences in post-entry processing of these serotypes might influence the efficiency of apical transduction. To this end, we tested the effectiveness of proteasome inhibitors to augment nuclear translocation and gene expression from the three serotypes. Augmentation of rAAV2/1 apical transduction of human polarized airway epithelia was 10-fold lower than that for rAAV2/2 and rAAV2/5. Cellular fractionation studies demonstrated that proteasome inhibitors more significantly enhanced rAAV2/2 and rAAV2/5 translocation to the nucleus than rAAV2/1. These results demonstrate that AAV1 transduction biology in human airway epithelia differs from that of AAV2 and AAV5 by virtue of altered ubiquitin/proteasome sensitivities that influence nuclear translocation.

Adeno-associated virus type 2 capsids with externalized VP1/VP2 trafficking domains are generated prior to passage through the cytoplasm and are maintained until uncoating occurs in the nucleus

Common features of parvovirus capsids are open pores at the fivefold symmetry axes that traverse the virion shell. Upon limited heat treatment in vitro, the pores can function as portals to externalize VP1/VP2 protein N-terminal sequences which harbor infection-relevant functional domains, such as a phospholipase A(2) catalytic domain. Here we show that adeno-associated virus type 2 (AAV2) also exposes its VP1/VP2 N termini in vivo during infection, presumably in the endosomal compartment. This conformational change is influenced by treatment with lysosomotropic reagents. While incubation of cells with bafilomycin A1 reduced exposure of VP1/VP2 N termini, incubation with chloroquine stimulated externalization transiently. N-terminally located basic amino acid clusters with nuclear localization activity also become exposed in this process and are accessible on the virus capsid when it enters the cytoplasm. This is an obligatory step in AAV2 infection. However, a direct role of these sequences in nuclear translocation of viral capsids could not be determined by microinjection of wild-type or mutant viruses. This suggests that further modifications of the capsid have to take place in a precytoplasmic entry step that prepares the virus for nuclear entry. Microinjection of several capsid-specific antibodies into the cell nucleus blocked AAV2 infection completely, supporting the conclusion that AAV2 capsids bring the infectious genome into the nucleus.

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.

Capsid modifications overcome low heterogeneous expression of heparan sulfate proteoglycan that limits AAV2-mediated gene transfer and therapeutic efficacy in human ovarian carcinoma

OBJECTIVES

Capsid-modified AAV vectors can mediate enhanced gene transfer to neoplasms characterized by low AAV receptor expression. Here we sought to determine the therapeutic potential of a capsid-modified AAV vector for gene therapy of ovarian carcinoma (OvCa).

METHODS

We tested a panel of OvCa cell lines for AAV2-mediated gene transduction and for sensitivity to ganciclovir (GCV) following AAVHSVtk administration. Levels of AAV internalization and attachment receptor were assessed by flow cytometry and immunohistochemistry. The role of receptors in AAV-mediated gene transfer was assessed by competition assays. Finally, we examined the ability of a modified vector with an integrin-binding RGD motif inserted into the AAV capsid to improve gene delivery to OvCa and enhance AAVHSVtk/GCV-mediated killing by cytotoxicity assay.

RESULTS

All OvCa cell lines were poorly transduced with AAV2 vectors and showed variably sensitive to AAVHSVtk/GCV. While OvCa cell lines expressed AAV2 internalization receptors (alphav integrins), expression of the AAV2 attachment receptor, HSPG, was variable and not detected on many lines. Analysis of archived clinical specimens showed no detectable HSPG expression on approximately 45% of primary human tumors. Gene transfer to OvCa was increased several fold using the RGD-modified vector. Gene transfer was independent of HSPG and specific to the targeted receptor. Importantly, the RGD-modified capsid markedly increased the ability of the AAVHSVtk to kill OvCa cells in the presence of GCV.

CONCLUSIONS

The development of AAV vectors targeted to cell surface receptors other than HSPG will be critical to the advancement of AAV-mediated gene therapy for treating OvCa.

