Endosomal processing limits gene transfer to polarized airway epithelia by adeno-associated virus

Adenovirus 5-fiber 35 chimeric vector mediates efficient apical correction of the cystic fibrosis transmembrane conductance regulator defect in cystic fibrosis primary airway epithelia

In vivo gene transfer to the human respiratory tract by adenovirus serotype 5 (Ad5) vectors has revealed their limitations related to inefficient gene transfer, host antiviral response, and innate adenoviral toxicity. In the present work, we compared the cytotoxicity and efficiency of Ad5 and a chimeric Ad5F35 vector with respect to CFTR gene transfer to cystic fibrosis (CF) and non-CF human airway epithelial cells. We found that high doses of Ad5 vector had an adverse effect on the function of exogenous and endogenous CFTR. Results obtained with Ad5 capsid mutants suggested that the RGD motifs on the penton base capsomers were responsible for the negative effect on CFTR function. This negative interference did not result from a lower level of biosynthesis and/or altered cellular trafficking of the CFTR protein, but rather from an indirect mechanism of functional blockage of CFTR, related to the RGD integrin-mediated endocytic pathway of Ad5. No negative interference with CFTR was observed for Ad5F35, an Ad5-based vector pseudotyped with fibers from Ad35, a serotype that uses another cell entry pathway. In vitro, Ad5F35 vector expressing the GFP-tagged CFTR (Ad5F35-GFP-CFTR) showed a 30-fold higher efficiency of transduction and chloride channel correction in CFTR-deficient cells, compared with Ad5GFP-CFTR. Ex vivo, Ad5F35-GFP-CFTR had the capacity to transduce efficiently reconstituted airway epithelia from patients with CF (CF-HAE) via the apical surface, restored chloride channel function at relatively low vector doses, and showed relatively stable expression of GFP-CFTR for several weeks.

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.

A capsid-encoded PPxY-motif facilitates adenovirus entry

Viruses use cellular machinery to enter and infect cells. In this study we address the cell entry mechanisms of nonenveloped adenoviruses (Ads). We show that protein VI, an internal capsid protein, is rapidly exposed after cell surface attachment and internalization and remains partially associated with the capsid during intracellular transport. We found that a PPxY motif within protein VI recruits Nedd4 E3 ubiquitin ligases to bind and ubiquitylate protein VI. We further show that this PPxY motif is involved in rapid, microtubule-dependent intracellular movement of protein VI. Ads with a mutated PPxY motif can efficiently escape endosomes but are defective in microtubule-dependent trafficking toward the nucleus. Likewise, depletion of Nedd4 ligases attenuates nuclear accumulation of incoming Ad particles and infection. Our data provide the first evidence that virus-encoded PPxY motifs are required during virus entry, which may be of significance for several other pathogens.

BAAV transcytosis requires an interaction with beta-1-4 linked- glucosamine and gp96

Cell surface carbohydrates play an important role in virus entry and intracellular trafficking. Bovine Adeno-Associated Virus (BAAV) uses plasma membrane gangliosides for transduction and infection. In addition, independent of the infectious pathway, BAAV also has the ability to pass through barrier epithelia and endothelia using a transcytosis pathway dependent upon the presence of cell surface carbohydrates. Thus, in order to better define the carbohydrate interactions that are necessary for BAAV infection or transcytosis, a glycan microarray composed of both natural and synthetic carbohydrates was probed with HA-tagged BAAV particles. This identified chitotriose, a trimer of β-1-4-linked N-acetyl glucosamine, as having an interaction with BAAV. Competition experiments showed that the BAAV interaction with this carbohydrate is not necessary for infection but is instead important in the transcytosis pathway. The β-1-4-linked N-acetyl glucosamine modification has been reported on gp96, a glycoprotein involved in the transcytosis of bacteria and toxins. Significantly, immunoprecipitation and competition experiments with an anti-gp96 antibody and a soluble form of gp96, respectively, showed this glycoprotein can also interact with BAAV to serve as a receptor for its transcytosis.

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.

