Production and characterization of adeno-associated viral vectors

AAV mediated GDNF secretion from retinal glia slows down retinal degeneration in a rat model of retinitis pigmentosa

Mutations in over 80 identified genes can induce apoptosis in photoreceptors, resulting in blindness with a prevalence of 1 in 3,000 individuals. This broad genetic heterogeneity of disease impacting a wide range of photoreceptor functions renders the design of gene-specific therapies for photoreceptor degeneration impractical and necessitates the development of mutation-independent treatments to slow photoreceptor cell death. One promising strategy for photoreceptor neuroprotection is neurotrophin secretion from Müller cells, the primary retinal glia. Müller glia are excellent targets for secreting neurotrophins as they span the entire tissue, ensheath all neuronal populations, are numerous, and persist through retinal degeneration. We previously engineered an adeno-associated virus (AAV) variant (ShH10) capable of efficient and selective glial cell transduction through intravitreal injection. ShH10-mediated glial-derived neurotrophic factor (GDNF) secretion from glia, generates high GDNF levels in treated retinas, leading to sustained functional rescue for over 5 months. This GDNF secretion from glia following intravitreal vector administration is a safe and effective means to slow the progression of retinal degeneration in a rat model of retinitis pigmentosa (RP) and shows significant promise as a gene therapy to treat human retinal degenerations. These findings also demonstrate for the first time that glia-mediated secretion of neurotrophins is a promising treatment that may be applicable to other neurodegenerative conditions.

Hydroxylation of 5-methylcytosine by TET1 promotes active DNA demethylation in the adult brain

Cytosine methylation is the major covalent modification of mammalian genomic DNA and plays important roles in transcriptional regulation. The molecular mechanism underlying the enzymatic removal of this epigenetic mark, however, remains elusive. Here, we show that 5-methylcytosine (5mC) hydroxylase TET1, by converting 5mCs to 5-hydroxymethylcytosines (5hmCs), promotes DNA demethylation in mammalian cells through a process that requires the base excision repair pathway. Though expression of the 12 known human DNA glycosylases individually did not enhance removal of 5hmCs in mammalian cells, demethylation of both exogenously introduced and endogenous 5hmCs is promoted by the AID (activation-induced deaminase)/APOBEC (apolipoprotein B mRNA-editing enzyme complex) family of cytidine deaminases. Furthermore, Tet1 and Apobec1 are involved in neuronal activity-induced, region-specific, active DNA demethylation and subsequent gene expression in the dentate gyrus of the adult mouse brain in vivo. Our study suggests a TET1-induced oxidation-deamination mechanism for active DNA demethylation in mammals.

Peptide affinity reagents for AAV capsid recognition and purification

We report the discovery of AAV capsid-binding peptides identified through phage panning. The heptapeptide motif GYVSRHP selectively recognized AAV serotype 8 capsids and blocked transduction in vitro. Recombinant AAV8 vectors were purified directly from crude cell lysate and supernatant through sequential application of peptide affinity and anion exchange chromatography. Peptide affinity reagents may serve as useful alternatives to monoclonal antibodies in AAV capsid recognition, and offer readily scalable solutions for purification of clinical grade AAV vectors.

RNA-driven genetic changes in bacteria and in human cells

As recently demonstrated in the yeast Saccharomyces cerevisiae model organism using synthetic RNA-containing oligonucleotides (oligos), RNA can serve as a template for DNA synthesis at the chromosomal level during the process of double-strand break (DSB) repair. Herein we show that the phenomenon of RNA-mediated DNA modification and repair is not limited to yeast cells. A tract of six ribonucleotides embedded in single-strand DNA oligos corresponding to either lagging or leading strand sequences could serve as a template to correct a defective lacZ marker gene in the chromosome of the bacterium Escherichia coli. In order to test the capacity of RNA to modify DNA in mammalian cells, we utilized DNA oligos containing an embedded tract of six ribonucleotides, as well as oligos mostly made of RNA. These oligos were designed to repair a chromosomal break generated within a copy of the green fluorescent protein (GFP) gene randomly integrated into the genome of human HEK-293 cells. We show that these RNA-containing oligos can serve as templates to repair a DSB in human cells and can introduce base changes into genomic or plasmid DNA. In both E. coli and human cells, the strand bias of chromosomal gene correction by the single-strand RNA-containing oligos was the same as that obtained for the corresponding DNA molecules. Therefore, the RNA-containing oligos are not converted into a cDNA before annealing with complementary DNA. Overall, we demonstrate that in both bacterial and human cells, as in yeast, RNA sequences can have a direct role in DNA genetic modification and remodeling.

The influence of epileptic neuropathology and prior peripheral immunity on CNS transduction by rAAV2 and rAAV5

Adeno-associated virus (AAV) provides a promising platform for clinical treatment of neurological disorders owing to its established efficacy and lack of apparent pathogenicity. To use viral vectors in treating neurological disease, however, transduction must occur under neuropathological conditions. Previous studies in rodents have shown that AAV5 more efficiently transduces cells in the hippocampus and piriform cortex than AAV2. Using the kainic acid (KA) model of temporal lobe epilepsy and AAV2 and 5 carrying a hybrid chicken β-actin promoter driving green fluorescent protein (GFP), we found that limbic seizure activity caused substantial neuropathology and resulted in a significant reduction in subsequent AAV5 transduction. Nonetheless, this reduced transduction still was greater than AAV2 transduction in control rats. Although KA seizures compromise blood-brain barrier function, potentially increasing exposure of target tissue to circulating neutralizing antibodies, we observed no interaction between KA seizure-induced damage and immunization status on AAV transduction. Finally, while we confirmed the near total neuronal-specific transgene expression for both serotypes in control rats, AAV5-GFP expression was increasingly localized to astrocytes in seizure-damaged areas. Thus, the pathological milieu of the injured brain can reduce transduction efficacy and alter viral tropism- both relevant concerns when considering viral vector gene therapy for neurological disorders.

AAV-mediated gene targeting methods for human cells

Gene targeting with adeno-associated virus (AAV) vectors has been demonstrated in multiple human cell types, with targeting frequencies ranging from 10(-5) to 10(-2) per infected cell. These targeting frequencies are 1-4 logs higher than those obtained by conventional transfection or electroporation approaches. A wide variety of different types of mutations can be introduced into chromosomal loci with high fidelity and without genotoxicity. Here we provide a detailed protocol for gene targeting in human cells with AAV vectors. We describe methods for vector design, stock preparation and titration. Optimized transduction protocols are provided for human pluripotent stem cells, mesenchymal stem cells, fibroblasts and transformed cell lines, as well as a method for identifying targeted clones by Southern blots. This protocol (from vector design through a single round of targeting and screening) can be completed in ∼10 weeks; each subsequent round of targeting and screening should take an additional 7 weeks.

