Basic fibroblast growth factor enhances transduction, distribution, and axonal transport of adeno-associated virus type 2 vector in rat brain

Recombinant Adeno-Associated Virus Vectors for Gene Therapy of the Central Nervous System: Delivery Routes and Clinical Aspects

The Central Nervous System (CNS) is vulnerable to a range of diseases, including neurodegenerative and oncological conditions, which present significant treatment challenges. The blood-brain barrier (BBB) restricts molecule penetration, complicating the achievement of therapeutic concentrations in the CNS following systemic administration. Gene therapy using recombinant adeno-associated virus (rAAV) vectors emerges as a promising strategy for treating CNS diseases, demonstrated by the registration of six gene therapy products in the past six years and 87 ongoing clinical trials. This review explores the implementation of rAAV vectors in CNS disease treatment, emphasizing AAV biology and vector engineering. Various administration methods-such as intravenous, intrathecal, and intraparenchymal routes-and experimental approaches like intranasal and intramuscular administration are evaluated, discussing their advantages and limitations in different CNS contexts. Additionally, the review underscores the importance of optimizing therapeutic efficacy through the pharmacokinetics (PK) and pharmacodynamics (PD) of rAAV vectors. A comprehensive analysis of clinical trials reveals successes and challenges, including barriers to commercialization. This review provides insights into therapeutic strategies using rAAV vectors in neurological diseases and identifies areas requiring further research, particularly in optimizing rAAV PK/PD.

AAV2 vector optimization for retinal ganglion cell-targeted delivery of therapeutic genes

Recombinant adeno-associated virus (AAV)-2 has significant potential as a delivery vehicle of therapeutic genes to retinal ganglion cells (RGCs), which are key interventional targets in optic neuropathies. Here we show that when injected intravitreally, AAV2 engineered with a reporter gene driven by cytomegalovirus (CMV) enhancer and chicken β-actin (CBA) promoters, displays ubiquitous and high RGC expression, similar to its synthetic derivative AAV8BP2. A novel AAV2 vector combining the promoter of the human RGC-selective γ-synuclein (hSNCG) gene and woodchuck hepatitis post-transcriptional regulatory element (WPRE) inserted upstream and downstream of a reporter gene, respectively, induces widespread transduction and strong transgene expression in RGCs. High transduction efficiency and selectivity to RGCs is further achieved by incorporating in the vector backbone a leading CMV enhancer and an SV40 intron at the 5' and 3' ends, respectively, of the reporter gene. As a delivery vehicle of hSIRT1, a 2.2-kb therapeutic gene with anti-apoptotic, anti-inflammatory and anti-oxidative stress properties, this recombinant vector displayed improved transduction efficiency, a strong, widespread and selective RGC expression of hSIRT1, and increased RGC survival following optic nerve crush. Thus, AAV2 vector carrying hSNCG promoter with additional regulatory sequences may offer strong potential for enhanced effects of candidate gene therapies targeting RGCs.

Overcoming barriers in non-viral gene delivery for neurological applications

Gene therapy for neurological disorders has attracted significant interest as a way to reverse or stop various disease pathologies. Typical gene therapies involving the central and peripheral nervous system make use of adeno-associated viral vectors whose questionable safety and limitations in manufacturing has given rise to extensive research into non-viral vectors. While early research studies have demonstrated limited efficacy with these non-viral vectors, investigation into various vector materials and functionalization methods has provided insight into ways to optimize these non-viral vectors to improve desired characteristics such as improved blood-brain barrier transcytosis, improved perfusion in brain region, enhanced cellular uptake and endosomal escape in neural cells, and nuclear transport of genetic material post- intracellular delivery. Using a combination of various strategies to enhance non-viral vectors, research groups have designed multi-functional vectors that have been successfully used in a variety of pre-clinical applications for the treatment of Parkinson's disease, brain cancers, and cellular reprogramming for neuron replacement. While more work is needed in the design of these multi-functional non-viral vectors for neural applications, much of the groundwork has been done and is reviewed here.

Systemic AAV6-synapsin-GFP administration results in lower liver biodistribution, compared to AAV1&2 and AAV9, with neuronal expression following ultrasound-mediated brain delivery

Non-surgical gene delivery to the brain can be achieved following intravenous injection of viral vectors coupled with transcranial MRI-guided focused ultrasound (MRIgFUS) to temporarily and locally permeabilize the blood-brain barrier. Vector and promoter selection can provide neuronal expression in the brain, while limiting biodistribution and expression in peripheral organs. To date, the biodistribution of adeno-associated viruses (AAVs) within peripheral organs had not been quantified following intravenous injection and MRIgFUS delivery to the brain. We evaluated the quantity of viral DNA from the serotypes AAV9, AAV6, and a mosaic AAV1&2, expressing green fluorescent protein (GFP) under the neuron-specific synapsin promoter (syn). AAVs were administered intravenously during MRIgFUS targeting to the striatum and hippocampus in mice. The syn promoter led to undetectable levels of GFP expression in peripheral organs. In the liver, the biodistribution of AAV9 and AAV1&2 was 12.9- and 4.4-fold higher, respectively, compared to AAV6. The percentage of GFP-positive neurons in the FUS-targeted areas of the brain was comparable for AAV6-syn-GFP and AAV1&2-syn-GFP. In summary, MRIgFUS-mediated gene delivery with AAV6-syn-GFP had lower off-target biodistribution in the liver compared to AAV9 and AAV1&2, while providing neuronal GFP expression in the striatum and hippocampus.