Adeno-associated virus serotype-9 mediated retinal outer plexiform layer transduction is mainly through the photoreceptors

Due to its high ocular transduction, low immune clearance and capability to bypass the brain blood barrier, adeno-associated virus-9 (AAV9) has been regarded as a promising vector for retinal disease gene therapy. We recently demonstrated that AAV9 efficiently transduces the retinal outer plexiform layer (OPL). The OPL consists of synapses formed between axons of the rod and cone photoreceptors (cell bodies in the outer nuclear layer, ONL) and dendrites of bipolar and horizontal cells (cell bodies in the inner nuclear layer, INL). It is not clear whether AAV9 transduces the OPL through the photoreceptors in the ONL or through bipolar and horizontal cells in the INL. To map the subcelluar pathway(s) involved in AAV9-mediated OPL transduction, we delivered subretinally AAV9.CMV.eGFP, an AAV vector carrying the enhanced green fluorescent protein gene (eGFP, 1 x 10(10) viral genome particles in microliter), to young (21-day-old) and adult (2- to 3-month-old) C57BL/6 mice. Four weeks after subretinal injection, eGFP expression was examined on retinal cryosections. PSD95 (postsynaptic density protein, a marker for photoreceptor terminals), CtBP2 (C-terminal binding protein 2, a marker for the photoreceptor synaptic ribbon), PKCalpha (protein kinase Calpha, a marker for rod bipolar cells), and calbindin (a marker for horizontal cells) were localized by immunofluorescence staining. In AAV9 infected retina, eGFP expression was seen in the retinal pigment epithelia, photoreceptor inner segments, ONL, OPL, Müller cells in the INL, inner plexiform layer and ganglion cell layer. Interestingly, eGFP expression co-localized with PSD95 and CtBP2, but not with PKCalpha and calbindin. Our results suggest that AAV9 transduces the photoreceptor side of the synapses in the OPL rather than the dendrites of bipolar and horizontal cells.

Ex vivo serotype-specific transduction of equine joint tissue by self-complementary adeno-associated viral vectors

Cell transplantation for the treatment of joint disease is an important clinical tool. Genetic modification of cells before transplantation has shown enhanced healing. Ex vivo genetic modification of joint tissue cells with various adeno-associated virus (AAV) serotypes has not been investigated. The transduction efficiencies of self-complementary AAV serotypes (1-6 and 8) were determined in joint tissue containing chondrocytes and synoviocytes isolated from equine models. When comparing scAAV serotypes for efficient transduction ex vivo, in chondrocytes versus synoviocytes, serotypes 6 and 2, and serotypes 3 and 2, respectively, appeared superior for gene expression. Unlike adenoviral vectors, no upregulation of inflammatory markers, such as matrix metalloproteinases and aggrecanase, was seen on treatment of joint tissue with AAV vectors ex vivo. Our findings also corroborate that ex vivo transduction of joint tissue can result in high transgene protein levels over time, and transplantation modalities might be feasible using AAV vectors in the treatment of joint-related diseases.

Generation of novel AAV variants by directed evolution for improved CFTR delivery to human ciliated airway epithelium

Recombinant adeno-associated virus (AAV) vectors expressing the cystic fibrosis transmembrane conductance regulator (CFTR) gene have been used to deliver CFTR to the airway epithelium of cystic fibrosis (CF) patients. However, no significant CFTR function has been demonstrated likely due to low transduction efficiencies of the AAV vectors. To improve AAV transduction efficiency for human airway epithelium (HAE), we generated a chimeric AAV library and performed directed evolution of AAV on an in vitro model of human ciliated airway epithelium. Two independent and novel AAV variants were identified that contained capsid components from AAV-1, AAV-6, and/or AAV-9. The transduction efficiencies of the two novel AAV variants for human ciliated airway epithelium were three times higher than that for AAV-6. The novel variants were then used to deliver CFTR to ciliated airway epithelium from CF patients. Here we show that our novel AAV variants, but not the parental, AAV provide sufficient CFTR delivery to correct the chloride ion transport defect to ~25% levels measured in non-CF cells. These results suggest that directed evolution of AAV on relevant in vitro models will enable further improvements in CFTR gene transfer efficiency and the development of an efficacious and safe gene transfer vector for CF lung disease.

Molecular evolution of adeno-associated virus for enhanced glial gene delivery

The natural tropism of most viral vectors, including adeno-associated viral (AAV) vectors, leads to predominant transduction of neurons and epithelia within the central nervous system (CNS) and retina. Despite the clinical relevance of glia for homeostasis in neural tissue, and as causal contributors in genetic disorders such as Alzheimer's and amyotrophic lateral sclerosis, efforts to develop more efficient gene delivery vectors for glia have met with limited success. Recently, viral vector engineering involving high-throughput random diversification and selection has enabled the rapid creation of AAV vectors with valuable new gene delivery properties. We have engineered novel AAV variants capable of efficient glia transduction by employing directed evolution with a panel of four distinct AAV libraries, including a new semi-random peptide replacement strategy. These variants transduced both human and rat astrocytes in vitro up to 15-fold higher than their parent serotypes, and injection into the rat striatum yielded astrocyte transduction levels up to 16% of the total transduced cell population, despite the human astrocyte selection platform. Furthermore, one variant exhibited a substantial shift in tropism toward Müller glia within the retina, further highlighting the general utility of these variants for efficient glia transduction in multiple species within the CNS and retina.

