Recombinant AAV serotype 1 transduction efficiency and tropism in the murine brain

Vector-Mediated Delivery of Transgenes and RNA Interference-Based Gene Silencing Sequences to Astrocytes for Disease Management: Advances and Prospectives

Astrocytes are a type of important glial cell in the brain that serve crucial functions in regulating neuronal activity, facilitating communication between neurons, and keeping everything in balance. In this abstract, we explore current methods and future approaches for using vectors to precisely target astrocytes in the fight against various illnesses. In order to deliver therapeutic cargo selectively to astrocytes, researchers have made tremendous progress by using viral vectors such as adeno-associated viruses (AAVs) and lentiviruses. It has been established that engineered viral vectors are capable of either crossing the blood-brain barrier (BBB) or being delivered intranasally, which facilitates their entrance into the brain parenchyma. These vectors are able to contain transgenes that code for neuroprotective factors, synaptic modulators, or anti-inflammatory medicines, which pave the way for multiple approaches to disease intervention. Strategies based on RNA interference (RNAi) make vector-mediated astrocyte targeting much more likely to work. Small interfering RNAs (siRNAs) and short hairpin RNAs (shRNAs) are two types of RNA that can be made to silence disease-related genes in astrocytes. Vector-mediated delivery in conjunction with RNAi techniques provides a powerful toolkit for investigating the complex biological pathways that contribute to disease development. However, there are still a number of obstacles to overcome in order to perfect the specificity, safety, and duration of vector-mediated astrocyte targeting. In order to successfully translate research findings into clinical practise, it is essential to minimise off-target effects and the risk of immunogenicity. To demonstrate the therapeutic effectiveness of these strategies, rigorous preclinical investigation and validation are required.

A custom-made AAV1 variant (AAV1-T593K) enables efficient transduction of Japanese quail neurons in vitro and in vivo

The widespread use of rodents in neuroscience has prompted the development of optimized viral variants for transduction of brain cells, in vivo. However, many of the viruses developed are less efficient in other model organisms, with birds being among the most resistant to transduction by current viral tools. Resultantly, the use of genetically-encoded tools and methods in avian species is markedly lower than in rodents; likely holding the field back. We sought to bridge this gap by developing custom viruses towards the transduction of brain cells of the Japanese quail. We first develop a protocol for culturing primary neurons and glia from quail embryos, followed by characterization of cultures via immunostaining, single cell mRNA sequencing, patch clamp electrophysiology and calcium imaging. We then leveraged the cultures for the rapid screening of various viruses, only to find that all yielded poor to no infection of cells in vitro. However, few infected neurons were obtained by AAV1 and AAV2. Scrutiny of the sequence of the AAV receptor found in quails led us to rationally design a custom-made AAV variant (AAV1-T593K; AAV1*) that exhibits improved transduction efficiencies in vitro and in vivo (14- and five-fold, respectively). Together, we present unique culturing method, transcriptomic profiles of quail's brain cells and a custom-tailored AAV1 for transduction of quail neurons in vitro and in vivo.

Various AAV Serotypes and Their Applications in Gene Therapy: An Overview

Despite scientific discoveries in the field of gene and cell therapy, some diseases still have no effective treatment. Advances in genetic engineering methods have enabled the development of effective gene therapy methods for various diseases based on adeno-associated viruses (AAVs). Today, many AAV-based gene therapy medications are being investigated in preclinical and clinical trials, and new ones are appearing on the market. In this article, we present a review of AAV discovery, properties, different serotypes, and tropism, and a following detailed explanation of their uses in gene therapy for disease of different organs and systems.

Delivery Platforms for CRISPR/Cas9 Genome Editing of Glial Cells in the Central Nervous System

Glial cells (astrocytes, oligodendrocytes, and microglia) are emerging as key players in several physiological and pathological processes of the central nervous system (CNS). Astrocytes and oligodendrocytes are not only supportive cells that release trophic factors or regulate energy metabolism, but they also actively modulate critical neuronal processes and functions in the tripartite synapse. Microglia are defined as CNS-resident cells that provide immune surveillance; however, they also actively contribute to shaping the neuronal microenvironment by scavenging cell debris or regulating synaptogenesis and pruning. Given the many interconnected processes coordinated by glial cells, it is not surprising that both acute and chronic CNS insults not only cause neuronal damage but also trigger complex multifaceted responses, including neuroinflammation, which can critically contribute to the disease progression and worsening of symptoms in several neurodegenerative diseases. Overall, this makes glial cells excellent candidates for targeted therapies to treat CNS disorders. In recent years, the application of gene editing technologies has redefined therapeutic strategies to treat genetic and age-related neurological diseases. In this review, we discuss the advantages and limitations of clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9-based gene editing in the treatment of neurodegenerative disorders, focusing on the development of viral- and nanoparticle-based delivery methods for in vivo glial cell targeting.