Adeno-associated virus serotype 2 mediated transduction and coexpression of the human apoAI and SR-BI gene in HepG2 cells

Cholesterol efflux is the first step in the reverse cholesterol transport (RCT) pathway, removing excess cholesterol from tissues, including the arterial wall, thus preventing the development of atherosclerosis. Adeno-associated virus (rAAV) has demonstrated significant promise as a DNA-delivery vector to treat serious human diseases. In this study, we constructed recombinant adeno-associated viruses coexpressing apoAI and SR-BI successfully, the double gene mRNA and protein were both strongly expressed in transduced HepG2 cells. A novel safe and efficient method of promoting the reverse cholesterol transport (RCT) may be established. These results may provide a new method for gene therapy of Arteriosclerosis.

Rapid, simple, and versatile manufacturing of recombinant adeno-associated viral vectors at scale

Adeno-associated viral (AAV) manufacturing at scale continues to hinder the application of AAV technology to gene therapy studies. Although scalable systems based on AAV-adenovirus, AAV-herpesvirus, and AAV-baculovirus hybrids hold promise for clinical applications, they require time-consuming generation of reagents and are not highly suited to intermediate-scale preclinical studies in large animals, in which several combinations of serotype and genome may need to be tested. We observed that during production of many AAV serotypes, large amounts of vector are found in the culture supernatant, a relatively pure source of vector in comparison with cell-derived material. Here we describe a high-yielding, recombinant AAV production process based on polyethylenimine (PEI)-mediated transfection of HEK293 cells and iodixanol gradient centrifugation of concentrated culture supernatant. The entire process can be completed in 1 week and the steps involved are universal for a number of different AAV serotypes. Process conditions have been optimized such that final purified yields are routinely greater than 1 x 10(14) genome copies per run, with capsid protein purity exceeding 90%. Initial experiments with vectors produced by the new process demonstrate equivalent or better transduction both in vitro and in vivo when compared with small-scale, CsCl gradient-purified vectors. In addition, the iodixanol gradient purification process described effectively separates infectious particles from empty capsids, a desirable property for reducing toxicity and unwanted immune responses during preclinical studies.

Engineering liver-detargeted AAV9 vectors for cardiac and musculoskeletal gene transfer

We report the generation of a new class of adeno-associated virus serotype 9 (AAV9)-derived vectors displaying selective loss of liver tropism and demonstrating potential for cardiac and musculoskeletal gene transfer applications. Random mutagenesis of residues within a surface-exposed region of the major AAV9 capsid protein yielded a capsid library with mutations clustered at the icosahedral threefold symmetry axis. Using a combination of sequence analysis, structural models, and in vivo screening, we identified several functionally diverse AAV9 variants. The latter were classified into three functional subgroups, with respect to parental AAV9 displaying: (i) decreased transduction efficiency across multiple tissues; (ii) a selective decrease in liver transduction, or (iii) a similar transduction profile. Notably, variants 9.45 and 9.61 (subgroup II) displayed 10- to 25-fold lower gene transfer efficiency in liver, while transducing cardiac and skeletal muscle as efficiently as AAV9. These results were further corroborated by quantitation of vector genome copies and histological analysis of reporter (tdTomato) gene expression. The study highlights the feasibility of generating AAV vectors with selectively ablated tissue tropism, which when combined with other targeting strategies could allow sharply segregated gene expression. Liver-detargeted AAV9 variants described herein are excellent candidates for preclinical evaluation in animal models of cardiac and musculoskeletal disease.

Terminal N-linked galactose is the primary receptor for adeno-associated virus 9

Sialylated glycans serve as cell surface attachment factors for a broad range of pathogens. We report an atypical example, where desialylation increases cell surface binding and infectivity of adeno-associated virus (AAV) serotype 9, a human parvovirus isolate. Enzymatic removal of sialic acid, but not heparan sulfate or chondroitin sulfate, increased AAV9 transduction regardless of cell type. Viral binding and transduction assays on mutant Chinese hamster ovary (CHO) cell lines defective in various stages of glycan chain synthesis revealed a potential role for core glycan residues under sialic acid in AAV9 transduction. Treatment with chemical inhibitors of glycosylation and competitive inhibition studies with different lectins suggest that N-linked glycans with terminal galactosyl residues facilitate cell surface binding and transduction by AAV9. In corollary, resialylation of galactosylated glycans on the sialic acid-deficient CHO Lec2 cell line with different sialyltransferases partially blocked AAV9 transduction. Quantitative analysis of AAV9 binding to parental, sialidase-treated or sialic acid-deficient mutant CHO cells revealed a 3-15-fold increase in relative binding potential of AAV9 particles upon desialylation. Finally, pretreatment of well differentiated human airway epithelial cultures and intranasal instillation of recombinant sialidase in murine airways enhanced transduction efficiency of AAV9 by >1 order of magnitude. Taken together, the studies described herein provide a molecular basis for low infectivity of AAV9 in vitro and a biochemical strategy to enhance gene transfer by AAV9 vectors in general.

Myocardial gene delivery using molecular cardiac surgery with recombinant adeno-associated virus vectors in vivo

We use a novel technique that allows for closed recirculation of vector genomes in the cardiac circulation using cardiopulmonary bypass, referred to here as molecular cardiac surgery with recirculating delivery (MCARD). We demonstrate that this platform technology is highly efficient in isolating the heart from the systemic circulation in vivo. Using MCARD, we compare the relative efficacy of single-stranded (ss) adeno-associated virus (AAV)6, ssAAV9 and self-complimentary (sc)AAV6-encoding enhanced green fluorescent protein, driven by the constitutive cytomegalovirus promoter to transduce the ovine myocardium in situ. MCARD allows for the unprecedented delivery of up to 48 green fluorescent protein genome copies per cell globally in the sheep left ventricular (LV) myocardium. We demonstrate that scAAV6-mediated MCARD delivery results in global, cardiac-specific LV gene expression in the ovine heart and provides for considerably more robust and cardiac-specific gene delivery than other available delivery techniques such as intramuscular injection or intracoronary injection; thus, representing a potential, clinically translatable platform for heart failure gene therapy.