Proteasome inhibitors decrease AAV2 capsid derived peptide epitope presentation on MHC class I following transduction

Adeno-associated viral (AAV) vectors are an extensively studied and highly used vector platform for gene therapy applications. We hypothesize that in the first clinical trial using AAV to treat hemophilia B, AAV capsid proteins were presented on the surface of transduced hepatocytes, resulting in clearance by antigen-specific CD8+ T cells and consequent loss of therapeutic transgene expression. It has been previously shown that proteasome inhibitors can have a dramatic effect on AAV transduction in vitro and in vivo. Here, we describe using the US Food and Drug Administration-approved proteasome inhibitor, bortezomib, to decrease capsid antigen presentation on hepatocytes in vitro, whereas at the same time, enhancing gene expression in vivo. Using an AAV capsid-specific T-cell reporter (TCR) line to analyze the effect of proteasome inhibitors on antigen presentation, we demonstrate capsid antigen presentation at low multiplicities of infection (MOIs), and inhibition of antigen presentation at pharmacologic levels of bortezomib. We also demonstrate that bortezomib can enhance Factor IX (FIX) expression from an AAV2 vector in mice, although the same effect was not observed for AAV8 vectors. A pharmacological agent that can enhance AAV transduction, decrease T-cell activation/proliferation, and decrease capsid antigen presentation would be a promising solution to obstacles to successful AAV-mediated, liver-directed gene transfer in humans.

Evidence for the failure of adeno-associated virus serotype 5 to package a viral genome > or = 8.2 kb

Limited packaging capacity hinders adeno-associated virus (AAV) gene therapy. A recent study seems to have provided a solution to this problem. Allocca et al. reported that AAV-5 could package an 8.9 kb vector genome. Here we tested whether this approach can be used to deliver a large genome for Duchenne muscular dystrophy (DMD) gene therapy. We first evaluated AAV-5 packaging of an 8.2 kb genome. This vector carries two independent reporter gene cassettes, one for alkaline phosphatase (AP) and another for LacZ. Viral yield was log-fold lower than that of a regular AAV-5. Nevertheless, both AP and LacZ genes were detected in purified virus. Injection to dystrophic muscle resulted in both AP and LacZ expression. On electron microscopy, virion structure appeared normal. Surprisingly, we did not find the full-length single-stranded viral genome by alkaline gel electrophoresis. Neither did we see the full-length double-stranded replication forms in adenovirus coinfected cells. We suspect that AP and LacZ expression may have come from partially packaged 5' or 3'-half of the genome. Additional studies revealed failure of AAV-5 to package and express an 8.7 kb minidystrophin gene cassette. In summary, our results do not support the extraordinary packaging capacity of AAV-5.

Genetic modification of adeno-associated viral vector type 2 capsid enhances gene transfer efficiency in polarized human airway epithelial cells

Cystic fibrosis (CF) is a common genetic disease characterized by defects in the expression of the CF transmembrane conductance regulator (CFTR) gene. Gene therapy offers better hope for the treatment of CF. Adeno-associated viral (AAV) vectors are capable of stable expression with low immunogenicity. Despite their potential in CF gene therapy, gene transfer efficiency by AAV is limited because of pathophysiological barriers in these patients. Although a few AAV serotypes have shown better transduction compared with the AAV2-based vectors, gene transfer efficiency in human airway epithelium has still not reached therapeutic levels. To engineer better AAV vectors for enhanced gene delivery in human airway epithelium, we developed and characterized mutant AAV vectors by genetic capsid modification, modeling the well-characterized AAV2 serotype. We genetically incorporated putative high-affinity peptide ligands to human airway epithelium on the GH loop region of AAV2 capsid protein. Six independent mutant AAV were constructed, containing peptide ligands previously reported to bind with high affinity for known and unknown receptors on human airway epithelial cells. The vectors were tested on nonairway cells and nonpolarized and polarized human airway epithelial cells for enhanced infectivity. One of the mutant vectors, with the peptide sequence THALWHT, not only showed the highest transduction in undifferentiated human airway epithelial cells but also indicated significant transduction in polarized cells. Interestingly, this modified vector was also able to infect cells independently of the heparan sulfate proteoglycan receptor. Incorporation of this ligand on other AAV serotypes, which have shown improved gene transfer efficiency in the human airway epithelium, may enhance the application of AAV vectors in CF gene therapy.