Hypothalamic gene transfer of BDNF promotes healthy aging

The aging process and age-related diseases all involve metabolic decline and impaired ability to cope with adversity. Environmental enrichment (EE)-a housing environment which recapitulates aspects of active lifestyle-exerts a wide range of health benefits in laboratory rodents. Brain-derived neurotrophic factor (BDNF) in the hypothalamus orchestrates autonomic and neuroendocrine processes, serving as one key brain mediator of EE-induced resistance to obesity, cancer, and autoimmunity. Recombinant adeno-associated virus (AAV)-mediated hypothalamic BDNF gene transfer alleviates obesity, diabetes, and metabolic syndromes in both diet-induced and genetic models. One recent study by our lab demonstrates the efficacy and safety of a built-in autoregulatory system to control transgene BDNF expression, mimicking the body's natural feedback systems in middle-age mice. Twelve-month old mice were treated with autoregulatory BDNF vector and monitored for 7months. BDNF gene transfer prevented age-associated metabolic decline by: reducing adiposity, preventing the decline of brown fat activity, increasing adiponectin while reducing leptin and insulin in circulation, improving glucose tolerance, increasing energy expenditure, alleviating hepatic steatosis, and suppressing inflammatory genes in the hypothalamus and adipose tissues. Furthermore, BDNF treatment reduced anxiety-like and depression-like behaviors. This chapter summarizes this work and discusses potential roles that hypothalamic BDNF might play in promoting healthy aging.

AAV Targeting of Glial Cell Types in the Central and Peripheral Nervous System and Relevance to Human Gene Therapy

Different glial cell types are found throughout the central (CNS) and peripheral nervous system (PNS), where they have important functions. These cell types are also involved in nervous system pathology, playing roles in neurodegenerative disease and following trauma in the brain and spinal cord (astrocytes, microglia, oligodendrocytes), nerve degeneration and development of pain in peripheral nerves (Schwann cells, satellite cells), retinal diseases (Müller glia) and gut dysbiosis (enteric glia). These cell type have all been proposed as potential targets for treating these conditions. One approach to target these cell types is the use of gene therapy to modify gene expression. Adeno-associated virus (AAV) vectors have been shown to be safe and effective in targeting cells in the nervous system and have been used in a number of clinical trials. To date, a number of studies have tested the use of different AAV serotypes and cell-specific promoters to increase glial cell tropism and expression. However, true glial-cell specific targeting for a particular glial cell type remains elusive. This review provides an overview of research into developing glial specific gene therapy and discusses some of the issues that still need to be addressed to make glial cell gene therapy a clinical reality.

Gene Therapy Vector Encoding Neuropeptide Y and Its Receptor Y2 for Future Treatment of Epilepsy: Preclinical Data in Rats

Gene therapy to treat pharmacoresistant temporal lobe epilepsy in humans is now being developed using an AAV vector (CG01) that encodes the combination of neuropeptide Y and its antiepileptic receptor Y2. With this in mind, the present study aimed to provide important preclinical data on the effects of CG01 on the duration of transgene expression, cellular tropism, and potential side effects on body weight and cognitive function. The CG01 vector was administered unilaterally into the dorsal and ventral hippocampus of adult male rats and expression of both transgenes was found to remain elevated without a sign of decline at 6 months post-injection. CG01 appeared to mediate expression selectively in hippocampal neurons, without expression in astrocytes or oligodendrocytes. No effects were seen on body weight as well as on short- or long-term memory as revealed by testing in the Y-maze or Morris water maze tests. Thus these data show that unilateral CG01 vector treatment as future gene therapy in pharmacoresistant temporal lobe epilepsy patients should result in stable and long-term expression predominantly in neurons and be well tolerated without side effects on body weight and cognitive function.

InVivo Cancer-Based Functional Genomics

Pinpointing the underlying mechanisms that drive tumorigenesis in human patients is a prerequisite for identifying suitable therapeutic targets for precision medicine. In contrast to cell culture systems, mouse models are highly favored for evaluating tumor progression and therapeutic response in a more realistic in vivo context. The past decade has witnessed a dramatic increase in the number of functional genomic studies using diverse mouse models, including in vivo clustered regularly interspaced short palindromic repeats (CRISPR) and RNA interference (RNAi) screens, and these have provided a wealth of knowledge addressing multiple essential questions in translational cancer research. We compare the multiple mouse systems and genomic tools that are commonly used for in vivo screens to illustrate their strengths and limitations. Crucial components of screen design and data analysis are also discussed.

Enhancer-Driven Gene Expression (EDGE) Enables the Generation of Viral Vectors Specific to Neuronal Subtypes

Although a variety of remarkable molecular tools for studying neural circuits have recently been developed, the ability to deploy them in particular neuronal subtypes is limited by the fact that native promoters are almost never specific enough. We recently showed that one can generate transgenic mice with anatomical specificity surpassing that of native promoters by combining enhancers uniquely active in particular brain regions with a heterologous minimal promoter, an approach we call EDGE (Enhancer-Driven Gene Expression). Here we extend this strategy to the generation of viral (rAAV) vectors, showing that some EDGE rAAVs can recapitulate the specificity of the corresponding transgenic lines in wild-type animals, even of another species. This approach thus holds the promise of enabling circuit-specific manipulations in wild-type animals, not only enhancing our understanding of brain function, but perhaps one day even providing novel therapeutic avenues to approach disorders of the brain.