Trans-Ocular Electric Current In Vivo Enhances AAV-Mediated Retinal Transduction in Large Animal Eye After Intravitreal Vector Administration

Purpose

Electric micro-current has been shown to enhance penetration and transduction of adeno-associated viral (AAV) vectors in mouse retina after intravitreal administration. We termed this: “electric-current vector mobility (ECVM).” The present study considered whether ECVM could augment retinal transduction efficiency of intravitreal AAV8-CMV-EGFP in normal rabbit and nonhuman primate (NHP) macaque. Potential mechanisms underlying enhanced retinal transduction by ECVM were also studied.

Methods

We applied an electric micro-current across the intact eye of normal rabbit and monkey in vivo for a brief period immediately after intravitreal injection of AAV8-CMV-EGFP. Retinal GFP expression was evaluated by fundus imaging in vivo. Retinal immunohistochemistry was performed to assess the distribution of retinal cells transduced by the AAV8-EGFP. Basic fibroblast growth factor (bFGF) was analyzed by quantitative RT-polymerase chain reaction (PCR). Müller glial reactivity and inner limiting membrane (ILM) were examined by the glial fibrillary acidic protein (GFAP) and vimentin staining in mouse retina, respectively.

Results

ECVM significantly increased the efficiency of AAV reaching and transducing the rabbit retina following intravitreal injection, with gene expression in inner nuclear layer, ganglion cells, and Müller cells. Similar trend of improvement was observed in the ECVM-treated monkey eye. The electric micro-current upregulated bFGF expression in Müller cells and vimentin showed ILM structural changes in mouse retina.

Conclusions

ECVM promotes the transduction efficiency of AAV8-CMV-GFP in normal rabbit and monkey retinas following intravitreal injection.

Translational Relevance

This work has potential translational relevance to human ocular gene therapy by increasing retinal expression of therapeutic vectors given by intravitreal administration.

Trans-ocular Electric Current In Vivo Enhances AAV-Mediated Retinal Gene Transduction after Intravitreal Vector Administration

Adeno-associated virus (AAV) vector-mediated gene delivery is a promising approach for therapy, but implementation in the eye currently is hampered by the need for delivering the vector underneath the retina, using surgical application into the subretinal space. This limits the extent of the retina that is treated and may cause surgical injury. Vector delivery into the vitreous cavity would be preferable because it is surgically less invasive and would reach more of the retina. Unfortunately, most conventional, non-modified AAV vector serotypes penetrate the retina poorly from the vitreous; this limits efficient transduction and expression by target cells (retinal pigment epithelium and photoreceptors). We developed a method of applying a small and safe electric current across the intact eye in vivo for a brief period following intravitreal vector administration. This significantly improved AAV-mediated transduction of retinal cells in wild-type mice following intravitreal delivery, with gene expression in retinal pigment epithelium and photoreceptor cells. The low-level current had no adverse effects on retinal structure and function. This method should be generally applicable for other AAV serotypes and may have broad application in both basic research and clinical studies.

Extensive Transduction and Enhanced Spread of a Modified AAV2 Capsid in the Non-human Primate CNS

The present study was designed to characterize transduction of non-human primate brain and spinal cord with a modified adeno-associated virus serotype 2, incapable of binding to the heparan sulfate proteoglycan receptor, referred to as AAV2-HBKO. AAV2-HBKO was infused into the thalamus, intracerebroventricularly or via a combination of both intracerebroventricular and thalamic delivery. Thalamic injection of this modified vector encoding GFP resulted in widespread CNS transduction that included neurons in deep cortical layers, deep cerebellar nuclei, several subcortical regions, and motor neuron transduction in the spinal cord indicative of robust bidirectional axonal transport. Intracerebroventricular delivery similarly resulted in widespread cortical transduction, with one striking distinction that oligodendrocytes within superficial layers of the cortex were the primary cell type transduced. Robust motor neuron transduction was also observed in all levels of the spinal cord. The combination of thalamic and intracerebroventricular delivery resulted in transduction of oligodendrocytes in superficial cortical layers and neurons in deeper cortical layers. Several subcortical regions were also transduced. Our data demonstrate that AAV2-HBKO is a powerful vector for the potential treatment of a wide number of neurological disorders, and highlight that delivery route can significantly impact cellular tropism and pattern of CNS transduction.

AAV viral vector delivery to the brain by shape-conforming MR-guided infusions

Gene transfer technology offers great promise as a potential therapeutic approach to the brain but has to be viewed as a very complex technology. Success of ongoing clinical gene therapy trials depends on many factors such as selection of the correct genetic and anatomical target in the brain. In addition, selection of the viral vector capable of transfer of therapeutic gene into target cells, along with long-term expression that avoids immunotoxicity has to be established. As with any drug development strategy, delivery of gene therapy has to be consistent and predictable in each study subject. Failed drug and vector delivery will lead to failed clinical trials. In this article, we describe our experience with AAV viral vector delivery system, that allows us to optimize and monitor in real time viral vector administration into affected regions of the brain. In addition to discussing MRI-guided technology for administration of AAV vectors we have developed and now employ in current clinical trials, we also describe ways in which infusion cannula design and stereotactic trajectory may be used to maximize the anatomical coverage by using fluid backflow. This innovative approach enables more precise coverage by fitting the shape of the infusion to the shape of the anatomical target.