Strategies to modulate immune responses: a new frontier for gene therapy

The success of gene therapy strategies to cure disease relies on the control of unwanted immune responses to transgene products, genetically modified cells and/or to the vector. Effective treatment of an established immune response is much harder to achieve than prevention of a response before it has had a chance to develop. However, preventive strategies are not always effective in avoiding immune responses, thus the use of drugs to induce immunosuppression (IS) is required. The growing discovery of novel drugs provides a conceptual shift from using generalized, moderately intensive immunosuppressive regimens towards a refined approach to attain the optimal balance of naive cells, effector cells, memory cells, and regulatory cells, harnessing the natural tolerance mechanisms of the body. We review several strategies based on transient IS coupled with gene therapy for sustained immune tolerance induction to the therapeutic transgene.

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.

Adeno-associated virus serotype-9 efficiently transduces the retinal outer plexiform layer

PURPOSE

Adeno-associated virus serotype-9 (AAV-9) is a promising gene delivery vector. In this study, we evaluated AAV-9 transduction in the mouse retina.

METHODS

Three different AAV vectors were used in our study: AAV-9.RSV.AP, AAV-9.CMV.eGFP, and AAV-9.CMV.R4-23/C. In these vectors, two different promoters (the cytomegalovirus promoter-CMV promoter and the Rous sarcoma virus-RSV promoter) were used to express three different transgenes including the alkaline phosphatase (AP) gene, the enhanced green fluorescent protein (eGFP) gene, and a therapeutic microdystrophin gene (the R4-23/C). Specifically, 1 microl AAV-9 reporter gene vectors (1x10(9) viral genome particles of AAV-9.RSV.AP or 1x10(10) viral genome particles of AAV-9.CMV.eGFP) were administered subretinally to young (2-3-week-old), adult (3-month-old), and old (12-month-old) C57BL/6J mice. To evaluate AAV-9 transduction in a diseased retina, we injected subretinally 1x10(9) viral genome particles of AAV-9.CMV.R4-23/C to mdx(3cv) mice, which we used as a model for Duchenne muscular dystrophy (DMD). Transgene expression was examined by histochemical as well as immunofluorescence staining at three and five weeks after injection. Electroretinograms were recorded five weeks after subretinal AAV-9.RSV.AP injection.

RESULTS

Subretinal injection yielded widespread transduction throughout the retina in all age groups. Robust expression was seen in the retinal pigment epithelium, outer nuclear layer, and in Müller cells. Interestingly a synaptic layer, the outer plexiform layer (OPL), also showed intensive expression. Transduction of the synaptic layer was further confirmed by immunostaining for C-terminal binding protein 2 (CtBP2), a marker for the photoreceptor synaptic ribbon. Dystrophin is normally expressed in the OPL photoreceptor terminals. This expression is lost in DMD patients and mdx(3cv) mice. Consistent with our findings in normal mice, we observed efficient microdystrophin expression in the OPL after AAV-9.CMV.R4-23/C infection. At five weeks after subretinal delivery of AAV-9.RSV.AP, no morphology or ERG abnormalities were observed.

CONCLUSIONS

We demonstrated that AAV-9 is a potent vector for retinal gene delivery. Furthermore, subretinal AAV-9 administration did not cause appreciable acute retinal damages. In summary, AAV-9-mediated OPL transduction holds promise for treating diseases that primarily affect this layer.

Capsid antigen presentation flags human hepatocytes for destruction after transduction by adeno-associated viral vectors

Adeno-associated virus (AAV) vectors are effective gene delivery vehicles mediating long-lasting transgene expression. Data from a clinical trial of AAV2-mediated hepatic transfer of the Factor IX gene (F9) into hemophilia B subjects suggests that CTL responses against AAV capsid can eliminate transduced hepatocytes and prevent long-term F9 expression. However, the capacity of hepatocytes to present AAV capsid-derived antigens has not been formally demonstrated, nor whether transduction by AAV sensitizes hepatocytes for CTL-mediated destruction. To investigate the fate of capsids after transduction, we engineered a soluble TCR for the detection of capsid-derived peptide:MHC I (pMHC) complexes. TCR multimers exhibited antigen and HLA specificity and possessed high binding affinity for cognate pMHC complexes. With this reagent, capsid pMHC complexes were detectable by confocal microscopy following AAV-mediated transduction of human hepatocytes. Although antigen presentation was modest, it was sufficient to flag transduced cells for CTL-mediated lysis in an in vitro killing assay. Destruction of hepatocytes was inhibited by soluble TCR, demonstrating a possible application for this reagent in blocking undesirable CTL responses. Together, these studies provide a mechanism for the loss of transgene expression and transient elevations in aminotransferases following AAV-mediated hepatic gene transfer in humans and a potential therapeutic intervention to abrogate these limitations imposed by the host T cell response.