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rAAVs with enhancers unique to a brain region specify cell types of that brain region

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This requires viral constructs optimized to express only with enhancers

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One rAAV distinguishes distinct subtypes of excitatory neurons in a cortical layer

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The same specificity is seen in wild-type animals of at least two species

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Hypothalamic gene transfer of BDNF promotes healthy aging in mice

The aging process and age-related diseases all involve perturbed energy adaption and impaired ability to cope with adversity. Brain-derived neurotrophic factor (BDNF) in the hypothalamus plays important role in regulation of energy balance. Our previous studies show that recombinant adeno-associated virus (AAV)-mediated hypothalamic BDNF gene transfer alleviates obesity, diabetes, and metabolic syndromes in both diet-induced and genetic models. Here we examined the efficacy and safety of a built-in autoregulatory system to control transgene BDNF expression mimicking the body's natural feedback systems in middle-aged mice. Twelve-month-old mice were treated with either autoregulatory BDNF vector or yellow fluorescence protein (YFP) control, maintained on normal diet, and monitored for 28 weeks. BDNF gene transfer prevented the development of aging-associated metabolic declines characterized by: preventing aging-associated weight gain, reducing adiposity, reversing the decline of brown fat activity, increasing adiponectin while reducing leptin and insulin in circulation, improving glucose tolerance, increasing energy expenditure, alleviating hepatic steatosis, and suppressing inflammatory genes in the hypothalamus and adipose tissues. Moreover, BDNF treatment reduced anxiety-like and depression-like behaviors. These safety and efficacy data provide evidence that hypothalamic BDNF is a target for promoting healthy aging.

Adeno-Associated Virus (AAV) as a Vector for Gene Therapy

There has been a resurgence in gene therapy efforts that is partly fueled by the identification and understanding of new gene delivery vectors. Adeno-associated virus (AAV) is a non-enveloped virus that can be engineered to deliver DNA to target cells, and has attracted a significant amount of attention in the field, especially in clinical-stage experimental therapeutic strategies. The ability to generate recombinant AAV particles lacking any viral genes and containing DNA sequences of interest for various therapeutic applications has thus far proven to be one of the safest strategies for gene therapies. This review will provide an overview of some important factors to consider in the use of AAV as a vector for gene therapy.

Destination Brain: the Past, Present, and Future of Therapeutic Gene Delivery

Neurological diseases and disorders (NDDs) present a significant societal burden and currently available drug- and biological-based therapeutic strategies have proven inadequate to alleviate it. Gene therapy is a suitable alternative to treat NDDs compared to conventional systems since it can be tailored to specifically alter select gene expression, reverse disease phenotype and restore normal function. The scope of gene therapy has broadened over the years with the advent of RNA interference and genome editing technologies. Consequently, encouraging results from central nervous system (CNS)-targeted gene delivery studies have led to their transition from preclinical to clinical trials. As we shift to an exciting gene therapy era, a retrospective of available literature on CNS-associated gene delivery is in order. This review is timely in this regard, since it analyzes key challenges and major findings from the last two decades and evaluates future prospects of brain gene delivery. We emphasize major areas consisting of physiological and pharmacological challenges in gene therapy, function-based selection of a ideal cellular target(s), available therapy modalities, and diversity of viral vectors and nanoparticles as vehicle systems. Further, we present plausible answers to key questions such as strategies to circumvent low blood-brain barrier permeability and most suitable CNS cell types for targeting. We compare and contrast pros and cons of the tested viral vectors in the context of delivery systems used in past and current clinical trials. Gene vector design challenges are also evaluated in the context of cell-specific promoters. Key challenges and findings reported for recent gene therapy clinical trials, assessing viral vectors and nanoparticles are discussed from the perspective of bench to bedside gene therapy translation. We conclude this review by tying together gene delivery challenges, available vehicle systems and comprehensive analyses of neuropathogenesis to outline future prospects of CNS-targeted gene therapies.

Recombinant Adeno-Associated Virus Serotype 6 (rAAV6) Potently and Preferentially Transduces Rat Astrocytes In vitro and In vivo

Recombinant adeno-associated virus vectors are an increasingly popular tool for gene delivery to the CNS because of their non-pathological nature, low immunogenicity, and ability to stably transduce dividing and non-dividing cells. One of the limitations of rAAVs is their preferential tropism for neuronal cells. Glial cells, specifically astrocytes, appear to be infected at low rates. To overcome this limitation, previous studies utilized rAAVs with astrocyte-specific promoters or assorted rAAV serotypes and pseudotypes with purported selectivity for astrocytes. Yet, the reported glial infection rates are not consistent from study to study. In the present work, we tested seven commercially available recombinant serotypes- rAAV1, 2, and 5 through 9, for their ability to transduce primary rat astrocytes [visualized via viral expression of green fluorescent protein (GFP)]. In cell cultures, rAAV6 consistently demonstrated the highest infection rates, while rAAV2 showed astrocytic transduction in some, but not all, of the tested viral batches. To verify that all rAAV constructs utilized by us were viable and effective, we confirmed high infectivity rates in retinal pigmented epithelial cells (ARPE-19), which are known to be transduced by numerous rAAV serotypes. Based on the in vitro results, we next tested the cell type tropism of rAAV6 and rAAV2 in vivo, which were both injected in the barrel cortex at approximately equal doses. Three weeks later, the brains were sectioned and immunostained for viral GFP and the neuronal marker NeuN or the astrocytic marker GFAP. We found that rAAV6 strongly and preferentially transduced astrocytes (>90% of cells in the virus-infected areas), but not neurons (∼10% infection rate). On the contrary, rAAV2 preferentially infected neurons (∼65%), but not astrocytes (∼20%). Overall, our results suggest that rAAV6 can be used as a tool for manipulating gene expression (either delivery or knockdown) in rat astrocytes in vivo.