Adeno-associated virus serotypes: vector toolkit for human gene therapy

Recombinant adeno-associated viral (AAV) vectors have rapidly advanced to the forefront of gene therapy in the past decade. The exponential progress of AAV-based vectors has been made possible by the isolation of several naturally occurring AAV serotypes and over 100 AAV variants from different animal species. These isolates are ideally suited to development into human gene therapy vectors due to their diverse tissue tropisms and potential to evade preexisting neutralizing antibodies against the common human AAV serotype 2. Despite their prolific application in several animal models of disease, the mechanisms underlying selective tropisms of AAV serotypes remain largely unknown. Efforts to understand cell surface receptor usage and intracellular trafficking pathways exploited by AAV continue to provide significant insight into the biology of AAV vectors. Such unique traits are thought to arise from differences in surface topology of the capsids of AAV serotypes and variants. In addition to the aforementioned naturally evolved AAV isolates, several strategies to engineer hybrid AAV serotype vectors have been formulated in recent years. The generation of mosaic or chimeric vectors through the transcapsidation or marker-rescue/domain-swapping approach, respectively, is notable in this regard. More recently, combinatorial strategies for engineering AAV vectors using error-prone PCR, DNA shuffling, and other molecular cloning techniques have been established. The latter library-based approaches can serve as powerful tools in the generation of low-immunogenic and cell/tissue type-specific AAV vectors for gene delivery. This review is focused on recent developments in the isolation of novel AAV serotypes and isolates, their production and purification, diverse tissue tropisms, mechanisms of cellular entry/trafficking, and capsid structure. Strategies for engineering hybrid AAV vectors derived from AAV serotypes and potential implications of the rapidly expanding AAV vector toolkit are discussed.

Viral interactions with the cytoskeleton: a hitchhiker's guide to the cell

The actin and microtubule cytoskeleton play important roles in the life cycle of every virus. During attachment, internalization, endocytosis, nuclear targeting, transcription, replication, transport of progeny subviral particles, assembly, exocytosis, or cell-to-cell spread, viruses make use of different cellular cues and signals to enlist the cytoskeleton for their mission. Viruses induce rearrangements of cytoskeletal filaments so that they can utilize them as tracks or shove them aside when they represent barriers. Viral particles recruit molecular motors in order to hitchhike rides to different subcellular sites which provide the proper molecular environment for uncoating, replicating and packaging viral genomes. Interactions between subviral components and cytoskeletal tracks also help to orchestrate virus assembly, release and efficient cell-to-cell spread. There is probably not a single virus that does not use cytoskeletal and motor functions in its life cycle. Being well informed intracellular passengers, viruses provide us with unique tools to decipher how a particular cargo recruits one or several motors, how these are activated or tuned down depending on transport needs, and how cargoes switch from actin tracks to microtubules to nuclear pores and back.

Scalable generation of high-titer recombinant adeno-associated virus type 5 in insect cells

We established a method for production of recombinant adeno-associated virus type 5 (rAAV5) in insect cells by use of baculovirus expression vectors. One baculovirus harbors a transgene between the inverted terminal repeat sequences of type 5, and the second expresses Rep78 and Rep52. Interestingly, the replacement of type 5 Rep52 with type 1 Rep52 generated four times more rAAV5 particles. We replaced the N-terminal portion of type 5 VP1 with the equivalent portion of type 2 to generate infectious AAV5 particles. The rAAV5 with the modified VP1 required alpha2-3 sialic acid for transduction, as revealed by a competition experiment with an analog of alpha2-3 sialic acid. rAAV5-GFP/Neo with a 4.4-kb vector genome produced in HEK293 cells or Sf9 cells transduced COS cells with similar efficiencies. Surprisingly, Sf9-produced humanized Renilla green fluorescent protein (hGFP) vector with a 2.4-kb vector genome induced stronger GFP expression than the 293-produced one. Transduction of murine skeletal muscles with Sf9-generated rAAV5 with a 3.4-kb vector genome carrying a human secreted alkaline phosphatase (SEAP) expression cassette induced levels of SEAP more than 30 times higher than those for 293-produced vector 1 week after injection. Analysis of virion DNA revealed that in addition to a 2.4- or 3.4-kb single-stranded vector genome, Sf9-rAAV5 had more-abundant forms of approximately 4.7 kb, which appeared to correspond to the monomer duplex form of hGFP vector or truncated monomer duplex SEAP vector DNA. These results indicated that rAAV5 can be generated in insect cells, although the difference in incorporated virion DNA may induce different expression patterns of the transgene.

rAAV2 traffics through both the late and the recycling endosomes in a dose-dependent fashion