Effects of recombinant adeno-associated viral vectors on angiopoiesis and osteogenesis in cultured rabbit bone marrow stem cells via co-expressing hVEGF and hBMP genes: a preliminary study in vitro

OBJECTIVE

VEGF and BMP play important roles in angiogenesis and osteogenesis. Combining these two factors may be a promising therapeutic strategy for avascular necrosis of the femoral head (ANFH).

METHODS

Rabbit bone marrow-derived mesenchymal stem cells (BMSCs) were isolated and purified by density gradient centrifugation combined with attachment culture methods. The purity and characteristics of the BMSCs were detected by cell surface antigen identification. The best MOI of BMSCs transfected with rAAV was detected by fluorescent cell counting, and cell viability was determined by MTT assay. Expression of the genes of interest was detected by GFP gene expression, RT-PCR assay, and ELISA assay. The biological activities of VEGF and BMP were detected by angiogenic and osteogenic assays.

RESULTS

The best MOI of BMSCs transfected with rAAV was 5 x 10(4)v.g./cell. Cell growth curves showed vigorous cell viability. Expressions of the GFP, VEGF165, and BMP(7) genes were detected 1 day post-transfection and peaked 14 days post-transfection. Expression of the genes of interest was sustained over 1 month. VEGF and BMP proteins secreted from BMSCs transfected with rAAV-hVEGF(165)-IRES-hBMP(7) enhanced angiogenesis and osteogenesis in vitro.

CONCLUSION

Recombinant adeno-associated viral vectors co-expressing the hVEGF(165) and hBMP(7) genes showed efficient gene expression ability. The VEGF(165) and BMP(7) proteins expressed from the vector have efficient biological activity in vitro.

Systemic gene transfer to skeletal muscle using reengineered AAV vectors

Gene therapy of musculoskeletal disorders warrants efficient gene transfer to a wide range of muscle groups. Reengineered adeno-associated viral (AAV) vectors that selectively transduce muscle tissue following systemic administration are attractive candidates for such applications. Here we provide examples of several lab-derived AAV vectors that display systemic tissue tropism in mice. Methods to evaluate the efficiency of gene transfer to skeletal muscle following intravenous or isolated limb infusion of AAV -vectors in mice are discussed in detail.

High efficiency of BRCA1 knockout using rAAV-mediated gene targeting: developing a pig model for breast cancer

Germline inactivating mutations of the breast cancer associated gene 1 (BRCA1) predispose to breast cancer and account for most cases of familiar breast and/or ovarian cancer. The pig is an excellent model for medical research as well as testing of new methods and drugs for disease prevention and treatment. We have generated cloned BRCA1 knockout (KO) Yucatan miniature piglets by targeting exon 11 using recombinant adeno-associated virus (rAAV)-mediated gene targeting and somatic cell nuclear transfer by Handmade Cloning (HMC). We found a very high targeting rate of rAAV-mediated BRCA1 KO. Approximately 35% of the selected cells were BRCA1 targeted. One BRCA1 KO cell clone (5D1), identified by PCR and Southern blot, was used as nuclear donor for HMC. Reconstructed embryos were transferred to three recipient sows which gave birth to 8 piglets in total. Genotyping identified seven piglets as BRCA1 heterozygotes (BRCA1(+/∆11)), and one as wild type. The BRCA1 expression was decreased at the mRNA level in BRCA1(+/∆11) fibroblasts. However, all BRCA1(+/∆11) piglets died within 18 days after birth. The causes of perinatal mortality remain unclear. Possible explanations may include a combination of the BRCA1 haploinsufficiency, problems of epigenetic reprogramming, presence of the marker gene, single cell clone effects, and/or the special genetic background of the minipigs.

AAV's anatomy: roadmap for optimizing vectors for translational success

Adeno-Associated Virus based vectors (rAAV) are advantageous for human gene therapy due to low inflammatory responses, lack of toxicity, natural persistence, and ability to transencapsidate the genome allowing large variations in vector biology and tropism. Over sixty clinical trials have been conducted using rAAV serotype 2 for gene delivery with a number demonstrating success in immunoprivileged sites, including the retina and the CNS. Furthermore, an increasing number of trials have been initiated utilizing other serotypes of AAV to exploit vector tropism, trafficking, and expression efficiency. While these trials have demonstrated success in safety with emerging success in clinical outcomes, one benefit has been identification of issues associated with vector administration in humans (e.g. the role of pre-existing antibody responses, loss of transgene expression in non-immunoprivileged sites, and low transgene expression levels). For these reasons, several strategies are being used to optimize rAAV vectors, ranging from addition of exogenous agents for immune evasion to optimization of the transgene cassette for enhanced therapeutic output. By far, the vast majority of approaches have focused on genetic manipulation of the viral capsid. These methods include rational mutagenesis, engineering of targeting peptides, generation of chimeric particles, library and directed evolution approaches, as well as immune evasion modifications. Overall, these modifications have created a new repertoire of AAV vectors with improved targeting, transgene expression, and immune evasion. Continued work in these areas should synergize strategies to improve capsids and transgene cassettes that will eventually lead to optimized vectors ideally suited for translational success.

Self-complementary AAV mediates gene targeting and enhances endonuclease delivery for double-strand break repair

Adeno-associated virus (AAV) mediates gene targeting in humans by providing exogenous DNA for allelic replacement through homologous recombination. In comparison to other methods of DNA delivery or alternative DNA substrates, AAV gene targeting is reported to be very efficient, perhaps due to its single-stranded DNA genome, the inverted terminal repeats (ITRs), and/or the consequence of induced cellular signals on infection or uncoating. These viral attributes were investigated in the presence and absence of an I-Sce endonuclease-induced double-strand break (DSB) within a chromosomal defective reporter in human embryonic kidney cells. Gene correction was evaluated using self-complementary (sc) AAV, which forms a duplexed DNA molecule and results in earlier and robust transgene expression compared with conventional single-strand (ss) AAV genomes. An scAAV repair substrate was modestly enhanced for reporter correction showing no dependency on ssAAV genomes for this process. The AAV ITR sequences were also investigated in a plasmid repair context. No correction was noted in the absence of a DSB, however, a modest inhibitory effect correlated with the increasing presence of ITR sequences. Similarly, signaling cascades stimulated upon recombinant AAV transduction had no effect on plasmid-mediated DSB repair. Noteworthy, was the 20-fold additional enhancement in reporter correction using scAAV vectors, over ss versions, to deliver both the repair substrate and the endonuclease. In this case, homologous recombination repaired the defective reporter in 4% of cells without any selection. This report provides novel insights regarding the recombination substrates used by AAV vectors in promoting homologous recombination and points to the initial steps in vector optimization that could facilitate their use in gene correction of genetic disorders.