Differential effects of peripheral and brain tumor necrosis factor on inflammation, sickness, emotional behavior and memory in mice

Tumor necrosis factor alpha (TNF) is increased in depression and clinical-trial evidence indicates that blocking peripheral TNF has some antidepressant efficacy. In rodents, peripheral or intracerebroventricular TNF results in sickness e.g. reduced body weight, altered emotional behavior and impaired memory. However, the underlying pathways and responsible brain regions are poorly understood. The aim of this mouse study was to increase understanding by comparing the effects of sustained increases in TNF in the circulation, in brain regions impacted by increased circulating TNF, or specific brain regions. Increased peripheral TNF achieved by repeated daily injection (IP-TNF) or osmotic pump resulted in decreased body weight, decreased saccharin (reward) consumption, and increased memory of an aversive conditioned stimulus. These effects co-occurred with increased plasma interleukin-6 and increased IP-derived TNF in brain peri-ventricular regions. An adenovirus-associated viral TNF vector (AAV-TNF) was constructed, brain injection of which resulted in dose-dependent, sustained and region-specific TNF expression, and was without effect on blood cytokine levels. Lateral ventricle AAV-TNF yielded increased TNF in the same brain regions as IP-TNF. In contrast to IP-TNF it was without effect on body weight, saccharin consumption and fear memory, although it did increase anxiety. Hippocampal AAV-TNF led to decreased body weight. It increased conditioning to but not subsequent memory of an aversive context, suggesting impaired consolidation; it also increased anxiety. Amygdala AAV-TNF was without effect on body weight and aversive stimulus learning-memory, but reduced saccharin consumption and increased anxiety. This study adds significantly to the evidence that both peripheral and brain region-specific increases in TNF lead to both sickness and depression- and anxiety disorder-relevant behavior and do so via different pathways. It thereby highlights the complexity in terms of indirect and direct pathways via which increased TNF can act and which need to be taken into account when considering it as a therapeutic target.

Gene therapy for the CNS using AAVs: The impact of systemic delivery by AAV9

Several attempts have been made to discover the ideal vector for gene therapy in central nervous system (CNS). Adeno-associated viruses (AAVs) are currently the preferred vehicle since they exhibit stable transgene expression in post-mitotic cells, neuronal tropism, low risk of insertional mutagenesis and diminished immune responses. Additionally, the discovery that a particular serotype, AAV9, bypasses the blood-brain barrier has raised the possibility of intravascular administration as a non-invasive delivery route to achieve widespread CNS gene expression. AAV9 intravenous delivery has already shown promising results for several diseases in animal models, including lysosomal storage disorders and motor neuron diseases, opening the way to the first clinical trial in the field. This review presents an overview of clinical trials for CNS disorders using AAVs and will focus on preclinical studies based on the systemic gene delivery using AAV9.

Widespread AAV1- and AAV2-mediated transgene expression in the nonhuman primate brain: implications for Huntington's disease

Huntington’s disease (HD) is caused by a toxic gain-of-function associated with the expression of the mutant huntingtin (htt) protein. Therefore, the use of RNA interference to inhibit Htt expression could represent a disease-modifying therapy. The potential of two recombinant adeno-associated viral vectors (AAV), AAV1 and AAV2, to transduce the cortico-striatal tissues that are predominantly affected in HD was explored. Green fluorescent protein was used as a reporter in each vector to show that both serotypes were broadly distributed in medium spiny neurons in the striatum and cortico-striatal neurons after infusion into the putamen and caudate nucleus of nonhuman primates (NHP), with AAV1-directed expression being slightly more robust than AAV2-driven expression. This study suggests that both serotypes are capable of targeting neurons that degenerate in HD, and it sets the stage for the advanced preclinical evaluation of an RNAi-based therapy for this disease.

Characterization of the Adeno-Associated Virus 1 and 6 Sialic Acid Binding Site

The adeno-associated viruses (AAVs), which are being developed as gene delivery vectors, display differential cell surface glycan binding and subsequent tissue tropisms. For AAV serotype 1 (AAV1), the first viral vector approved as a gene therapy treatment, and its closely related AAV6, sialic acid (SIA) serves as their primary cellular surface receptor. Toward characterizing the SIA binding site(s), the structure of the AAV1-SIA complex was determined by X-ray crystallography to 3.0 Å. Density consistent with SIA was observed in a pocket located at the base of capsid protrusions surrounding icosahedral 3-fold axes. Site-directed mutagenesis substitution of the amino acids forming this pocket with structurally equivalent residues from AAV2, a heparan sulfate binding serotype, followed by cell binding and transduction assays, further mapped the critical residues conferring SIA binding to AAV1 and AAV6. For both viruses five of the six binding pocket residues mutated (N447S, V473D, N500E, T502S, and W503A) abolished SIA binding, whereas S472R increased binding. All six mutations abolished or decreased transduction by at least 50% in AAV1. Surprisingly, the T502S substitution did not affect transduction efficiency of wild-type AAV6. Furthermore, three of the AAV1 SIA binding site mutants-S472R, V473D, and N500E-escaped recognition by the anti-AAV1 capsid antibody ADK1a. These observations demonstrate that common key capsid surface residues dictate both virus binding and entry processes, as well as antigenic reactivity. This study identifies an important functional capsid surface "hot spot" dictating receptor attachment, transduction efficiency, and antigenicity which could prove useful for vector engineering.