Inefficient trafficking of recombinant adeno-associated virus type-2 (rAAV2) to the nucleus is a major barrier for transduction. Using imaging and subcellular fractionation techniques, we evaluated the extent of rAAV2 movement through the late (Rab7) and recycling (Rab11) endosomes. Following rAAV2 infection of HeLa cells, immunoisolation of HA-Rab7- or HA-Rab11-tagged endosomes and intracellular colocalization of Cy3-labeled rAAV2 with EGFP-Rab7 or EGFP-Rab11 markers demonstrated dose-dependent trafficking of rAAV2 through the recycling and late endosomal compartments. At low multiplicities of infection (m.o.i. 100 genomes/cell), rAAV2 predominantly trafficked to the Rab7 compartment. In contrast, rAAV2 predominantly trafficked to the recycling endosome at 100-fold higher m.o.i. siRNA studies inhibiting either Rab7 or Rab11 demonstrated that reducing Rab11 protein levels more significantly inhibited rAAV2 transduction on a per genome basis compared to inhibition of Rab7. Dose-response curves, comparing the m.o.i. of AV2Luc infection to relative transduction, also supported the hypothesis that viral movement through the Rab11 compartment at high m.o.i. is more competent for transgene expression ( approximately 100-fold) than virus that moves through the Rab7 compartment at low m.o.i. These findings suggest that strategies to shunt viral movement from the late to the recycling endosome may be effective at increasing viral transduction for gene therapy.

Species-specific differences in mouse and human airway epithelial biology of recombinant adeno-associated virus transduction

Differences in airway epithelial biology between mice and humans have presented challenges to evaluating gene therapies for cystic fibrosis (CF) using murine models. In this context, recombinant adeno-associated virus (rAAV) type 2 and rAAV5 vectors have very different transduction efficiencies in human air-liquid interface (ALI) airway epithelia (rAAV2 approximately = rAAV5) as compared with mouse lung (rAAV5 >> rAAV2). It is unclear if these differences are due to species-specific airway biology or limitations of ALI cultures to reproduce in vivo airway biology. To this end, we compared rAAV2 and rAAV5 transduction biology in mouse and human ALI cultures, and investigated the utility of murine deltaF508 cystic fibrosis transmembrane conductance regulator (CFTR) ALI epithelia to study CFTR complementation. Our results demonstrate that mouse ALI epithelia retain in vivo preferences for rAAV serotype transduction from the apical membrane (rAAV5 >> rAAV2) not seen in human epithelia (rAAV2 approximately = rAAV5). Viral binding of rAAV2 and rAAV5 to the apical surface of mouse ALI airway epithelia was not significantly different, and proteasome-modulating agents significantly enhanced rAAV2 transduction to a level equivalent to that of rAAV5 in the presence of these agents, suggesting that the ubiquitin/proteasome pathway represents a more significant intracellular block for rAAV2 transduction of mouse airway epithelia. Interestingly, cAMP-inducible chloride currents were enhanced in deltaF508CFTR mouse ALI cultures, making this model incompatible with CFTR complementation studies. These studies emphasize species-specific differences in airway biology between mice and humans that significantly influence the use of mice as surrogate models for rAAV transduction and gene therapy for CF.

Parvovirus uncoating in vitro reveals a mechanism of DNA release without capsid disassembly and striking differences in encapsidated DNA stability

The uncoating mechanism of parvoviruses is unknown. Their capsid robustness and increasing experimental data would suggest an uncoating mechanism without capsid disassembly. We have developed an in vitro system to detect and quantify viral DNA externalization and applied the assay on two parvoviruses with important differences in capsid structure, human B19 and minute virus of mice (MVM). Upon briefly treating the capsids to increasing temperatures, the viral genome became accessible in its full-length in a growing proportion of virions. Capsid disassembly started at temperatures above 60 degrees C for B19 and 70 degrees C for MVM. For both viruses, the externalization followed an all-or-nothing mechanism, without transitions exposing only a particular genomic region. However, the heat-induced DNA accessibility was remarkably more pronounced in B19 than in MVM. This difference was also evident under conditions mimicking endosomal acidification (pH 6.5 to 5), which triggered the externalization of B19-DNA but not of MVM-DNA. The externalized ssDNA was a suitable template for the full second-strand synthesis. Immunoprecipitation with antibodies against conformational epitopes and quantitative PCR revealed that the DNA externalized by heat was mostly dissociated from its capsid, however, the low pH-induced DNA externalization of B19 was predominantly capsid-associated. These results provide new insights into parvovirus uncoating suggesting a mechanism by which the full-length viral genome is released without capsid disassembly. The remarkable instability of the encapsidated B19 DNA, which is easily released from its capsid, would also explain the faster heat inactivation of B19 when compared to other parvoviruses.