Network mechanisms of theta related neuronal activity in hippocampal CA1 pyramidal neurons

Although hippocampal theta oscillations represent a prime example of temporal coding in the mammalian brain, little is known about the specific biophysical mechanisms. Intracellular recordings support a particular abstract oscillatory interference model of hippocampal theta activity, the soma-dendrite interference model. To gain insight into the cellular and circuit level mechanisms of theta activity, we implemented a similar form of interference using the actual hippocampal network in mice in vitro. We found that pairing increasing levels of phasic dendritic excitation with phasic stimulation of perisomatic projecting inhibitory interneurons induced a somatic polarization and action potential timing profile that reproduced most common features. Alterations in the temporal profile of inhibition were required to fully capture all features. These data suggest that theta-related place cell activity is generated through an interaction between a phasic dendritic excitation and a phasic perisomatic shunting inhibition delivered by interneurons, a subset of which undergo activity-dependent presynaptic modulation.

Changes in adeno-associated virus-mediated gene delivery in retinal degeneration

Gene therapies for retinal degeneration have relied on subretinal delivery of viral vectors carrying therapeutic DNA. The subretinal injection is clearly not ideal as it limits the viral transduction profile to a focal region at the injection site and negatively affects the neural retina by detaching it from the supportive retinal pigment epithelium (RPE). We assessed changes in adeno-associated virus (AAV) dispersion and transduction in the degenerating rat retina after intravitreal delivery. We observed a significant increase in AAV-mediated gene transfer in the diseased compared with normal retina, the extent of which depends on the AAV serotype injected. We also identified key structural changes that correspond to increased viral infectivity. Particle diffusion and transgene accumulation in normal and diseased retina were monitored via fluorescent labeling of viral capsids and quantitative PCR. Viral particles were observed to accumulate at the vitreoretinal junction in normal retina, whereas particles spread into the outer retina and RPE in degenerated tissue. Immunohistochemistry illustrates remarkable changes in the architecture of the inner limiting membrane, which are likely to underlie the increased viral transduction in diseased retina. These data highlight the importance of characterizing gene delivery vectors in diseased tissue as structural and biochemical changes can alter viral vector transduction patterns. Furthermore, these results indicate that gene delivery to the outer nuclear layer may be achieved by noninvasive intravitreal AAV administration in the diseased state.

Restoration of cone vision in the CNGA3-/- mouse model of congenital complete lack of cone photoreceptor function

Congenital absence of cone photoreceptor function is associated with strongly impaired daylight vision and loss of color discrimination in human achromatopsia. Here, we introduce viral gene replacement therapy as a potential treatment for this disease in the CNGA3(-/-) mouse model. We show that such therapy can restore cone-specific visual processing in the central nervous system even if cone photoreceptors had been nonfunctional from birth. The restoration of cone vision was assessed at different stages along the visual pathway. Treated CNGA3(-/-) mice were able to generate cone photoreceptor responses and to transfer these signals to bipolar cells. In support, we found morphologically that treated cones expressed regular cyclic nucleotide-gated (CNG) channel complexes and opsins in outer segments, which previously they did not. Moreover, expression of CNGA3 normalized cyclic guanosine monophosphate (cGMP) levels in cones, delayed cone cell death and reduced the inflammatory response of Müller glia cells that is typical of retinal degenerations. Furthermore, ganglion cells from treated, but not from untreated, CNGA3(-/-) mice displayed cone-driven, light-evoked, spiking activity, indicating that signals generated in the outer retina are transmitted to the brain. Finally, we demonstrate that this newly acquired sensory information was translated into cone-mediated, vision-guided behavior.

Angiopoiesis and bone regeneration via co-expression of the hVEGF and hBMP genes from an adeno-associated viral vector in vitro and in vivo

AIM

To investigate the therapeutic potential of adeno-associated virus (AAV)-mediated expression of vascular endothelial growth factor (VEGF) and bone morphogenetic protein (BMP).

METHODS

Four experimental groups were administered the following AAV vector constructs: rAAV-hVEGF(165)-internal ribosome entry site (IRES)-hBMP-7 (AAV-VEGF/BMP), rAAV-hVEGF(165)-GFP (AAV-VEGF), rAAV-hBMP-7-GFP (AAV-BMP), and rAAV-IRES-GFP (AAV-GFP). VEGF(165) and BMP-7 gene expression was detected using RT-PCR. The VEGF(165) and BMP-7 protein expression was determined by Western blotting and ELISA. The rabbit ischemic hind limb model was adopted and rAAV was administered intramuscularly into the ischemic limb.

RESULTS

Rabbit bone marrow-derived mesenchymal stem cells (BMSCs) were cultured and infected with the four viral vectors. The expression of GFP increased from the 7th day of infection and could be detected on the 28th day post-infection. In the AAV-VEGF/BMP group, the levels of VEGF165 and BMP-7 increased with prolonged infection time. The VEGF(165) and BMP-7 secreted from BMSCs in the AAV-VEGF/BMP group enhanced HUVEC tube formation and resulted in a stronger osteogenic ability, respectively. In rabbit ischemic hind limb model, GFP expression increased from the 4th week and could be detected at 8 weeks post-injection. The rAAV vector had superior gene expressing activity. Eight weeks after gene transfer, the mean blood flow was significantly higher in the AAV-VEGF/BMP group. Orthotopic ossification was radiographically evident, and capillary growth and calcium deposits were obvious in this group.

CONCLUSION

AAV-mediated VEGF and BMP gene transfer stimulates angiogenesis and bone regeneration and may be a new therapeutic technique for the treatment of avascular necrosis of the femoral head (ANFH).