Functional cystic fibrosis transmembrane conductance regulator expression in cystic fibrosis airway epithelial cells by AAV6.2-mediated segmental trans-splicing

Cystic fibrosis is characterized by deficiency of the cystic fibrosis transmembrane conductance regulator (CFTR), a Cl(-) transporter. The packaging constraints of adeno-associated viral (AAV) vectors preclude delivery of both an active promoter and CFTR cDNA to target cells. We hypothesized that segmental trans-splicing, in which two AAV vectors deliver the 5' and 3' halves of the CFTR cDNA, could mediate splicing of two pre-mRNAs into a full-length, functional CFTR mRNA. Using a segmental trans-splicing 5' donor-3' acceptor pair that split the CFTR cDNA between exons 14a and 14b, cotransfection of donor and acceptor plasmids into CFTR(-) cells resulted in full-length CFTR message and protein. Microinjection of plasmids into CFTR(-) cells produced cAMP-activated Cl(-) conductance. Vectors created with an engineered human serotype, AAV6.2, were used to deliver CFTR donor and acceptor constructs, resulting in full-length CFTR mRNA and protein as well as cAMP-activated Cl(-) conductance in CFTR(-) cells, including human CF airway epithelial IB3-1 cells. Thus, segmental trans-splicing can be used with AAV vectors to mediate expression of CFTR, a strategy potentially applicable to individuals with CF.

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.

Gene therapy for cystic fibrosis

Cystic Fibrosis (CF) is an autosomal recessive disorder due to mutations in the CF transmembrane conductance regulator (CFTR) gene that lead to defective ion transport in the conducting pulmonary airways and exocrine glands. Through a process that is not fully understood, CFTR defects predispose affected patients to chronic endobronchial infections with organisms such as Pseudomonas aeruginosa and Staphylococcus aureus. Following the discovery of the CFTR gene in 1989, CF became one of the primary targets for gene therapy research. Early enthusiasm surrounded the new field of gene therapy during most of the 1990s and it led academics and clinicians on a big effort to apply gene therapy for cystic fibrosis. Clinical studies have been pursued using recombinant adenovirus, recombinant adeno-associated virus, cationic liposomes, and cationic polymer vectors. Although to this date no dramatic therapeutic benefits have been observed, a lot of information has been gained from the pre-clinical and clinical studies that were performed. This learning curve has led to the optimization of vector technology and an appreciation of immune and mechanical barriers that have to be overcome for successful delivery.

Involvement of glycosaminoglycans in vesicular stomatitis virus G glycoprotein pseudotyped lentiviral vector-mediated gene transfer into airway epithelial cells

BACKGROUND

The involvement of surface molecules in HIV-1-derived lentivirus (LV)-mediated transduction of airway epithelial cells has not been studied so far. The present study aimed to evaluate the role of glycosaminoglycans (GAGs) in gene transfer mediated by a third generation vesicular stomatitis virus G glycoprotein (VSV-G) pseudotyped LV vector in an in vitro model of polarized airway epithelial cells.

METHODS

Human bronchial (16HBE-S1) and tracheal (CFT1-C2) epithelial cells were grown either on plastic or on filters and transduced with the LV vector polypurine tract (PPT)-green fluoresecent protein (GFP). Zonula Occludens (ZO)-1, a marker of tight junction, and GAG localization were assessed by cytofluorimetry and confocal microscopy. Soluble GAGs and removal of cell surface GAGs were used to affect LV-mediated transduction.

RESULTS

Extensive optimization of experimental parameters (presence of polybrene during the infection, the incubation time in the presence of LV particles, period of time intercurring between infection and gene expression analysis) was carried out in plastic-adherent cells. Polybrene resulted to be cytotoxic and was not further used. In CFT1-C2 polarized cells, EGTA treatment determined a 20% decrease in transepithelial resistance, a diminished ZO-1 localization at the tight junction location and a 31% increase in GFP positive cells. Heparane sulfate was distributed evenly on the cell surface. Heparin and soluble chondroitin sulfate A and B inhibited LV-mediated transduction in a dose-dependent fashion. These results were confirmed upon enzymatic removal of GAGs from the cell surface.

CONCLUSIONS

Taken together, these results show that GAGs are involved in VSV-G LV transduction of airway epithelial cells.