Intracellular trafficking of adeno-associated viral vectors

Adeno-associated virus (AAV) has attracted considerable interest as a gene therapy vector over the past decade. In all, 85% of the current 2052 PubMed references on AAV (as of December 2004) have been published in the last 10 years. As researchers have moved forward with using this vector system for gene delivery, an increased appreciation for the complexities of AAV biology has emerged. The biology of recombinant AAV (rAAV) transduction has demonstrated considerable diversity in different cell types and target tissues. This review will summarize the current understanding of events that control rAAV transduction following receptor binding and leading to nuclear uptake. These stages are broadly classified as intracellular trafficking and have been found to be a major rate-limiting step in rAAV transduction for many cell types. Advances in understanding this area of rAAV biology will help to improve the efficacy of this vector system for the treatment of inherited and acquired diseases.

Proteasome inhibition enhances AAV-mediated transgene expression in human synoviocytes in vitro and in vivo

To explore the potential applicability of recombinant adeno-associated virus (rAAV) vectors in the treatment of rheumatoid arthritis (RA), primary human fibroblast-like synoviocytes (FLS) derived from patients with RA were infected with rAAV encoding mouse IL-10 under the control of the CMV promoter. Addition of the proteasome inhibitor carbobenzoxy-l-leucyl-l-leucyl-l-leucinal (zLLL) to the cultures dramatically enhanced expression of the IL-10 transgene, in a dose-dependent manner. The increased expression was transient, peaking at 3 days and returning to near baseline by 7 days. The enhancement was observed even when zLLL was added 13 days after infection with rAAV. The effect of zLLL was not specific to either the mIL-10 transgene or the CMV promoter, as similar findings were observed using an rAAV construct encoding alpha1-anti-trypsin under the control of the chick beta-actin promoter or GFP, driven by the CMV promoter. Transgene expression could be repeatedly induced by reexposure to zLLL. Transgene mRNA levels increased in parallel with protein levels. Transgene expression could also be repeatedly induced in vivo by administering zLLL to SCID mice previously injected with rAAV-infected FLS. These data demonstrate that proteasome inhibition can dramatically enhance transgene expression in human RA FLS following infection with rAAV and suggest a possible approach to regulating synovial transgene expression in vivo.

AAV8-Mediated Hepatic Expression of Acid Sphingomyelinase Corrects the Metabolic Defect in the Visceral Organs of a Mouse Model of Niemann–Pick Disease

Acid sphingomyelinase deficiency is a lysosomal storage disorder in which the defective lysosomal hydrolase fails to degrade sphingomyelin. The resulting accumulation of substrate in the lysosomes of histiocytic cells leads to hepatosplenomegaly and severe pulmonary inflammation. Administration of a recombinant AAV1 vector encoding human acid sphingomyelinase to acid sphingomyelinase knockout (ASMKO) mice effectively reduced the accumulated substrate in all of the affected visceral organs. However, more complete and rapid clearance of sphingomyelin was observed when an AAV8-based serotype vector was used in lieu of AAV1. Importantly, AAV8-mediated hepatic expression of higher and sustained levels of the enzyme also corrected the abnormal cellularity, cell differentials, and levels of the chemokine MIP-1alpha in the bronchoalveolar lavage fluids of the ASMKO mice. Treatment also reversed the morphological aberrations associated with the alveolar macrophages of ASMKO mice and restored their phagocytic activity. No antibodies to the expressed enzyme were detected when the viral vectors were used in conjunction with a transcription cassette harboring a liver-restricted enhancer/promoter. Together, these data support the continued development of AAV8-mediated hepatic gene transfer as an approach to treat the visceral manifestations observed in individuals with acid sphingomyelinase deficiency.