Two-photon single-cell optogenetic control of neuronal activity by sculpted light

Recent advances in optogenetic techniques have generated new tools for controlling neuronal activity, with a wide range of neuroscience applications. The most commonly used approach has been the optical activation of the light-gated ion channel channelrhodopsin-2 (ChR2). However, targeted single-cell-level optogenetic activation with temporal precessions comparable to the spike timing remained challenging. Here we report fast (< or = 1 ms), selective, and targeted control of neuronal activity with single-cell resolution in hippocampal slices. Using temporally focused laser pulses (TEFO) for which the axial beam profile can be controlled independently of its lateral distribution, large numbers of channels on individual neurons can be excited simultaneously, leading to strong (up to 15 mV) and fast (< or = 1 ms) depolarizations. Furthermore, we demonstrated selective activation of cellular compartments, such as dendrites and large presynaptic terminals, at depths up to 150 microm. The demonstrated spatiotemporal resolution and the selectivity provided by TEFO allow manipulation of neuronal activity, with a large number of applications in studies of neuronal microcircuit function in vitro and in vivo.

Multi-array silicon probes with integrated optical fibers: light-assisted perturbation and recording of local neural circuits in the behaving animal

Recordings of large neuronal ensembles and neural stimulation of high spatial and temporal precision are important requisites for studying the real-time dynamics of neural networks. Multiple-shank silicon probes enable large-scale monitoring of individual neurons. Optical stimulation of genetically targeted neurons expressing light-sensitive channels or other fast (milliseconds) actuators offers the means for controlled perturbation of local circuits. Here we describe a method to equip the shanks of silicon probes with micron-scale light guides for allowing the simultaneous use of the two approaches. We then show illustrative examples of how these compact hybrid electrodes can be used in probing local circuits in behaving rats and mice. A key advantage of these devices is the enhanced spatial precision of stimulation that is achieved by delivering light close to the recording sites of the probe. When paired with the expression of light-sensitive actuators within genetically specified neuronal populations, these devices allow the relatively straightforward and interpretable manipulation of network activity.

Comparison of adeno-associated virus pseudotype 1, 2, and 8 vectors administered by intramuscular injection in the treatment of murine phenylketonuria

Phenylketonuria (PKU) is caused by hepatic phenylalanine hydroxylase (PAH) deficiency and is associated with systemic accumulation of phenylalanine (Phe). Previously we demonstrated correction of murine PKU after intravenous injection of a recombinant type 2 adeno-associated viral vector pseudotyped with type 8 capsid (rAAV2/8), which successfully directed hepatic transduction and Pah gene expression. Here, we report that liver PAH activity and phenylalanine clearance were also restored in PAH-deficient mice after simple intramuscular injection of either AAV2 pseudotype 1 (rAAV2/1) or rAAV2/8 vectors. Serotype 2 AAV vector (rAAV2/2) was also investigated, but long-term phenylalanine clearance has been observed only for pseudotypes 1 and 8. Therapeutic correction was shown in both male and female mice, albeit more effectively in males, in which correction lasted for the entire period of the experiment (>1 year). Although phenylalanine levels began to rise in female mice at about 8-10 months after rAAV2/8 injection they remained only mildly hyperphenylalaninemic thereafter and subsequent supplementation with synthetic tetrahydrobiopterin resulted in a transient decrease in blood phenylalanine. Alternatively, subsequent administration of a second vector with a different AAV pseudotype to avoid immunity against the previously administrated vector was also successful for long-term treatment of female PKU mice. Overall, this relatively less invasive gene transfer approach completes our previous studies and allows comparison of complementary strategies in the development of efficient PKU gene therapy protocols.

Near-perfect infectivity of wild-type AAV as benchmark for infectivity of recombinant AAV vectors

Viral vectors derived from adeno-associated viruses (AAV) are widely used for gene transfer both in vitro and in vivo. The increasing use of AAV as a gene transfer vector, as well as recently demonstrated immunological complications in clinical trials, highlight the necessity to define the specific activity of vector preparations beyond current standards. In this report, we determined the infectious, physical and genome-containing particle titers of several wild-type AAV type 2 (wtAAV2) and recombinant AAV type 2 (rAAV2) preparations that were produced and purified by standard methods. We found that the infectivity of wtAAV2 approaches a physical-to-infectious particle ratio of one. This near-perfect physical-to-infectious particle ratio defines a “ceiling” for the theoretically achievable quality of recombinant AAV vectors. In comparison, for rAAV2, only approximately 50 out of 100 viral particles contained a genome and more strikingly only approximately one of the 100 viral particles was infectious. Our findings suggest that current strategies for rAAV vector design, production and/or purification should be amenable to improvements. Ultimately, this could result in the generation of near-perfect vector particles, a prospect with significant implications for gene therapy.

Directed evolution of a novel adeno-associated virus (AAV) vector that crosses the seizure-compromised blood-brain barrier (BBB)

DNA shuffling and directed evolution were employed to develop a novel adeno-associated virus (AAV) vector capable of crossing the seizure-compromised blood-brain barrier (BBB) and transducing cells in the brain. Capsid DNA from AAV serotypes 1-6, 8, and 9 were shuffled and recombined to create a library of chimeric AAVs. One day after kainic acid-induced limbic seizure activity in rats, the virus library was infused intravenously (i.v.), and 3 days later, neuron-rich cells were mechanically dissociated from seizure-sensitive brain sites, collected and viral DNA extracted. After three cycles of selection, green fluorescent protein (GFP)-packaged clones were administered directly into brain or i.v. 1 day after kainic acid-induced seizures. Several clones that were effective after intracranial administration did not transduce brain cells after the i.v. administration. However, two clones (32 and 83) transduced the cells after direct brain infusion and after i.v. administration transduced the cells that were localized to the piriform cortex and ventral hippocampus, areas exhibiting a seizure-compromised BBB. No transduction occurred in areas devoid of BBB compromise. Only one parental serotype (AAV8) exhibited a similar expression profile, but the biodistribution of 32 and 83 diverged dramatically from this parental serotype. Thus, novel AAV vectors have been created that can selectively cross the seizure-compromised BBB and transduce cells.

Development of viral vectors for use in cardiovascular gene therapy

Cardiovascular disease represents the most common cause of mortality in the developed world but, despite two decades of promising pre-clinical research and numerous clinical trials, cardiovascular gene transfer has so far failed to demonstrate convincing benefits in the clinical setting. In this review we discuss the various targets which may be suitable for cardiovascular gene therapy and the viral vectors which have to date shown the most potential for clinical use. We conclude with a summary of the current state of clinical cardiovascular gene therapy and the key trials which are ongoing.