Trans-neuronal transduction of spinal neurons following cortical injection and anterograde axonal transport of a bicistronic AAV1 vector

Adeno-associated viral (AAV) vectors are one of the most promising gene delivery systems to the central nervous system. We now report, that AAV1 can be used to express transgenes trans-neuronally in neurons distant from the injection site. Specifically, intracortical injection of a bicistronic AAV1 vector trans-neuronally transduced spinal neurons as shown by fluorescence microscopy, the presence of AAV genome and AAV transcript in the contralateral spinal cord. Prior pyramidotomy abolished spinal transduction, confirming anterograde axonal transport of AAV1 in the corticospinal tract. These observations demonstrate the potential of bicistronic AAV1 for trans-neuronal expression of therapeutic transgenes in neurological disorders or reporter genes in connectivity studies.

Gene therapy for the nervous system: challenges and new strategies

Current clinical treatments for central nervous system (CNS) diseases, such as Parkinson's disease and glioblastoma do not halt disease progression and have significant treatment morbidities. Gene therapy has the potential to "permanently" correct disease by bringing in a normal gene to correct a mutant gene deficiency, knocking down mRNA of mutant alleles, and inducing cell-death in cancer cells using transgenes encoding apoptosis-inducing proteins. Promising results in clinical trials of eye disease (Leber's congenital aumorosis) and Parkinson's disease have shown that gene-based neurotherapeutics have great potential. The recent development of genome editing technology, such as zinc finger nucleases, TALENS, and CRISPR, has made the ultimate goal of gene correction a step closer. This review summarizes the challenges faced by gene-based neurotherapeutics and the current and recent strategies designed to overcome these barriers. We have chosen the following challenges to focus on in this review: (1) delivery vehicles (both virus and nonviral), (2) use of promoters for vector-mediated gene expression in CNS, and (3) delivery across the blood-brain barrier. The final section (4) focuses on promising pre-clinical/clinical studies of neurotherapeutics.

Intraneural convection enhanced delivery of AAVrh20 for targeting primary sensory neurons

Gene therapy using adeno-associated virus (AAV) is an attractive strategy to treat disorders of the peripheral nervous system (PNS), such as chronic pain or peripheral neuropathies. Although intrathecal (IT) administration of AAV has been the standard in the field for targeting the PNS, it lacks anatomical specificity and results in wide rostro-caudal distribution of the vector. An alternative approach is to deliver AAV directly to the peripheral nerve axon. The present study employed convection-enhanced delivery (CED) of a novel AAV serotype, AAVrh20, expressing enhanced green fluorescent protein (EGFP) into rat sciatic nerve investigating its efficacy, anatomical selectivity, and safety, compared to the IT route. Intraneural CED resulted in transduction confined to the ipsilateral L4 and L5 DRG while IT administration led to promiscuous DRG transduction encompassing the entire lumbar region bilaterally. The transduction rate for intraneural AAV administration was similar to IT delivery (24% for L4 and 31.5% for L5 DRG versus 50% for L4 and 19.5% for L5 DRG). The use of hyperosmotic diluent did not further improve the transduction efficiency. AAVrh20 was superior to reference serotypes previously described to be most active for each route. Intraneural CED of AAV was associated with transient allodynia that resolved spontaneously. These findings establish intraneural CED as an alternative to IT administration for AAV mediated gene transfer to the PNS and, based on a reference rodent model, suggest AAVrh20 as a superior serotype for targeting the PNS.

Viral vector-based models of Parkinson's disease

In order to study the molecular pathways of Parkinson's disease (PD) and to develop novel therapeutic strategies, scientific investigators rely on animal models. The identification of PD-associated genes has led to the development of genetic PD models as an alternative to toxin-based models. Viral vector-mediated loco-regional gene delivery provides an attractive way to express transgenes in the central nervous system. Several vector systems based on various viruses have been developed. In this chapter, we give an overview of the different viral vector systems used for targeting the CNS. Further, we describe the different viral vector-based PD models currently available based on overexpression strategies for autosomal dominant genes such as α-synuclein and LRRK2, and knockout or knockdown strategies for autosomal recessive genes, such as parkin, DJ-1, and PINK1. Models based on overexpression of α-synuclein are the most prevalent and extensively studied, and therefore the main focus of this chapter. Many efforts have been made to increase the expression levels of α-synuclein in the dopaminergic neurons. The best α-synuclein models currently available have been developed from a combined approach using newer AAV serotypes and optimized vector constructs, production, and purification methods. These third-generation α-synuclein models show improved face and predictive validity, and therefore offer the possibility to reliably test novel therapeutics.