Spliceosome-Mediated RNATrans-Splicing with Recombinant Adeno-Associated Virus Partially Restores Cystic Fibrosis Transmembrane Conductance Regulator Function to Polarized Human Cystic Fibrosis Airway Epithelial Cells

We previously reported that spliceosome-mediated RNA trans-splicing (SMaRT), using recombinant adenoviral vectors expressing pre-trans-splicing molecules (PTMs), could partially restore cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel activity to polarized human DeltaF508 CF airway epithelia. Although these studies proved that SMaRT could correct CFTR mRNA defects, recombinant adenoviral infection from the basolateral surface was required because of inefficient infection from the apical membrane. Hence, applications of SMaRT technology for CF gene therapy require further testing with alternative, more clinically viable, vector systems. Furthermore, because recombinant adeno-associated virus (rAAV) vectors have packing limitations with respect to the size of the CFTR transgene insert, SMaRT correction of CFTR has the added attraction of a smaller transgene cassette. In the present study, we investigated whether rAAV vectors could effectively rescue CFTR chloride conductance in polarized human CF airway epithelial cells, using a SMaRT approach. AAV vectors were generated to carry a PTM engineered to bind intron 9 of CFTR pre-mRNA and then trans-splice the normal sequence for human CFTR exons 10-24 into the endogenous pre-mRNA. Human CF polarized airway epithelia were infected from the apical membrane with rAAV2 or rAAV5 CFTR-PTM vectors in the presence of proteasome-modulating agents (doxorubicin and N-acetyl-L-leucinyl-L-leucinyl-L-norleucinal) to enhance transduction. Epithelia were then evaluated for cAMP-sensitive short-circuit currents 2 weeks postinfection. Levels of CFTR correction seen with rAAV2 (1.07 +/- 0.24 microA) and rAAV5 (0.90 +/- 0.20 microA) CFTR-PTM vectors were similar, representing conductance equivalent to 14.2 and 13.6% of that observed in non-CF human polarized epithelia, respectively. RT-PCR analysis demonstrated the existence of wild-type CFTR transcript in CFTR-PTM-corrected epithelia, whereas only DeltaF508 mRNA was detected in polarized cells infected with control rAAV LacZ-PTM vectors. These results provide evidence that rAAV vectors are capable of using SMaRT to correct CFTR function after apical infection of human CF airway epithelia. The ability of CFTR-PTM-mediated correction to maintain endogenous CFTR regulation of the transgene product may further improve the efficacy of gene therapy for CF.

Packaging capacity of adeno-associated virus serotypes: impact of larger genomes on infectivity and postentry steps

The limited packaging capacity of adeno-associated virus (AAV) precludes the design of vectors for the treatment of diseases associated with larger genes. Autonomous parvoviruses, such as minute virus of mice and B19, while identical in size (25 nm), are known to package larger genomes of 5.1 and 5.6 kb, respectively, compared to AAV genomes of 4.7 kb. One primary difference is the fact that wild-type (wt) AAV utilizes three capsid subunits instead of two to form the virion shell. In this study, we have characterized the packaging capacity of AAV serotypes 1 through 5 with and without the Vp2 subunit. Using reporter transgene cassettes that range in size from 4.4 to 6.0 kb, we determined that serotypes 1 through 5 with and without Vp2 could successfully package, replicate in, and transduce cells. Dot blot analysis established that packaging efficiency was similar for all vector cassettes and that the integrity of encapsidated genomes was intact regardless of size. Although physical characterization determined that virion structures were indistinguishable from wt, transduction experiments determined that all serotype vectors carrying larger genomes (5.3 kb and higher) transduced cells less efficiently (within a log) than AAV encapsidating wt size genomes. This result was not unique to reporter genes and was observed for CFTR vector cassettes ranging in size from 5.1 to 5.9 kb. No apparent advantage in packaging efficiency was observed when Vp2 was present or absent from the virion. Further analysis determined that a postentry step was responsible for the block in infection and specific treatment of cells upon infection with proteasome inhibitors increased transduction of AAV encapsidating larger DNA templates to wt levels, suggesting a preferential degradation of virions encapsidating larger-than-wt genomes. This study illustrates that AAV is capable of packaging and protecting recombinant genomes as large as 6.0 kb but the larger genome-containing virions are preferentially degraded by the proteasome and that this block can be overcome by the addition of proteasome inhibitors.