Reengineering a receptor footprint of adeno-associated virus enables selective and systemic gene transfer to muscle

Reengineering the receptor footprints of adeno-associated virus (AAV) isolates may yield variants with improved properties for clinical applications. We generated a panel of synthetic AAV2 vectors by replacing a hexapeptide sequence in a previously identified heparan sulfate receptor footprint with corresponding residues from other AAV strains. This approach yielded several chimeric capsids displaying systemic tropism after intravenous administration in mice. Of particular interest, an AAV2/AAV8 chimera designated AAV2i8 displayed an altered antigenic profile, readily traversed the blood vasculature, and selectively transduced cardiac and whole-body skeletal muscle tissues with high efficiency. Unlike other AAV serotypes, which are preferentially sequestered in the liver, AAV2i8 showed markedly reduced hepatic tropism. These features of AAV2i8 suggest that it is well suited to translational studies in gene therapy of musculoskeletal disorders.

Comparative transduction efficiency of AAV vector serotypes 1-6 in the substantia nigra and striatum of the primate brain

Vectors derived from adeno-associated virus (AAV) are promising candidates for neural cell transduction in vivo because they are nonpathogenic and achieve long-term transduction in the central nervous system. AAV serotype 2 (AAV2) is the most widely used AAV vector in clinical trials based largely on its ability to transduce neural cells in the rodent and primate brain. Prior work in rodents suggests that other serotypes might be more efficient; however, a systematic evaluation of vector transduction efficiency has not yet been performed in the primate brain. In this study, AAV viral vectors of serotypes 1-6 with an enhanced green-fluorescent protein (GFP) reporter gene were generated at comparable titers, and injected in equal amounts into the brains of Chlorocebus sabaeus. Vector injections were placed in the substantia nigra (SN) and the caudate nucleus (CD). One month after injection, immunohistochemistry for GFP was performed and the total number of GFP+ cells was calculated using unbiased stereology. AAV5 was the most efficient vector, not only transducing significantly more cells than any other serotype, but also transducing both NeuN+ and glial-fibrillary-acidic protein positive (GFAP+) cells. These results suggest that AAV5 is a more effective vector than AAV2 at delivering potentially therapeutic transgenes to the nigrostriatal system of the primate brain.

High AAV vector purity results in serotype- and tissue-independent enhancement of transduction efficiency

The purity of adeno-associated virus (AAV) vector preparations has important implications for both safety and efficacy of clinical gene transfer. Early-stage screening of candidates for AAV-based therapeutics ideally requires a purification method that is flexible and also provides vectors comparable in purity and potency to the prospective investigational product manufactured for clinical studies. The use of cesium chloride (CsCl) gradient-based protocols provides the flexibility for purification of different serotypes; however, a commonly used first-generation CsCl-based protocol was found to result in AAV vectors containing large amounts of protein and DNA impurities and low transduction efficiency in vitro and in vivo. Here, we describe and characterize an optimized, second-generation CsCl protocol that incorporates differential precipitation of AAV particles by polyethylene glycol, resulting in higher yield and markedly higher vector purity that correlated with better transduction efficiency observed with several AAV serotypes in multiple tissues and species. Vectors purified by the optimized CsCl protocol were found to be comparable in purity and functional activity to those prepared by more scalable, but less flexible serotype-specific purification processes developed for manufacture of clinical vectors, and are therefore ideally suited for pre-clinical studies supporting translational research.

AAV recombineering with single strand oligonucleotides

Adeno-associated virus (AAV) transduction initiates a signaling cascade that culminates in a transient DNA damage response. During this time, host DNA repair proteins convert the linear single-strand AAV genomes to double-strand circular monomers and concatemers in processes stimulated by the AAV inverted terminal repeats (ITRs). As the orientation of AAV genome concatemerization appears unbiased, the likelihood of concatemerization in a desired orientation is low (less than 1 in 6). Using a novel recombineering method, Oligo-Assisted AAV Genome Recombination (OAGR), this work demonstrates the ability to direct concatemerization specifically to a desired orientation in human cells. This was achieved by a single-strand DNA oligonucleotide (oligo) displaying homology to distinct AAV genomes capable of forming an intermolecular bridge for recombination. This DNA repair process results in concatemers with genomic junctions corresponding to the sequence of oligo homology. Furthermore, OAGR was restricted to single-strand, not duplexed, AAV genomes suggestive of replication-dependent recombination. Consistent with this process, OAGR demonstrated oligo polarity biases in all tested configurations except when a portion of the oligo targeted the ITR. This approach, in addition to being useful for the elucidation of intermolecular homologous recombination, may find eventual relevance for AAV mediated large gene therapy.

Viral vectors for neurotrophic factor delivery: a gene therapy approach for neurodegenerative diseases of the CNS

The clinical manifestation of most diseases of the central nervous system results from neuronal dysfunction or loss. Diseases such as stroke, epilepsy and neurodegeneration (e.g. Alzheimer's disease and Parkinson's disease) share common cellular and molecular mechanisms (e.g. oxidative stress, endoplasmic reticulum stress, mitochondrial dysfunction) that contribute to the loss of neuronal function. Neurotrophic factors (NTFs) are secreted proteins that regulate multiple aspects of neuronal development including neuronal maintenance, survival, axonal growth and synaptic plasticity. These properties of NTFs make them likely candidates for preventing neurodegeneration and promoting neuroregeneration. One approach to delivering NTFs to diseased cells is through viral vector-mediated gene delivery. Viral vectors are now routinely used as tools for studying gene function as well as developing gene-based therapies for a variety of diseases. Currently, many clinical trials using viral vectors in the nervous system are underway or completed, and seven of these trials involve NTFs for neurodegeneration. In this review, we discuss viral vector-mediated gene transfer of NTFs to treat neurodegenerative diseases of the central nervous system.

Delivering multiple gene products in the brain from a single adeno-associated virus vector

For certain gene therapy applications, the simultaneous delivery of multiple genes would allow for novel therapies. In the case of adeno-associated virus (AAV) vectors, the limited packaging capacity greatly restricts current methods of carrying multiple transgene cassettes. To address this issue, a recombinant AAV (rAAV) vector was designed such that a furin proteolytic cleavage site (RKRRKR) was placed between the coding sequences of two genes (green fluorescent protein (GFP) and galanin), to allow cleavage of the chimeric protein into two fragments. In addition, these constructs contained the fibronectin secretory signal sequence that causes the gene products to be constitutively secreted from transduced cells. In vitro studies show that after transfection of HEK293 cells, the appropriate cleavage and constitutive secretion occurred regardless of the order of the genes in the transgene cassette. In vivo, infusion of rAAV vectors into the piriform cortex resulted in both GFP expression and significant galanin attenuation of kainic acid-induced seizure activity. Thus, the present results establish the utility of a proteolytic approach for the expression and secretion of multiple gene products from a single AAV vector transgene cassette.