Transient cytochalasin-D treatment induces apically administered rAAV2 across tight junctions for transduction of enterocytes

Enteropathogens are known to disrupt apical actin filaments and/or tight-junction barriers of intestinal epithelial cells to promote infection. In this study, we show that a controlled, cytochalasin-D (Cyto-D)-mediated disruption of actin filaments and tight junctions enhanced the apical delivery of the gene-therapy vector recombinant adeno-associated virus serotype 2 (rAAV2). This increase in transduction efficiency can be attributed to the enhanced delivery of rAAV2 across the Cyto-D disrupted tight junctions, allowing basolateral entry of rAAV2. Previously, we have shown that MG101 and doxorubicin are capable of overcoming proteasome-mediated transduction barriers of rAAV2 in enterocytes. In this study, when Cyto-D was combined with MG101 and doxorubicin in apical delivery of rAAV2 to transduce the differentiated Caco-2 enterocytes, a synergistic >2300-fold increase in transgene expression was achieved. We conclude that Cyto-D is capable of permeating the polarized enterocytes for rAAV2 transduction, which may potentially be a useful device to facilitate intestinal gene transfer via the gut lumen.

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.

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.

Tailoring the AAV vector capsid for gene therapy

A number of preclinical studies have shown the adeno-associated virus (AAV) to be an efficient vehicle for gene therapy. Clinical studies successfully demonstrated its potential for in vivo gene transfer. The complexity of host-vector interactions when progressing from small to large animal models, and eventually to humans, has impeded translation of AAV technology to the clinic. One approach to address this complexity has been to explore the biological characteristics of variations in AAV capsid structure. Initial strategies characterized the naturally occurring capsid variants from mammalian species. The structural and functional knowledge gathered on these natural AAV variants as vectors has led to the first series of second-generation vectors that aim at specifically improving certain properties by rational design of the capsid. A third exciting approach uses directed evolution to isolate vectors that are able to overcome selective pressures applied in the laboratory and thereby steer the capsid to evolve toward improved functionality.

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.

Recent developments in adeno-associated virus vector technology

Adeno-associated virus (AAV), a single-stranded DNA parvovirus, is emerging as one of the leading gene therapy vectors owing to its nonpathogenicity and low immunogenicity, stability and the potential to integrate site-specifically without known side-effects. A portfolio of recombinant AAV vector types has been developed with the aim of optimizing efficiency, specificity and thereby also the safety of in vitro and in vivo gene transfer. More and more information is now becoming available about the mechanism of AAV/host cell interaction improving the efficacy of recombinant AAV vector (rAAV) mediated gene delivery. This review summarizes the current knowledge of the infectious biology of AAV, provides an overview of the latest developments in the field of AAV vector technology and discusses remaining challenges.

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.

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.

Engineering adeno-associated virus 2 vectors for targeted gene delivery to atherosclerotic lesions

Targeted delivery of biological agents to atherosclerotic plaques may provide a novel treatment and/or useful tool for imaging of atherosclerosis in vivo. However, there are no known viral vectors that possess the desired tropism. Two plaque-targeting peptides, CAPGPSKSC (CAP) and CNHRYMQMC (CNH) were inserted into the capsid of adeno-associated virus 2 (AAV2) to assess vector retargeting. AAV2-CNH produced significantly higher levels of transduction than unmodified AAV2 in human, murine and rat endothelial cells, whereas transduction of nontarget HeLa cells was unaltered. Transduction studies and surface plasmon resonance suggest that AAV2-CNH uses membrane type 1 matrix metalloproteinase as a surface receptor. AAV2-CAP only produced higher levels of transduction in rat endothelial cells, possibly because the virus was found to be affected by proteasomal degradation. In vivo substantially higher levels of both peptide-modified AAV2 vectors was detected in the brachiocephalic artery (site of advanced atherosclerotic plaques) and aorta, whereas reduced levels were detected in all other organs examined. These results suggest that in the AAV2 platform the peptides are exposed on the capsid surface in a way that enables efficient receptor binding and so creates effective atherosclerotic plaque targeted vectors.

A novel class of intrinsic proteasome inhibitory gene transfer peptides

Proteasomes are multisubunit enzymes responsible for the degradation of many cytosolic proteins. The inhibition of the proteasome has been the subject of intense interest in the development of drug therapies. We have previously demonstrated that simultaneous administration of a tripeptide aldehyde proteasome inhibitor (MG115 or MG132) with a peptide (Cys-Trp-Lys18) DNA condensate boosted gene expression by 30-fold in cell culture. In the present study, we have developed a convergent synthesis to allow the incorporation of a proteasome inhibitor tripeptide into the C-terminal end of a gene delivery peptide. The resulting peptides formed DNA condensates that mediated a 100-fold enhancement in gene expression over a control peptide lacking all or part of the tripeptide inhibitor. Gene transfer peptides possessing intrinsic proteasome inhibitors were also found to be nontoxic to cells in culture. These results suggest that intrinsic proteasome inhibition may also be used to boost the efficiency of peptide-mediated nonviral gene delivery systems in vivo.