Gene transfer to the CNS using recombinant adeno-associated virus

Recombinant adeno-associated virus (rAAV) vectors are great tools for gene transfer due to their ability to mediate long-term gene expression. rAAVs have been used successfully as gene transfer vehicles in multiple animal models of CNS disorders, and several clinical trials are currently underway. rAAV vectors have been used at various stages of development with no apparent toxicity. There are multiple ways of delivering AAV vectors to the mouse CNS, depending on the stage of development. In neonates, intravascular injections into the facial vein are often used. In adults, direct injections into target regions of the brain are achieved with great spatiotemporal control through stereotaxic surgeries. Recently, discoveries of new AAV vectors with the ability to cross the blood brain barrier have made it possible to target the adult CNS by intravascular injections.

Gene therapy for misfolding protein diseases of the central nervous system

Protein aggregation as a result of misfolding is a common theme underlying neurodegenerative diseases. Accordingly, most recent studies aim to prevent protein misfolding and/or aggregation as a strategy to treat these pathologies. For instance, state-of-the-art approaches, such as silencing protein overexpression by means of RNA interference, are being tested with positive outcomes in preclinical models of animals overexpressing the corresponding protein. Therapies designed to treat central nervous system diseases should provide accurate delivery of the therapeutic agent and long-term or chronic expression by means of a nontoxic delivery vehicle. After several years of technical advances and optimization, gene therapy emerges as a promising approach able to fulfill those requirements. In this review we will summarize the latest improvements achieved in gene therapy for central nervous system diseases associated with protein misfolding (e.g., amyotrophic lateral sclerosis, Alzheimer's, Parkinson's, Huntington's, and prion diseases), as well as the most recent approaches in this field to treat these pathologies.

Gene therapy for Parkinson's disease: where are we now and where are we going?

The number of patients worldwide who have received some kind of gene therapy is now in the thousands. A subset of that number have received intracranial injections of adeno-associated viruses encoding various therapeutic genes directed at ameliorating Parkinson's disease (PD). In this article we briefly examine the current status of Phase I and Phase II trials of gene therapy for PD and preview some of the improvements in delivery technology that promise to make adeno-associated-virus-based gene therapy for PD safer and more accessible to interventional neurologists around the world.

Anterograde axonal transport of AAV2-GDNF in rat basal ganglia

We elucidated the effects of parkinsonian degeneration on trafficking of AAV2-GDNF in the nigro-striatum (nigro-ST) of unilaterally 6-hydroxydopamine (6-OHDA)-lesioned rats. Vector infused into striatum (ST) was transported to substantia nigra (SN), both pars compacta (SNc), and pars reticulata (SNr). In the lesioned hemisphere, glial cell line-derived neurotrophic factor (GDNF) immunoreactivity was only found in SNr consistent with elimination of SNc dopaminergic (DA) neurons by 6-OHDA. Further analysis showed that striatal delivery of AAV2-GDNF resulted in GDNF expression in globus pallidus (GP), entopeduncular nucleus (EPN), and subthalamic nucleus (STN) in both lesioned and unlesioned hemispheres. Injection of vector into SN, covering both SNc and SNr, resulted in striatal expression of GDNF in the unlesioned hemisphere but not in the lesioned hemisphere. No expression was seen in GP or EPN. We conclude that adeno-associated virus serotype 2 (AAV2) is transported throughout the nigro-ST exclusively by anterograde transport. This transport phenomenon directs GDNF expression throughout the basal ganglia in regions that are adversely affected in Parkinson's disease (PD) in addition to SNc. Delivery of vector to SN, however, does not direct expression of GDNF in ST, EPN, or GP. On this basis, we believe that striatal delivery of AAV2-GDNF is the preferred course of action for trophic rescue of DA function.

Adeno-associated viral vector serotypes 1 and 5 targeted to the neonatal rat and pig striatum induce widespread transgene expression in the forebrain

Viral vector-mediated gene transfer has emerged as a powerful means to target transgene expression in the central nervous system. Here we characterized the efficacy of serotypes 1 and 5 recombinant adeno-associated virus (rAAV) vectors encoding green fluorescent protein (GFP) after stereotaxic delivery to the neonatal rat and minipig striatum. The efficiency of GFP expression and the phenotype of GFP-positive cells were assessed within the forebrain at different time points up to 12 months after surgery. Both rAAV1-GFP and rAAV5-GFP delivery resulted in transduction of the striatum as well as striatal input and output areas, including large parts of the cortex. In both species, rAAV5 resulted in a more widespread transgene expression compared to rAAV1. In neonatal rats, rAAV5 also transduced several other areas such as the olfactory bulbs, hippocampus, and septum. Phenotypic analysis of the GFP-positive cells, performed using immunohistochemistry and confocal microscopy, showed that most of the GFP-positive cells by either serotype were NeuN-positive neuronal profiles. The rAAV5 vector further displayed the ability to transduce non-neuronal cell types in both rats and pigs, albeit at a low frequency. Our results show that striatal delivery of rAAV5 vectors in the neonatal brain represents a useful tool to express genes of interest both in the basal ganglia and the neocortex. Furthermore, we apply, for the first time, viral vector-mediated gene transfer to the pig brain providing the opportunity to study effects of genetic manipulation in this non-primate large animal species. Finally, we generated an atlas of the Göttingen minipig brain for guiding future studies in this large animal species.