Inner limiting membrane barriers to AAV-mediated retinal transduction from the vitreous

Adeno-associated viral gene therapy has shown great promise in treating retinal disorders, with three promising clinical trials in progress. Numerous adeno-associated virus (AAV) serotypes can infect various cells of the retina when administered subretinally, but the retinal detachment accompanying this injection induces changes that negatively impact the microenvironment and survival of retinal neurons. Intravitreal administration could circumvent this problem, but only AAV2 can infect retinal cells from the vitreous, and transduction is limited to the inner retina. We therefore sought to investigate and reduce barriers to transduction from the vitreous. We fluorescently labeled several AAV serotype capsids and followed their retinal distribution after intravitreal injection. AAV2, 8, and 9 accumulate at the vitreoretinal junction. AAV1 and 5 show no accumulation, indicating a lack of appropriate receptors at the inner limiting membrane (ILM). Importantly, mild digestion of the ILM with a nonspecific protease enabled substantially enhanced transduction of multiple retinal cell types from the vitreous, with AAV5 mediating particularly remarkable expression in all retinal layers. This protease treatment has no effect on retinal function as shown by electroretinogram (ERG) and visual cortex cell population responses. These findings may help avoid limitations, risks, and damage associated with subretinal injections currently necessary for clinical gene therapy.

Efficient gene transfer to periodontal ligament cells and human gingival fibroblasts by adeno-associated virus vectors

OBJECTIVES

We explored for the first time the possibility to deliver a reporter gene (Green Fluorescence Protein) to human primary periodontal ligament (PDL) cells and human gingival fibroblasts (HGF) using shuttle vectors derived from adeno-associated virus (AAV). Since AAV transduction rates on other human primary fibroblasts have been previously shown to depend on the particular cell lineage and on the employed viral serotype, we determined the most effective AAV variant for periodontal cells comparing different vector types.

METHODS

AAV serotypes 1-5 encoding GFP in single stranded (ss) and self-complementary (sc) vector genome conformations were used to infect primary HGF and PDL cells. Two days post-infection, the percentage of GFP expressing cells was determined by flow cytometry.

RESULTS

Highest transduction rates for both cell types were achieved with self-complementary vectors derived from AAV-2, resulting in GFP expression in up to 86% of PDL cells and 50% of HGF. Transgene expression could be observed by optical microscopy for 2 months after infection. Lower but detectable rates were obtained with serotypes 1, 3 and 5.

CONCLUSIONS

The efficacy demonstrated here and the safety and versatility of AAV technology indicated in previous studies clearly suggest the potential of AAV vectors as tools for gene transfer to periodontal tissues.

Adeno-associated virus type 5 exploits two different entry pathways in human embryo fibroblasts

The helper-dependent adeno-associated viruses (AAVs) have attracted great interest as vectors for gene therapy. Uptake and intracellular trafficking pathways of AAV are of importance, since they are often rate-limiting steps in infection. Here, we have investigated the entry of AAV type 5 (AAV5) in primary human embryo fibroblasts. At low binding temperatures, numerous virions are concentrated between cells, at contact points between cells and cellular protrusions, and at filopodia. When the temperature is raised to 37 degrees C, uptake of AAV5 takes place but up to 80 % of the bound virions dissociate from the cells. Uptake is achieved by cellular structures that are part of at least two different entry pathways. In addition to the common clathrin-dependent route, caveolar endocytosis and caveosome-like organelles are involved in a second pathway not yet described for parvoviruses. Both pathways can be used in parallel to enter an individual cell.

Gene therapy of the rheumatic diseases: 1998 to 2008

During the decade since the launch of Arthritis Research, the application of gene therapy to the rheumatic diseases has experienced the same vicissitudes as the field of gene therapy as a whole. There have been conceptual and technological advances and an increase in the number of clinical trials. However, funding has been unreliable and a small number of high-profile deaths in human trials, including one in an arthritis gene therapy trial, have provided ammunition to skeptics. Nevertheless, steady progress has been made in a number of applications, including rheumatoid arthritis and osteoarthritis, Sjögren syndrome, and lupus. Clinical trials in rheumatoid arthritis have progressed to phase II and have provided the first glimpses of possible efficacy. Two phase I protocols for osteoarthritis are under way. Proof of principle has been demonstrated in animal models of Sjögren syndrome and lupus. For certain indications, the major technological barriers to the development of genetic therapies seem to have been largely overcome. The translational research necessary to turn these advances into effective genetic medicines requires sustained funding and continuity of effort.

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.

Designing heart performance by gene transfer

The birth of molecular cardiology can be traced to the development and implementation of high-fidelity genetic approaches for manipulating the heart. Recombinant viral vector-based technology offers a highly effective approach to genetically engineer cardiac muscle in vitro and in vivo. This review highlights discoveries made in cardiac muscle physiology through the use of targeted viral-mediated genetic modification. Here the history of cardiac gene transfer technology and the strengths and limitations of viral and nonviral vectors for gene delivery are reviewed. A comprehensive account is given of the application of gene transfer technology for studying key cardiac muscle targets including Ca(2+) handling, the sarcomere, the cytoskeleton, and signaling molecules and their posttranslational modifications. The primary objective of this review is to provide a thorough analysis of gene transfer studies for understanding cardiac physiology in health and disease. By comparing results obtained from gene transfer with those obtained from transgenesis and biophysical and biochemical methodologies, this review provides a global view of cardiac structure-function with an eye towards future areas of research. The data presented here serve as a basis for discovery of new therapeutic targets for remediation of acquired and inherited cardiac diseases.

Transduction of PACAP38 protects primary cortical neurons from neurotoxic injury

Neurotrophic factors such as pituitary adenylate cyclase activating polypeptide (PACAP38) are promising therapeutics for neurodegenerative diseases. However, delivery of trophic factors into brain neurons remains a challenge. The objective of this study is to determine whether adeno-associated virus (AAV) can mediate PACAP38 gene delivery into neurons in vitro and if transduction of AAV/PACAP38 into cortical neurons protects cells against neurotoxic insult. Primary cortical neuronal cultures are transduced with rAAV/PACAP38/GFP and cell survival against the nitric oxide releasing neurotoxin sodium nitroprusside (SNP) determined. GFP expression, a surrogate marker for successful transduction, is detected using fluorescent microscopy. The results show expression of GFP transgene and AAV capsid proteins in neurons. PACAP38 transduction significantly increases cell survival of neurons exposed to SNP. These results support the feasibility of using AAV-mediated delivery of PACAP38 to enhance neuronal survival and suggest that AAV-delivered PACAP38 maybe a therapeutic strategy for neurodegenerative diseases.