The Mre11/Rad50/Nbs1 complex limits adeno-associated virus transduction and replication

Adeno-associated virus (AAV) is a parvovirus with a small single-stranded DNA genome that relies on cellular replication machinery together with functions supplied by coinfecting helper viruses. The impact of host factors on AAV infection is not well understood. We explored the connection between AAV helper functions supplied by adenovirus and cellular DNA repair proteins. The adenoviral E1b55K/E4orf6 proteins induce degradation of the cellular Mre11 repair complex (MRN) to promote productive adenovirus infection. These viral proteins also augment recombinant AAV transduction and provide crucial helper functions for wild-type AAV replication. Here, we show that MRN poses a barrier to AAV and that the helper function provided by E1b55K/E4orf6 involves MRN degradation. Using a fluorescent method to visualize the viral genome, we show an effect at the viral DNA level. MRN components accumulate at AAV replication centers and recognize the viral inverted terminal repeats. Together, our data suggest that AAV is targeted by MRN and has evolved to exploit adenoviral proteins that degrade these cellular factors.

Biological Differences in rAAV Transduction of Airway Epithelia in Humans and in Old World Non-human Primates

Non-human primates (NHPs) are considered to be among the most relevant animal models for pre-clinical testing of human therapies, on the basis of their close evolutionary relatedness to humans in terms of organ cell biology and physiology. In this study, we sought to investigate whether NHP models accurately reflect the effectiveness of recombinant adeno-associated virus (rAAV)-mediated gene delivery to the airway in humans. In order to do this, we utilized an identical model system of differentiated airway epithelia from Indian Rhesus monkeys and from humans, cultured at an air-liquid interface (ALI). In addition to assessing the biology of rAAV-mediated transduction for three serotypes, we characterized the bioelectric properties as a reference for biological similarities and differences between the cell cultures from the two species. Our results demonstrate that airway epithelia from NHPs and humans have very similar Na(+) and Cl(-) transport properties. In contrast, rAAV transduction of airway epithelia of NHPs demonstrated significant differences to those in humans with regard to the efficiency of apical and/or basal transduction with three rAAV serotypes (AAV1, AAV2, AAV5). These findings suggest that the IndianRhesusmonkey may not be the best model for preclinical testing of rAAV-mediated gene therapy to the airway in humans.

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.

Existence of transient functional double-stranded DNA intermediates during recombinant AAV transduction

Previous studies have documented that 0.1 approximately 1% of input recombinant adeno-associated virus (rAAV) vectors could be stabilized and lead to transgene expression. To characterize the steps involving massive AAV DNA loss, we designed an"AAV footprinting" strategy that can track newly formed AAV dsDNA genomes. This strategy is based on an ROSA26R mouse model or cell line that carries a lacZ gene flanked by two loxP sites. When it is transduced by a rAAV vector carrying the Cre recombinase, the lacZ gene can be activated and remain active even when rAAV genomes are later lost. By using this sensitive AAV footprinting technique, we confirmed the existence of transient AAV dsDNA that went undetected by conventional DNA methods. Although these dsDNA intermediates could be efficiently formed in almost every cell and were competent for mRNA transcription and protein synthesis in vivo, they got lost continuously. Only a small fraction was eventually stabilized for sustained gene expression. Although both rAAV2 and rAAV8 can potentially have similar levels of dsDNA formation, AAV8 dsDNA was formed much faster than that of AAV2, which explains why rAAV8 is more efficient than rAAV2 in transducing the liver. Collectively, our studies suggested that rather than receptor binding, viral entry, and ssDNA to dsDNA conversion, the instability of newly formed AAV dsDNA was the primary contributing factor for the low rAAV transduction efficacy. The uncoating step significantly influenced the stability of AAV transient dsDNA. The identification of transient AAV dsDNA provided a new pathway for improving rAAV transduction.

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.

Sendai virus-mediated CFTR gene transfer to the airway epithelium

The potential for gene therapy to be an effective treatment for cystic fibrosis has been hampered by the limited gene transfer efficiency of current vectors. We have shown that recombinant Sendai virus (SeV) is highly efficient in mediating gene transfer to differentiated airway epithelial cells, because of its capacity to overcome the intra- and extracellular barriers known to limit gene delivery. Here, we have identified a novel method to allow the cystic fibrosis transmembrane conductance regulator (CFTR) cDNA sequence to be inserted within SeV (SeV-CFTR). Following in vitro transduction with SeV-CFTR, a chloride-selective current was observed using whole-cell and single-channel patch-clamp techniques. SeV-CFTR administration to the nasal epithelium of cystic fibrosis (CF) mice (Cftr(G551D) and Cftr(tm1Unc)TgN(FABPCFTR)#Jaw mice) led to partial correction of the CF chloride transport defect. In addition, when compared to a SeV control vector, a higher degree of inflammation and epithelial damage was found in the nasal epithelium of mice treated with SeV-CFTR. Second-generation transmission-incompetent F-deleted SeV-CFTR led to similar correction of the CF chloride transport defect in vivo as first-generation transmission-competent vectors. Further modifications to the vector or the host may make it easier to translate these studies into clinical trials of cystic fibrosis.