Transduction of nonhuman primate brain with adeno-associated virus serotype 1: vector trafficking and immune response

We used convection-enhanced delivery (CED) to characterize gene delivery mediated by adeno-associated virus type 1 (AAV1) by tracking expression of hrGFP (humanized green fluorescent protein from Renilla reniformis) into the striatum, basal forebrain, and corona radiata of monkey brain. Four cynomolgus monkeys received single infusions into corona radiata, putamen, and caudate. The other group (n = 4) received infusions into basal forebrain. Thirty days after infusion animals were killed and their brains were processed for immunohistochemical evaluation. Volumetric analysis of GFP-positive brain areas was performed. AAV1-hrGFP infusions resulted in approximately 550, 700, and 73 mm(3) coverage after infusion into corona radiata, striatum, and basal forebrain, respectively. Aside from targeted regions, other brain structures also showed GFP signal (internal and external globus pallidus, subthalamic nucleus), supporting the idea that AAV1 is actively trafficked to regions distal from the infusion site. In addition to neuronal transduction, a significant nonneuronal cell population was transduced by AAV1 vector; for example, oligodendrocytes in corona radiata and astrocytes in the striatum. We observed a strong humoral and cell-mediated response against AAV1-hrGFP in transduced monkeys irrespective of the anatomic location of the infusion, as evidenced by induction of circulating anti-AAV1 and anti-hrGFP antibodies, as well as infiltration of CD4(+) lymphocytes and upregulation of MHC-II in regions infused with vector. We conclude that transduction of antigen-presenting cells within the CNS is a likely cause of this response and that caution is warranted when foreign transgenes are used as reporters in gene therapy studies with vectors with broader tropism than AAV2.

Efficient gene therapy-based method for the delivery of therapeutics to primate cortex

Transduction of the primate cortex with adeno-associated virus (AAV)-based gene therapy vectors has been challenging, because of the large size of the cortex. We report that a single infusion of AAV2 vector into thalamus results in widespread expression of transgene in the cortex through transduction of widely dispersed thalamocortical projections. This finding has important implications for the treatment of certain genetic and neurodegenerative diseases.

Controlled dissemination of AAV vectors in the primate brain

Adeno-associated viral (AAV) vectors are currently the preeminent gene therapy vehicles for neurological application. However, issues regarding the trafficking of AAV vectors within the primate brain, and consequently control over the targeting of transgene expression, remain a matter of investigation. Studies in nonhuman primates have shown that distribution of AAV vectors is largely mediated by the flow of cerebrospinal fluid within perivascular space, trafficking of vector along axonal projections, and AAV receptor binding. Together these processes can result in transduction of cells in areas distant from the parenchymal site of infusion. Additionally, we have addressed the unique surgical issues concerning delivery of AAV vectors by convection-enhanced delivery and are working toward tailored delivery by means of real-time MRI.

Biodistribution of adeno-associated virus type-2 in nonhuman primates after convection-enhanced delivery to brain

A combination treatment of AAV2-hAADC with oral levodopa is a novel therapeutic approach that is being developed for late-stage Parkinson's disease. Biodistribution of AAV2-hAADC was assessed over a wide range of vector dose in 12 monkeys with parkinsonian syndrome, 6 months after intraputamenal infusion. Quantitative PCR (Q-PCR) from all the major neuroanatomical regions of the brain indicated a dose-dependent increase in vector DNA, with 99% being detected in the target site and other basal ganglia tissues. Within these tissues, the distribution varied widely between the putamen (PT) and the globus pallidus, and this was attributed to differences in vector transport. Q-PCR and immunocytochemistry were consistent with results reported earlier for various measures of transgene expression including aromatic L-amino acid decarboxylase (AADC) activity assays, behavioral response, and in vivo imaging with positron emission tomography (PET). Outside of the brain, trace amounts of vector DNA were detected in the spleens of animals in the two highest dose groups, but not in any other peripheral tissue, blood, or cerebrospinal fluid. Some increase in neutralizing antibody titers to adeno-associated virus type-2 (AAV2) capsid protein was observed in monkeys that received high doses of AAV2-hAADC or control AAV2-GFP. This study further validates convection-enhanced delivery (CED) as the preferred method of viral vector delivery to the brain, and supports a Phase I clinical testing of AAV2-hAADC in humans with Parkinson's disease.

Brain area, age and viral vector-specific glial cell-line-derived neurotrophic factor expression and transport in rat

We investigated the feasibility of viral vector-mediated expression and axonal transport of the glial cell-line-derived neurotrophic factor, a potential antiepileptic agent, to the hippocampus and the piriform cortex, areas involved in the induction and spread of seizure activity. Glial cell-line-derived neurotrophic factor overexpression was induced by injections of recombinant vectors derived from serotype 2 adeno-associated virus or lentivirus. We found that recombinant adeno-associated viral vector was able to effectively transduce mitral cells of the olfactory bulb and pyramidal cells of CA1, resulting in transport of glial cell-line-derived neurotrophic factor to the piriform cortex and to the contralateral CA1 area, respectively. These data suggest that the recombinant adeno-associated viral vector vector system is an optimal alternative for therapeutic glial cell-line-derived neurotrophic factor gene transduction and transport of the protein to the epileptogenic brain areas.