Optimizing gene delivery vectors for the treatment of heart disease

BACKGROUND

Cardiac gene therapy is approaching reality, with clinical trials entering Phase II/III. Even so, challenges exist to improve the efficacy of even the most successful therapies.

OBJECTIVE

The merits of different gene therapy vectors are weighed to assess the current feasibility of each in specific cardiac applications. Major obstacles are discussed, along with recent advances in vector development to overcome or circumvent those difficulties.

METHODS

This review focuses primarily on gene delivery via naked DNA, adenovirus, lentivirus, and adeno-associated virus (AAV) vectors.

CONCLUSION

Gene therapy via adenovirus and AAV vectors has developed into a promising option for the treatment of heart disease. The merits of gene therapy compared with emerging stem cell and microRNA-based treatments are discussed.

Bioluminescent virion shells: new tools for quantitation of AAV vector dynamics in cells and live animals

Current technologies for visualizing infectious pathways of viruses rely on fluorescent labeling of capsid proteins by chemical conjugation or genetic manipulation. For noninvasive in vivo imaging of such agents in mammalian tissue, we engineered bioluminescent Gaussia luciferase-tagged Adeno-associated viral (gLuc/AAV) vectors. The enzyme was incorporated into recombinant AAV serotypes 1, 2 and 8 capsids by fusing to the N-terminus of the VP2 capsid subunit to yield bioluminescent virion shells. The gLuc/AAV vectors were used to quantify kinetics of cell-surface-binding by AAV2 capsids in vitro. Bioluminescent virion shells displayed an exponential decrease in luminescent signal following cellular uptake in vitro. A similar trend was observed following intramuscular injection in vivo, although the rate of decline in bioluminescent signal varied markedly between AAV serotypes. gLuc/AAV1 and gLuc/AAV8 vectors displayed rapid decrease in bioluminescent signal to background levels within 30 min, whereas the signal from gLuc/AAV2 vectors persisted for over 2 h. Bioluminescent virion shells might be particularly useful in quantifying dynamics of viral vector uptake in cells and peripheral tissues in live animals.

Arthritis gene therapy's first death

In July 2007 a subject died while enrolled in an arthritis gene therapy trial. The study was placed on clinical hold while the circumstances surrounding this tragedy were investigated. Early in December 2007 the Food and Drug Administration removed the clinical hold, allowing the study to resume with minor changes to the protocol. In the present article we collate the information we were able to obtain about this clinical trial and discuss it in the wider context of arthritis gene therapy.

AAV2-mediated gene transfer of GDNF to the striatum of MPTP monkeys enhances the survival and outgrowth of co-implanted fetal dopamine neurons

Neural transplantation offers the potential of treating Parkinson's disease by grafting fetal dopamine neurons to depleted regions of the brain. However, clinical studies of neural grafting in Parkinson's disease have produced only modest improvements. One of the main reasons for this is the low survival rate of transplanted neurons. The inadequate supply of critical neurotrophic factors in the adult brain is likely to be a major cause of early cell death and restricted outgrowth of fetal grafts placed into the mature striatum. Glial derived neurotrophic factor (GDNF) is a potent neurotrophic factor that is crucial to the survival, outgrowth and maintenance of dopamine neurons, and so is a candidate for protecting grafted fetal dopamine neurons in the adult brain. We found that implantation of adeno-associated virus type 2 encoding GDNF (AAV2-GDNF) in the normal monkey caudate nucleus induced overexpression of GDNF that persisted for at least 6 months after injection. In a 6-month within-animal controlled study, AAV2-GDNF enhanced the survival of fetal dopamine neurons by 4-fold, and increased the outgrowth of grafted fetal dopamine neurons by almost 3-fold in the caudate nucleus of MPTP-treated monkeys, compared with control grafts in the other caudate nucleus. Thus, the addition of GDNF gene therapy to neural transplantation may be a useful strategy to improve treatment for Parkinson's disease.

Genetic modification of cells for transplantation

Progress in gene therapy has produced promising results that translate experimental research into clinical treatment. Gene modification has been extensively employed in cell transplantation. The main barrier is an effective gene delivery system. Several viral vectors were utilized in end-stage differentiated cells. Recently, successful applications were described with adenovirus-associated vectors. As an alternative, embryonic stem cell- and stem cell-like systems were established for generation of tissue-specified gene-modified cells. Owing to the feasibility for genetic manipulations and the self-renewing potency of these cells they can be used in a way enabling large-scale in vitro production. This approach offers the establishment of in vitro cell culture systems that will deliver sufficient amounts of highly purified, immunoautologous cells suitable for application in regenerative medicine. In this review, the current technology of gene delivery systems to cells is recapitulated and the latest developments for cell transplantation are discussed.

Designer gene delivery vectors: molecular engineering and evolution of adeno-associated viral vectors for enhanced gene transfer

Gene delivery vectors based on adeno-associated virus (AAV) are highly promising due to several desirable features of this parent virus, including a lack of pathogenicity, efficient infection of dividing and non-dividing cells, and sustained maintenance of the viral genome. However, several problems should be addressed to enhance the utility of AAV vectors, particularly those based on AAV2, the best characterized AAV serotype. First, altering viral tropism would be advantageous for broadening its utility in various tissue or cell types. In response to this need, vector pseudotyping, mosaic capsids, and targeting ligand insertion into the capsid have shown promise for altering AAV specificity. In addition, library selection and directed evolution have recently emerged as promising approaches to modulate AAV tropism despite limited knowledge of viral structure-function relationships. Second, pre-existing immunity to AAV must be addressed for successful clinical application of AAV vectors. "Shielding" polymers, site-directed mutagenesis, and alternative AAV serotypes have shown success in avoiding immune neutralization. Furthermore, directed evolution of the AAV capsid is a high throughput approach that has yielded vectors with substantial resistance to neutralizing antibodies. Molecular engineering and directed evolution of AAV vectors therefore offer promise for generating 'designer' gene delivery vectors with enhanced properties.