IMPORTANCE

The adeno-associated virus (AAV) vector gene delivery system has shown promise in several clinical trials and an AAV1-based vector has been approved as the first gene therapy treatment. However, limitations still exist with respect to transduction efficiency and the detrimental effects of preexisting host antibodies. This study aimed to identify key capsid regions which can be engineered to overcome these limitations. A sialic glycan receptor recognition pocket was identified in AAV1 and its closely related AAV6, using X-ray crystallography. The site was confirmed by mutagenesis followed by cell binding and transduction assays. Significantly, residues controlling gene expression efficiency, as well as antibody escape variants, were also identified. This study thus provides, at the amino acid level, information for rational structural engineering of AAV vectors with improved therapeutic efficacy.

Movement and structure of mitochondria in oligodendrocytes and their myelin sheaths

Mitochondria play several crucial roles in the life of oligodendrocytes. During development of the myelin sheath they are essential providers of carbon skeletons and energy for lipid synthesis. During normal brain function their consumption of pyruvate will be a key determinant of how much lactate is available for oligodendrocytes to export to power axonal function. Finally, during calcium-overload induced pathology, as occurs in ischemia, mitochondria may buffer calcium or induce apoptosis. Despite their important functions, very little is known of the properties of oligodendrocyte mitochondria, and mitochondria have never been observed in the myelin sheaths. We have now used targeted expression of fluorescent mitochondrial markers to characterize the location and movement of mitochondria within oligodendrocytes. We show for the first time that mitochondria are able to enter and move within the myelin sheath. Within the myelin sheath the highest number of mitochondria was in the cytoplasmic ridges along the sheath. Mitochondria moved more slowly than in neurons and, in contrast to their behavior in neurons and astrocytes, their movement was increased rather than inhibited by glutamate activating NMDA receptors. By electron microscopy we show that myelin sheath mitochondria have a low surface area of cristae, which suggests a low ATP production. These data specify fundamental properties of the oxidative phosphorylation system in oligodendrocytes, the glial cells that enhance cognition by speeding action potential propagation and provide metabolic support to axons.

AAV2/1 CD74 Gene Transfer Reduces β-amyloidosis and Improves Learning and Memory in a Mouse Model of Alzheimer's Disease

Modulation of the amyloid-β (Aβ) trafficking pathway heralds a new therapeutic frontier for Alzheimer's disease (AD). As CD74 binds to the amyloid-β precursor protein (APP) and can suppresses Aβ processing, we investigated whether recombinant adeno-associated virus (AAV) delivery of CD74 could reduce Aβ production and affect disease outcomes. This idea was tested in a mouse AD model. Cotransduction of AAV-tetracycline-controlled transactivator (tTA) and AAV-tet-response element (TRE)-CD74 resulted in CD74 expression, reduced Aβ production in mouse neurons containing the human APP with familial AD-linked mutations. Stereotaxic injection of AAV-TRE-GFP or CD74 into the hippocampi of an AD mouse, defined as a TgCRND8 × calmodulin-dependent protein kinase II derived promoter-tTA double-transgenic, reduced Aβ loads and pyramidal neuronal Aβ accumulation in the hippocampus. Immunofluorescent studies showed that APP colocalization with Lamp1 was increased in CD74-expressing neurons. Moreover, Morris water maze tasks demonstrated that mice treated with AAV-TRE-CD74 showed improved learning and memory compared to AAV-TRE-GFP control animals. These results support the idea that CD74-induced alteration of Aβ processing could improve AD-associated memory deficits as shown in mouse models of human disease.

Use of Adeno-Associated and Herpes Simplex Viral Vectors for In Vivo Neuronal Expression in Mice

Adeno-associated viruses and the herpes simplex virus are the two most widely used vectors for the in vivo expression of exogenous genes. Advances in the development of these vectors have enabled remarkable temporal and spatial control of gene expression. This unit provides methods for storing, delivering, and verifying expression of adeno-associated and herpes simplex viruses in the adult mouse brain. It also describes important considerations for experiments using in vivo expression of these viral vectors, including serotype and promoter selection, as well as timing of expression. Additional protocols are provided that describe methods for preliminary experiments to determine the appropriate conditions for in vivo delivery.

Enzyme replacement in the CSF to treat metachromatic leukodystrophy in mouse model using single intracerebroventricular injection of self-complementary AAV1 vector

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease caused by a functional deficiency in human arylsulfatase A (hASA). We recently reported that ependymal cells and the choroid plexus are selectively transduced by intracerebroventricular (ICV) injection of adeno-associated virus serotype 1 (AAV1) vector and serve as a biological reservoir for the secretion of lysosomal enzymes into the cerebrospinal fluid (CSF). In the present study, we examined the feasibility of this AAV-mediated gene therapy to treat MLD model mice. Preliminary experiments showed that the hASA level in the CSF after ICV injection of self-complementary (sc) AAV1 was much higher than in mice injected with single-stranded AAV1 or scAAV9. However, when 18-week-old MLD mice were treated with ICV injection of scAAV1, the concentration of hASA in the CSF gradually decreased and was not detectable at 12 weeks after injection, probably due to the development of anti-hASA antibodies. As a result, the sulfatide levels in brain tissues of treated MLD mice were only slightly reduced compared with those of untreated MLD mice. These results suggest that this approach is potentially promising for treating MLD, but that controlling the immune response appears to be crucial for long-term expression of therapeutic proteins in the CSF.