AAV hybrid serotypes: improved vectors for gene delivery

In recent years, significant efforts have been made on studying and engineering adeno-associated virus (AAV) capsid, in order to increase efficiency in targeting specific cell types that are non-permissive to wild type (wt) viruses and to improve efficacy in infecting only the cell type of interest. With our previous knowledge of the viral properties of the naturally occurring serotypes and the elucidation of their capsid structures, we can now generate capsid mutants, or hybrid serotypes, by various methods and strategies. In this review, we summarize the studies performed on AAV retargeting, and categorize the available hybrid serotypes to date, based on the type of modification: 1) transcapsidation, 2) adsorption of bi-specific antibody to capsid surface, 3) mosaic capsid, and 4) chimeric capsid. Not only these hybrid serotypes could achieve high efficiency of gene delivery to a specific targeted cell type, which can be better-tailored for a particular clinical application, but also serve as a tool for studying AAV biology such as receptor binding, trafficking and genome delivery into the nucleus.

Proteasome modulating agents induce rAAV2-mediated transgene expression in human intestinal epithelial cells

Intestinal gene transfer offers promise as a therapeutic option for treatment of both intestinal and non-intestinal diseases. Recombinant adeno-associated virus serotype 2, rAAV2, based vectors have been utilized to transduce lung epithelial cells in culture and in human subjects. rAAV2 transduction of intestinal epithelial cells, however, is limited both in culture and in vivo. Proteasome-inhibiting agents have recently been shown to enhance rAAV2-mediated transgene expression in airway epithelial cells. We hypothesized that similar inhibition of proteasome-related cellular processes can function to induce rAAV2 transduction of intestinal epithelial cells. Our results demonstrate that combined treatment with proteasome-modulating agents MG101 (N-acetyl-L-leucyl-L-leucyl-L-norleucine) and Doxorubicin synergistically induces rAAV2-mediated luciferase transgene expression by >400-fold in undifferentiated Caco-2 cells. In differentiated Caco-2 monolayers, treatment with MG101 and Doxorubicin induces transduction preferentially from the basolateral cell surface. In addition to Caco-2 cells, treatment with MG101 and Doxorubicin also results in enhanced rAAV2 transduction of HT-29, T84, and HCT-116 human intestinal epithelial cell lines. We conclude that MG101 and Doxorubicin mediate generic effects on intestinal epithelial cells that result in enhanced rAAV2 transduction. Use of proteasome-modulating agents to enhance viral transduction may facilitate the development of more efficient intestinal gene transfer protocols.

Recombinant adeno-associated virus type 2-mediated gene transfer into human keratinocytes is influenced by both the ubiquitin/proteasome pathway and epidermal growth factor receptor tyrosine kinase

Efficient gene delivery into keratinocytes is a prerequisite for successful skin gene therapy. Vectors based on recombinant adeno-associated virus type 2 (rAAV-2) offer several promising features that make them attractive for cutaneous applications. However, highly efficient gene delivery may be hampered by different cellular factors, including lack of viral receptors, impairment of cytoplasmic trafficking or limitations in viral second-strand synthesis. This study was undertaken to find factors that influence rAAV-2-mediated in vitro gene transfer into human keratinocytes and, consequently, ways to optimize gene delivery. Transduction experiments using rAAV-2 vectors expressing green fluorescent protein (GFP) demonstrated that impaired cellular trafficking of vector particles and high levels of autophosphorylation at epidermal growth factor receptor tyrosine kinase (EGF-R TK) have a negative influence on gene transfer into keratinocytes. Treatment of keratinocytes with proteasome inhibitor MG132 resulted in a transient augmentation of GFP expression in up to 37% of cells. Treatment with EGF-R TK inhibitors (quinazoline type) enhanced transgene expression in 10-14.5% of the cells. Gene expression was stable for more than 10 weeks and persisted until proliferative senescence occurred. This stable gene expression allows speculation that keratinocyte stem cells have initially been transduced. These findings might have relevance for the use of rAAV-2 vectors in skin gene therapy: transient enhancement of rAAV-2 transduction with proteasome inhibitors might be useful for genetic promotion of wound healing or skin-directed vaccination. Treatment with quinazolines may increase rAAV-2 transduction of keratinocyte stem cells, which is important for gene therapy approaches to inherited diseases.

Recombinant adeno-associated virus vectors for gene therapy

Recombinant adeno-associated virus (rAAV) vectors are based on a non-pathogenic human parvovirus (AAV) that is unique in its ability to persist in human cells without causing any pathologic effects. Studies of the potential barriers to rAAV-mediated transduction of relatively resistant cells has led to an understanding of the mechanisms of cell attachment and entry, cytoplasmic translocation, nuclear entry, conversion to active double-stranded DNA, activation of transcription and establishment of persistent molecular forms. Each of these areas is individually discussed, as are recent applications in vivo in preclinical models and clinical trials.