Engineering adeno-associated virus for one-step purification via immobilized metal affinity chromatography

Adeno-associated virus (AAV) is a promising vehicle for gene therapy, which will rely on the generation of high-titer, high-purity recombinant vectors. However, numerous purification protocols can involve challenging optimization or scalability issues, and most AAV serotypes do not bind heparin or sialic acid, used for AAV2/3 or AAV4/5 purification, requiring the development of new chromatography strategies. Immobilized metal affinity chromatography (IMAC) allows for robust protein purification via affinity tags such as the hexahistidine (His(6)) sequence. Through the combination of a diverse AAV2 library and rational peptide insertions, we have located an optimal His(6) tag insertion site within the viral capsid. This mutant and a related AAV8 variant can be purified from clarified cell lysate in a single gravity column step at infectious particle yields exceeding 90%. Furthermore, injection of IMAC-purified vector into the brain demonstrates that it mediates high-efficiency gene delivery in vivo, equivalent to that of wild-type capsid, with minimal immune cell activation. This affinity chromatography method may offer advantages in ease of purification, final vector purity, and process scalability. Moreover, a combined rational design and high-throughput library selection approach can aid in the design of enhanced viral gene delivery vectors.

Enhanced gene transfer efficiency in the murine striatum and an orthotopic glioblastoma tumor model, using AAV-7- and AAV-8-pseudotyped vectors

In this study, recombinant AAV vectors pseudotyped with viral capsids derived from AAV serotypes 7 and 8 were evaluated for gene transfer in the murine striatum relative to vectors pseudotyped with AAV serotypes 2, 5, and 6. In comparison with rAAV serotype 2, pseudotyped vectors derived from AAV-7 and AAV-8 have increased transduction efficiency in the murine CNS, with the rank order rAAV-7 > rAAV-8 > rAAV-5 > rAAV-2 = rAAV-6, with all vectors demonstrating a marked tropism for neuronal transduction. Pseudotyped rAAV vector gene transfer in the brain after preimplantation of a murine 4C8 glioblastoma tumor was also evaluated. Efficiency of gene transfer to the orthotopic tumor was increased when using AAV-6, -7, and -8 capsid proteins in comparison with serotype 2, with the order rAAV-8 = rAAV-7 > rAAV-6 > rAAV-2 > rAAV-5. The increased gene transfer efficiency of rAAV vectors pseudotyped with the rAAV-8 capsid also provided enhanced therapeutic efficacy in a mouse model of glioblastoma multiforme, using vectors encoding an inhibitor of the vascular endothelial growth factor pathway. These studies demonstrate that rAAV vectors pseudotyped with capsids derived from AAV serotypes 7 and 8 provide enhanced gene transfer in the murine CNS and may offer increased therapeutic efficacy in the treatment of neurological disease.

Detection of adeno-associated virus 2 and parvovirus B19 in the human dorsolateral prefrontal cortex

Although animal parvoviruses have long been recognized as causes of brain pathology in multiple animal models, especially during early development, human parvoviruses are rarely thought of as neurotropic or causes of neuropathology in humans. However, several recent case reports have suggested possible associations of parvovirus B19 (B19) infection with various neurological and neuropsychiatric symptoms. Adeno-associated virus 2 (AAV2) is related to B19 but has thus far not been shown to be associated with any human disease but is of clinical interest because of the recent use of recombinant AAV vectors in human gene therapy, including gene delivery to the brain. To date, there have been no large-scale studies of the propensity of wild-type human parvoviruses to infect the brain. The Stanley Medical Research Institute Brain Collection offered a unique opportunity to study a large sample (n = 104) of dorsolateral prefrontal cortex (DLPC) DNAs isolated from unaffected control, schizophrenic, and bipolar disorder brains for the presence of parvoviral sequences. This is the first investigator-blinded study to document the presence of parvoviral sequences in the DLPC by utilizing highly sensitive nested polymerase chain reaction (nPCR) and DNA sequencing. Of the overall sample, 6.7% to 12.5% were positive for AAV2, and 14.4% to 42.3% were positive for B19 sequences, with no statistical differences among subgroups. This is the first report to demonstrate the presence of human parvoviruses in a large cohort of adult DLPC, which underscores the need to gain a better insight into the basic biology of parvovirus-brain interactions, including mechanisms of infection and persistence.

Tissue affinity of the infusate affects the distribution volume during convection-enhanced delivery into rodent brains: Implications for local drug delivery

Convection-enhanced delivery (CED) is a recently developed technique for local delivery of agents to a large volume of tissue in the central nervous system (CNS). We have previously reported that this technique can be applied to CNS delivery of nanoparticles including viruses and liposomes. In this paper, we describe the impact of key physical and chemical properties of infused molecules on the extent of CED-mediated delivery. For simple infusates, CED distribution was significantly increased if the infusate was more hydrophilic or had less tissue affinity. Encapsulation of tissue-affinitive molecules by neutral liposomes significantly increased their tissue distribution. The poorer brain distribution observed with cationic liposomes, due to their greater tissue affinity, was completely overcome by PEGylation, which provides steric stabilization and reduced surface charge. Finally, liposomal encapsulation of doxorubicin reduced its tissue affinity and substantially increased its distribution within brain tumor tissue. Taken together, the physical and chemical properties of drugs, small molecules and macromolecular carriers determine the tissue affinity of the infusate and strongly affect the distribution of locally applied agents. Thus, an increased and more predictable tissue distribution can be achieved by reducing the tissue affinity of the infusate using appropriately engineered liposomes or other nanoparticles.