Single nucleotide seed modification restores in vivo tolerability of a toxic artificial miRNA sequence in the mouse brain

Huntington's disease is a fatal neurodegenerative disease caused by polyglutamine-expansion in huntingtin (HTT). Recent work showed that gene silencing approaches, including RNA interference (RNAi), improve disease readouts in mice. To advance RNAi to the clinic, we designed miHDS1, with robust knockdown of human HTT and minimized silencing of unintended transcripts. In Rhesus macaque, AAV delivery of miHDS1 to the putamen reduced HTT expression with no adverse effects on neurological status including fine and gross motor skills, no immune activation and no induction of neuropathology out to 6 weeks post injection. Others showed safety of a different HTT-targeting RNAi in monkeys for 6 months. Application of miHDS1 to Huntington's patients requires further safety testing in normal rodents, despite the fact that it was optimized for humans. To satisfy this regulatory requirement, we evaluated normal mice after AAV.miHDS1 injection. In contrast to monkeys, neurological deficits occurred acutely in mice brain and was attributed to off-target silencing through interactions of miHDS1 with the 3′UTR of other transcripts. While we resolved miHDS1 toxicity in mouse brain and maintained miHDS1-silencing efficacy, these studies highlight that optimizing nucleic acid-based medicines for safety in humans presents challenges for safety testing in rodents or other distantly related species.

Adeno-associated virus serotypes 1, 8, and 9 share conserved mechanisms for anterograde and retrograde axonal transport

Adeno-associated virus (AAV) vectors often undergo long-distance axonal transport after brain injection. This leads to transduction of brain regions distal to the injection site, although the extent of axonal transport and distal transduction varies widely among AAV serotypes. The mechanisms driving this variability are poorly understood. This is a critical problem for applications that require focal gene expression within a specific brain region, and also impedes the utilization of vector transport for applications requiring widespread delivery of transgene to the brain. Here, we compared AAV serotypes 1 and 9, which frequently demonstrate distal transduction, with serotype 8, which rarely spreads beyond the injection site. To examine directional AAV transport in vitro, we used a microfluidic chamber to apply dye-labeled AAV to the axon termini or to the cell bodies of primary rat embryonic cortical neurons. All three serotypes were actively transported along axons, with transport characterized by high velocities and prolonged runs in both the anterograde and retrograde directions. Coinfection with pairs of serotypes indicated that AAV1, 8, and 9 share the same intracellular compartments for axonal transport. In vivo, both AAV8 and 9 demonstrated anterograde and retrograde transport within a nonreciprocal circuit after injection into adult mouse brain, with highly similar distributions of distal transduction. However, in mass-cultured neurons, we found that AAV1 was more frequently transported than AAV8 or 9, and that the frequency of AAV9 transport could be enhanced by increasing receptor availability. Thus, while these serotypes share conserved mechanisms for axonal transport both in vitro and in vivo, the frequency of transport can vary among serotypes, and axonal transport can be markedly increased by enhancing vector uptake. This suggests that variability in distal transduction in vivo likely results from differential uptake at the plasma membrane, rather than fundamental differences in transport mechanisms among AAV serotypes.

Adeno-associated viral (AAV) vectors do not efficiently target muscle satellite cells

Adeno-associated viral (AAV) vectors are becoming an important tool for gene therapy of numerous genetic and other disorders. Several recombinant AAV vectors (rAAV) have the ability to transduce striated muscles in a variety of animals following intramuscular and intravascular administration, and have attracted widespread interest for therapy of muscle disorders such as the muscular dystrophies. However, most studies have focused on the ability to transduce mature muscle cells, and have not examined the ability to target myogenic stem cells such as skeletal muscle satellite cells. Here we examined the relative ability of rAAV vectors derived from AAV6 to target myoblasts, myocytes, and myotubes in culture and satellite cells and myofibers in vivo. AAV vectors are able to transduce proliferating myoblasts in culture, albeit with reduced efficiency relative to postmitotic myocytes and myotubes. In contrast, quiescent satellite cells are refractory to transduction in adult mice. These results suggest that while muscle disorders characterized by myofiber regeneration can be slowed or halted by AAV transduction, little if any vector transduction can be obtained in myogenic stems cells that might other wise support ongoing muscle regeneration.

Efficient gene delivery and selective transduction of astrocytes in the mammalian brain using viral vectors

Astrocytes are now considered as key players in brain information processing because of their newly discovered roles in synapse formation and plasticity, energy metabolism and blood flow regulation. However, our understanding of astrocyte function is still fragmented compared to other brain cell types. A better appreciation of the biology of astrocytes requires the development of tools to generate animal models in which astrocyte-specific proteins and pathways can be manipulated. In addition, it is becoming increasingly evident that astrocytes are also important players in many neurological disorders. Targeted modulation of protein expression in astrocytes would be critical for the development of new therapeutic strategies. Gene transfer is valuable to target a subpopulation of cells and explore their function in experimental models. In particular, viral-mediated gene transfer provides a rapid, highly flexible and cost-effective, in vivo paradigm to study the impact of genes of interest during central nervous system development or in adult animals. We will review the different strategies that led to the recent development of efficient viral vectors that can be successfully used to selectively transduce astrocytes in the mammalian brain.