Inverted terminal repeat sequences are important for intermolecular recombination and circularization of adeno-associated virus genomes

The relatively small package capacity (less than 5 kb) of adeno-associated virus (AAV) vectors has been effectively doubled with the development of dual-vector heterodimerization approaches. However, the efficiency of such dual-vector systems is limited not only by the extent to which intermolecular recombination occurs between two independent vector genomes, but also by the directional bias required for successful transgene reconstitution following concatemerization. In the present study, we sought to evaluate the mechanisms by which inverted terminal repeat (ITR) sequences mediate intermolecular recombination of AAV genomes, with the goal of engineering more efficient vectors for dual-vector trans-splicing approaches. To this end, we generated a novel AAV hybrid-ITR vector characterized by an AAV-2 and an AAV-5 ITR at opposite ends of the viral genome. This hybrid genome was efficiently packaged into either AAV-2 or AAV-5 capsids to generate infectious virions. Hybrid AV2:5 ITR viruses had a significantly lower capacity to form circular intermediates in infected cells than homologous AV2:2 and AV5:5 ITR vectors despite their similar capacity to express an encoded enhanced green fluorescent protein (EGFP) transgene. To examine whether the divergent ITR sequences contained within hybrid AV2:5 ITR vectors could direct intermolecular recombination in a tail-to-head fashion, we generated two hybrid ITR trans-splicing vectors (AV5:2LacZdonor and AV2:5LacZacceptor). Each delivered one exon of a beta-galactosidase minigene flanked by donor or acceptor splice sequences. These hybrid trans-splicing vectors were compared to homologous AV5:5 and AV2:2 trans-splicing vector sets for their ability to reconstitute beta-galactosidase gene expression. Results from this comparison demonstrated that hybrid ITR dual-vector sets had a significantly enhanced trans-splicing efficiency (6- to 10-fold, depending on the capsid serotype) compared to homologous ITR vectors. Molecular studies of viral genome structures suggest that hybrid ITR vectors provide more efficient directional recombination due to an increased abundance of linear-form genomes. These studies provide direct evidence for the importance of ITR sequences in directing intermolecular and intramolecular homologous recombination of AAV genomes. The use of hybrid ITR AAV vector genomes provides new strategies to manipulate viral genome conversion products and to direct intermolecular recombination events required for efficient dual-AAV vector reconstitution of the transgene.

Adeno-associated virus serotypes 1 to 5 mediated tumor cell directed gene transfer and improvement of transduction efficiency

BACKGROUND

Gene therapy is an attractive new approach for the treatment of cancer. Therefore, the development of efficient vector systems is of crucial importance in this field. Different adeno-associated virus (AAV) serotypes have been characterized so far, which show considerable differences in tissue tropism. Consequently, we aimed to characterize the most efficient serotype for this application.

METHODS

To exclude all influences other than those provided by the capsid, all serotypes contained the same transgene cassette flanked by the AAV2 inverted terminal repeats. We systematically compared these vectors for efficiency in human cancer cell directed gene transfer. In order to identify limiting steps, the influence of second-strand synthesis and proteasomal degradation of AAV in a poorly transducible cell line were examined.

RESULTS

AAV2 was the most efficient serotype in all solid tumor cells and primary melanoma cells with transduction rates up to 98 +/- 0.3%. Transduction above 70% could be reached with serotypes 1 (in cervical and prostate carcinoma) and 3 (in cervical, breast, prostate and colon carcinoma) using 1000 genomic particles per cell. In the colon carcinoma cell line HT-29 proteasomal degradation limited AAV1-AAV4-mediated gene transfer. Moreover, inefficient second-strand synthesis prevents AAV2-mediated transgene expression in this cell line.

CONCLUSIONS

Recent advances in AAV-vector technology suggest that AAV-based vectors can be used for cancer gene therapy. Our comparative analysis revealed that, although AAV2 is the most promising candidate for such an application, serotypes 1 and 3 are valid alternatives. Furthermore, the use of self-complementary AAV vectors and proteasome inhibitors significantly improves cancer cell transduction.