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 vectors forin vivogene delivery to vascular tissue

Although some success has been achieved with gene delivery in animal models of vascular disorders, the results from some clinical trials have been less promising, possibly due, in part, to the use of suboptimal vectors for in vivo gene transfer. Non-viral vectors have a very low transfection efficiency so are largely unsuitable for most in vivo applications, and the relatively broad tropism of many of the commonly used viral vectors can limit efficient gene delivery specifically to target vascular tissues. However, characterisation of novel virus serotypes and advances in techniques that enable vectors to be targeted to the required tissue have led to progress in the development of novel vectors that could be utilised for gene delivery for vascular disorders.

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.

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.

Efficient whole-body transduction with trans-splicing adeno-associated viral vectors

Limited packaging capacity has hampered adeno-associated virus (AAV)-mediated gene therapy for many common genetic diseases such as cystic fibrosis (CF) and Duchenne muscular dystrophy (DMD). Trans-splicing AAV (tsAAV) vectors double AAV packaging capacity but their transduction efficiency has been too low to be useful. We have recently overcome this hurdle by rational vector design. We have shown that a pair of optimized mini-dystrophin tsAAV vectors can reach the same transduction efficiency as that of a single AAV vector after local injection in dystrophic muscle. However, global gene transfer is required to treat diseases like DMD. To test whether systemic delivery can be achieved with tsAAV vectors, we generated a set of optimized alkaline phosphatase (AP) tsAAV vectors. We delivered AAV serotype 9 pseudotyped AP tsAAV intravenously to newborn mice. Six weeks later, we observed high-level transduction in all body skeletal muscle and the heart, the tissues that are affected in DMD. We also detected efficient transduction in the lung, the primary organ affected in CF. Our results provide the first evidence of whole-body transduction with tsAAV vectors and further raise the hope of tsAAV gene therapy for DMD and CF.

Specific adeno-associated virus serotypes facilitate efficient gene transfer into human and non-human primate mesenchymal stromal cells

Mesenchymal stromal cells (MSCs) show great promise for ex vivo gene and cell-mediated therapies. The immunophenotype and in vitro differentiation capacity of primary baboon MSCs was demonstrated to be near-identical to that observed in human MSCs. To optimize gene transfer efficiency, we compared the efficiency of serotypes 1, 2, 3, 4, 5, 6, and 8 of adeno-associated virus (AAV) vectors for their ability to mediate transduction of human and baboon MSCs. AAV serotype 2 vectors were the most efficient in transducing MSCs from humans and baboons. As a reference, human Ad293 cells were transduced with these seven AAV serotypes, and were found to have the highest transduction levels followed by baboon MSCs, and then human MSCs. The order of increasing transduction efficiency for the serotypes tested was similar for human and baboon MSCs, but was different for human Ad293 cells. The transduction efficiency of MSCs isolated from different individuals was comparable within the same species. We also demonstrated that baboon MSCs transduced with AAV serotype 2 vectors retain their potential to differentiate into adipocytes in vitro, and can incorporate into injured muscle tissue of NODSCID mice in vivo. We detected beta-galactosidase reporter gene expression in host muscle tissue for up to 9 weeks in this study, indicating engraftment of transduced baboon MSCs and sustained transgene expression in vivo.

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.

Peptide-guided gene delivery

Although currently less efficient than their viral counterparts, nonviral vectors are under intense investigation as a safer alternative for gene therapy. For successful delivery, the nonviral vector must be able to overcome many barriers to protect DNA and specifically deliver it for efficient gene expression in target cells. The use of peptides as gene delivery vectors is advantageous over other nonviral agents in that they are able to achieve all of these goals. This review will focus on the application of peptides to mediate nonviral gene delivery. By examining the literature over the past 20 years, it becomes clear that no other class of biomolecules are simultaneously capable of DNA condensation, blocking metabolism, endosomal escape, nuclear localization, and receptor targeting. Based on virtually limitless diversity of peptide sequence and function information from nature, it is increasingly clear that peptide-guided gene delivery is still in its infancy.

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.