Recent advances in RNA interference therapeutics for CNS diseases

Over the last decade, RNA interference technology has shown therapeutic promise in rodent models of dominantly inherited brain diseases, including those caused by polyglutamine repeat expansions in the coding region of the affected gene. For some of these diseases, proof-of concept studies in model organisms have transitioned to safety testing in larger animal models, such as the nonhuman primate. Here, we review recent progress on RNA interference-based therapies in various model systems. We also highlight outstanding questions or concerns that have emerged as a result of an improved (and ever advancing) understanding of the technologies employed.

Transgenic models of Alzheimer's disease: better utilization of existing models through viral transgenesis

Animal models have been used for decades in the Alzheimer's disease (AD) research field and have been crucial for the advancement of our understanding of the disease. Most models are based on familial AD mutations of genes involved in the amyloidogenic process, such as the amyloid precursor protein (APP) and presenilin 1 (PS1). Some models also incorporate mutations in tau (MAPT) known to cause frontotemporal dementia, a neurodegenerative disease that shares some elements of neuropathology with AD. While these models are complex, they fail to display pathology that perfectly recapitulates that of the human disease. Unfortunately, this level of pre-existing complexity creates a barrier to the further modification and improvement of these models. However, as the efficacy and safety of viral vectors improves, their use as an alternative to germline genetic modification is becoming a widely used research tool. In this review we discuss how this approach can be used to better utilize common mouse models in AD research. This article is part of a Special Issue entitled: Animal Models of Disease.

Viral vectors for gene delivery to the central nervous system

The potential benefits of gene therapy for neurological diseases such as Parkinson's, Amyotrophic Lateral Sclerosis (ALS), Epilepsy, and Alzheimer's are enormous. Even a delay in the onset of severe symptoms would be invaluable to patients suffering from these and other diseases. Significant effort has been placed in developing vectors capable of delivering therapeutic genes to the CNS in order to treat neurological disorders. At the forefront of potential vectors, viral systems have evolved to efficiently deliver their genetic material to a cell. The biology of different viruses offers unique solutions to the challenges of gene therapy, such as cell targeting, transgene expression and vector production. It is important to consider the natural biology of a vector when deciding whether it will be the most effective for a specific therapeutic function. In this review, we outline desired features of the ideal vector for gene delivery to the CNS and discuss how well available viral vectors compare to this model. Adeno-associated virus, retrovirus, adenovirus and herpesvirus vectors are covered. Focus is placed on features of the natural biology that have made these viruses effective tools for gene delivery with emphasis on their application in the CNS. Our goal is to provide insight into features of the optimal vector and which viral vectors can provide these features.

Merits of combination cortical, subcortical, and cerebellar injections for the treatment of Niemann-Pick disease type A

Niemann-Pick disease Type A (NPA) is a neuronopathic lysosomal storage disease (LSD) caused by the loss of acid sphingomyelinase (ASM). The goals of the current study are to ascertain the levels of human ASM that are efficacious in ASM knockout (ASMKO) mice, and determine whether these levels can be attained in non-human primates (NHPs) using a multiple parenchymal injection strategy. Intracranial injections of different doses of AAV1-hASM in ASMKO mice demonstrated that only a small amount of enzyme (<0.5 mg hASM/g tissue) was sufficient to increase survival, and that increasing the amount of hASM did not enhance this survival benefit until a new threshold level of >10 mg hASM/g tissue was reached. In monkeys, injection of 12 tracts of AAV1-hASM resulted in efficacious levels of enzyme in broad regions of the brain that was aided, in part, by axonal transport of adeno-associated virus (AAV) and movement through the perivascular space. This study demonstrates that a combination cortical, subcortical, and cerebellar injection protocol could provide therapeutic levels of hASM to regions of the NHP brain that are highly affected in NPA patients. The information from this study might help design new AAV-mediated enzyme replacement protocols for NPA and other neuronopathic LSDs in future clinical trials.

Correction of brain oligodendrocytes by AAVrh.10 intracerebral gene therapy in metachromatic leukodystrophy mice

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder characterized by accumulation of sulfatides in glial cells and neurons, the result of an inherited deficiency of arylsulfatase A (ARSA; EC 3.1.6.8) and myelin degeneration in the central and peripheral nervous systems. No effective treatment is currently available for the most frequent late infantile (LI) form of MLD, which results in rapid neurological degradation and early death after the onset of clinical manifestations. To potentially arrest or reverse disease progression, ARSA enzyme must be rapidly delivered to brain oligodendrocytes of patients with LI MLD. We previously showed that brain gene therapy with adeno-associated virus serotype 5 (AAV5) driving the expression of human ARSA cDNA under the control of the murine phosphoglycerate kinase (PGK) promoter alleviated most long-term disease manifestations in MLD mice. Herein, we evaluated the short-term effects of AAVrh.10 driving the expression of human ARSA cDNA under the control of the cytomegalovirus/β-actin hybrid (CAG/cu) promoter in 8-month-old MLD mice that already show marked sulfatide accumulation and brain pathology. Within 2 months, and in contrast to results with the AAV5-PGK-ARSA vector, a single intrastriatal injection of AAVrh.10cuARSA resulted in correction of brain sulfatide storage, accumulation of specific sulfatide species in oligodendrocytes, and associated brain pathology in the injected hemisphere. Better potency of the AAVrh.10cuARSA vector was mediated by higher neuronal and oligodendrocyte transduction, axonal transport of the AAVrh.10 vector and ARSA enzyme, as well as higher CAG/cu promoter driven expression of ARSA enzyme. These results strongly support the use of AAVrh.10cuARSA vector for intracerebral gene therapy in rapidly progressing early-onset forms of MLD.