The "perivascular pump" driven by arterial pulsation is a powerful mechanism for the distribution of therapeutic molecules within the brain

We investigated the movement of interstitially infused macromolecules within the central nervous system (CNS) in rats with high and low blood pressure (BP)/heart rate and in rats euthanized immediately before infusion (no heart action). Adeno-associated virus 2 (AAV2), fluorescent liposomes, or bovine serum albumin was infused into rat striatum (six hemispheres per group) by convection-enhanced delivery (CED). After infusion, distribution volumes were evaluated. The rats with high BP/heart rate displayed a significantly larger distribution of the infused molecules within the injected site and more extensive transport of those molecules to the globus pallidus. This difference was particularly apparent for AAV2, for which a 16.5-fold greater distribution of viral capsids was observed in the rats with high BP/heart rate than in the rats with no heartbeat. Similar results were observed for liposomes, despite their larger diameter. The distribution of all infused molecules in all rats that had low or no blood flow was confined to the space around brain blood vessels. These findings show that fluid circulation within the CNS through the perivascular space is the primary mechanism by which viral particles and other therapeutic agents administered by CED are spread within the brain and that cardiac contractions power this process.

Intracranial delivery of CLN2 reduces brain pathology in a mouse model of classical late infantile neuronal ceroid lipofuscinosis

Classical late infantile neuronal ceroid lipofuscinosis (cLINCL) is a lysosomal storage disorder caused by mutations in CLN2, which encodes lysosomal tripeptidyl peptidase I (TPP1). Lack of TPP1 results in accumulation of autofluorescent storage material and curvilinear bodies in cells throughout the CNS, leading to progressive neurodegeneration and death typically in childhood. In this study, we injected adeno-associated virus (AAV) vectors containing the human CLN2 cDNA into the brains of CLN2(-/-) mice to determine therapeutic efficacy. AAV2CUhCLN2 or AAV5CUhCLN2 were stereotaxically injected into the motor cortex, thalamus, and cerebellum of both hemispheres at 6 weeks of age, and mice were then killed at 13 weeks after injection. Mice treated with AAV2CUhCLN2 and AAV5CUhCLN2 contained TPP1 activity at each injection tract that was equivalent to 0.5- and 2-fold that of CLN2(+/+) control mice, respectively. Lysosome-associated membrane protein 1 immunostaining and confocal microscopy showed intracellular targeting of TPP1 to the lysosomal compartment. Compared with control animals, there was a marked reduction of autofluorescent storage in the AAV2CUhCLN2 and AAV5CUhCLN2 injected brain regions, as well as adjacent regions, including the striatum and hippocampus. Analysis by electron microscopy confirmed a significant decrease in pathological curvilinear bodies in cells. This study demonstrates that AAV-mediated TPP1 enzyme replacement corrects the hallmark cellular pathologies of cLINCL in the mouse model and raises the possibility of using AAV gene therapy to treat cLINCL patients.

Treatment of human disease by adeno-associated viral gene transfer

During the past decade, in vivo administration of viral gene transfer vectors for treatment of numerous human diseases has been brought from bench to bedside in the form of clinical trials, mostly aimed at establishing the safety of the protocol. In preclinical studies in animal models of human disease, adeno-associated viral (AAV) vectors have emerged as a favored gene transfer system for this approach. These vectors are derived from a replication-deficient, non-pathogenic parvovirus with a single-stranded DNA genome. Efficient gene transfer to numerous target cells and tissues has been described. AAV is particularly efficient in transduction of non-dividing cells, and the vector genome persists predominantly in episomal forms. Substantial correction, and in some instances complete cure, of genetic disease has been obtained in animal models of hemophilia, lysosomal storage disorders, retinal diseases, disorders of the central nervous system, and other diseases. Therapeutic expression often lasted for months to years. Treatments of genetic disorders, cancer, and other acquired diseases are summarized in this review. Vector development, results in animals, early clinical experience, as well as potential hurdles and challenges are discussed.

Novel caprine adeno-associated virus (AAV) capsid (AAV-Go.1) is closely related to the primate AAV-5 and has unique tropism and neutralization properties

Preexisting humoral immunity to adeno-associated virus (AAV) vectors may limit their clinical utility in gene delivery. We describe a novel caprine AAV (AAV-Go.1) capsid with unique biological properties. AAV-Go.1 capsid was cloned from goat-derived adenovirus preparations. Surprisingly, AAV-Go.1 capsid was 94% identical to the human AAV-5, with differences predicted to be largely on the surface and on or under the spike-like protrusions. In an in vitro neutralization assay using human immunoglobulin G (IgG) (intravenous immune globulin [IVIG]), AAV-Go.1 had higher resistance than AAV-5 (100-fold) and resistance similar to that of AAV-4 or AAV-8. In an in vivo model, SCID mice were pretreated with IVIG to generate normal human IgG plasma levels prior to the administration of AAV human factor IX vectors. Protein expression after intramuscular administration of AAV-Go.1 was unaffected in IVIG-pretreated mice, while it was reduced 5- and 10-fold after administration of AAV-1 and AAV-8, respectively. In contrast, protein expression after intravenous administration of AAV-Go.1 was reduced 7.1-fold, similar to the 3.8-fold reduction observed after AAV-8 administration in IVIG-pretreated mice, and protein expression was essentially extinguished after AAV-2 administration in mice pretreated with much less IVIG (15-fold). AAV-Go.1 vectors also demonstrated a marked tropism for lung when administered intravenously in SCID mice. The pulmonary tropism and high neutralization resistance to human preexisting antibodies suggest novel therapeutic uses for AAV-Go.1 vectors, including targeting diseases such as cystic fibrosis. Nonprimate sources of AAVs may be useful to identify additional capsids with distinct tropisms and high resistance to neutralization by human preexisting antibodies.

Distribution of AAV2-hAADC-transduced cells after 3 years in Parkinsonian monkeys

The present report describes for the first time, the stability of recombinant adeno-associated virus serotype 2 (AAV2) human aromatic L-amino acid decarboxylase (hAADC) gene transfer after 3-year survival time in a non-human primate model of Parkinson's disease. 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned monkeys were treated with six injections of 30 microl/site of AAV2-hAADC at a concentration of 2 x 10(12) vg/ml into the caudate and putamen. Stereological analysis revealed a 46.6% increase in the total number of AAV2-hAADC-transduced cells in the striatum between 8 weeks and 3 years after gene transfer survival time. In the 8-week animals, the distribution of the AADC+ cells was dispersed and heterogeneous, whereas in the 3-year animals it was widespread and homogenous. Confocal analysis demonstrated that approximately 85% of the AADC+ cells were neuronal nuclei immunoreactive.

Adeno-associated virus vectors serotyped with AAV8 capsid are more efficient than AAV-1 or -2 serotypes for widespread gene delivery to the neonatal mouse brain

Adeno-associated virus (AAV) vectors have gained a preeminent position in the field of gene delivery to the normal brain through their ability to achieve extensive transduction of neurons and to mediate long-term gene expression with no apparent toxicity. In adult animals direct infusion of AAV vectors into the brain parenchyma results in highly efficient transduction of target structures. However AAV-mediated global delivery to the adult brain has been an elusive goal. In contrast, widespread global gene delivery has been obtained by i.c.v. injection of AAV1 or AAV2 in neonates. Among the novel AAV serotypes cloned and engineered for production of recombinant vectors, AAV8 has shown a tremendous potential for in vivo gene delivery with nearly complete transduction of many tissues in rodents after intravascular infusion. Here we compare the efficiency of an AAV8 serotyped vector with that of AAV1 and AAV2 serotyped vectors for the extent of gene delivery to the brain after neonatal injection into the lateral ventricles. The vectors all encoded green fluorescent protein (GFP) under control of a hybrid CMV enhancer/chicken beta-actin promoter with AAV2 inverted terminal repeats, but differed from each other with respect to the capsid type. A total of 6.8 x 10(10) genome copies were injected into the lateral ventricles of postnatal day 0 mice. Mice were killed at postnatal day 30 and brains analyzed for distribution of GFP-positive cells. AAV8 proved to be more efficient than AAV1 or AAV2 vectors for gene delivery to all of the structures analyzed, including the cerebral cortex, hippocampus, olfactory bulb, and cerebellum. Moreover the intensity of gene expression, assessed using a microarray reader, was considerably higher for AAV8 in all structures analyzed. In conclusion, the enhanced transduction achieved by AAV8 compared with AAV1 and AAV2 indicates that AAV8 is the superior serotype for gene delivery to the CNS.

Recombinant adeno-associated viral vectors as therapeutic agents to treat neurological disorders

Recombinant adeno-associated virus (rAAV) is derived from a small human parvovirus with an excellent safety profile. In addition, this viral vector efficiently transduces and supports long-term transgene expression in the nervous system. These properties make rAAV a reasonable candidate vector for treating neurological disorders. Indeed, rAAV is currently being used in five early stage clinical trials for various neurodegenerative disorders. Therefore, we will review the currently available preclinical data using rAAV in animal models of central nervous system (CNS) disorders. Moreover, potential caveats for rAAV-based gene therapy in the CNS are also presented.

Recombinant adeno-associated viral vectors in the nervous system

Recombinant adeno-associated virus 2 (rAAV2) has been extensively used as a gene delivery vector for the nervous system. It targets primarily neurons in the nervous system and results in sustained long-term expression of transgenes. New rAAV serotypes have been characterized and demonstrated to have improved transduction efficiencies in various regions of the brain and spinal cord. This review discusses some properties of rAAV that have been studied in the nervous system such as cell tropism, duration of transgene expression, and distribution of viral transduction, as well as immunity and regulation of transgene expression issues, all of which are important for optimization of the use of rAAV in the nervous system.