Recombinant adeno-associated virus: clinical application and development as a gene-therapy vector

Gene therapy is gaining momentum as a method of treating human disease. Initially conceived as a strategy to complement defective genes in monogenic disorders, the scope of gene therapy has expanded to encompass a variety of applications. Likewise, the molecular tools for gene delivery have evolved and diversified to meet these various therapeutic needs. Recombinant adeno-associated virus (rAAV) has made significant strides toward clinical application with an excellent safety profile and successes in several clinical trials. This review covers the basic biology of rAAV as a gene therapy vector as well as its advantages compared with other methods of gene delivery. The status of clinical trials utilizing rAAV is also discussed in detail. In conclusion, methods of engineering the vector to overcome challenges identified from these trials are covered, with emphasis on modification of the viral capsid to increase the tissue/cell-specific targeting and transduction efficiency.

Rapid transgene expression in multiple precursor cell types of adult rat subventricular zone mediated by adeno-associated type 1 vectors

The adult rat brain subventricular zone (SVZ) contains proliferative precursors that migrate to the olfactory bulb (OB) and differentiate into mature neurons. Recruitment of precursors constitutes a potential avenue for brain repair. We have investigated the kinetics and cellular specificity of transgene expression mediated by AAV2/1 vectors (i.e., adeno-associated virus type 2 pseudotyped with AAV1 capsid) in the SVZ. Self-complementary (sc) and single-stranded (ss) AAV2/1 vectors mediated efficient GFP expression, respectively, at 17 and 24 hr postinjection. Transgene expression was efficient in all the rapidly proliferating cells types, that is, Mash1(+) precursors (30% of the GFP(+) cells), Dlx2(+) neuronal progenitors (55%), Olig2(+) oligodendrocyte progenitors (35%), and doublecortin-positive (Dcx(+)) migrating cells (40%), but not in the slowly proliferating glial fibrillary acidic protein-positive (GFAP(+)) neural stem cell pool (5%). Because cell cycle arrest by wild-type and recombinant AAV has been described in primary cultures, we examined SVZ proliferative activity after vector injection. Indeed, cell proliferation was reduced immediately after vector injection but was normal after 1 month. In contrast, migration and differentiation of GFP(+) precursors were unaltered. Indeed, the proportion of Dcx(+) cells was similar in the injected and contralateral hemispheres. Furthermore, 1 month after vector injection into the SVZ, GFP(+) cells, found, as expected, in the OB granular cell layer, were mature GABAergic neurons. In conclusion, the rapid and efficient transgene expression in SVZ neural precursors mediated by scAAV2/1 vectors underlines their potential usefulness for brain repair via recruitment of immature cells. The observed transient precursor proliferation inhibition, not affecting their migration and differentiation, will likely not compromise this strategy.

Intracellular transport of recombinant adeno-associated virus vectors

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

Hematopoietic stem cell and gene therapy corrects primary neuropathology and behavior in mucopolysaccharidosis IIIA mice

Mucopolysaccharidosis IIIA (MPS IIIA or Sanfilippo disease) is a neurodegenerative disorder caused by a deficiency in the lysosomal enzyme sulfamidase (SGSH), catabolizing heparan sulfate (HS). Affected children present with severe behavioral abnormalities, sleep disturbances, and progressive neurodegeneration, leading to death in their second decade. MPS I, a similar neurodegenerative disease accumulating HS, is treated successfully with hematopoietic stem cell transplantation (HSCT) but this treatment is ineffectual for MPS IIIA. We compared HSCT in MPS IIIA mice using wild-type donor cells transduced ex vivo with lentiviral vector-expressing SGSH (LV-WT-HSCT) versus wild-type donor cell transplant (WT-HSCT) or lentiviral-SGSH transduced MPS IIIA cells (LV-IIIA-HSCT). LV-WT-HSCT results in 10% of normal brain enzyme activity, near normalization of brain HS and GM2 gangliosides, significant improvements in neuroinflammation and behavioral correction. Both WT-HSCT and LV-IIIA-HSCT mediated improvements in GM2 gangliosides and neuroinflammation but were less effective at reducing HS or in ameliorating abnormal HS sulfation and had no significant effect on behavior. This suggests that HS may have a more significant role in neuropathology than neuroinflammation or GM2 gangliosides. These data provide compelling evidence for the efficacy of gene therapy in conjunction with WT-HSCT for neurological correction of MPS IIIA where conventional transplant is ineffectual.

The advent of AAV9 expands applications for brain and spinal cord gene delivery

INTRODUCTION

Straightforward studies compared adeno-associated virus (AAV) serotypes to determine the most appropriate one for robust expression in the CNS. AAV9 was efficient when directly injected into the brain, but more surprisingly, AAV9 produced global expression in the brain and spinal cord after a peripheral, systemic route of administration to neonatal mice.

AREAS COVERED

Topics include AAV9 gene delivery from intraparenchymal, intravenous, intrathecal and intrauterine routes of administration, and related preclinical studies and disease models. Systemic AAV9 gene transfer yields remarkably consistent neuronal expression, though only in early development. AAV9 is versatile to study neuropathological proteins: microtubule-associated protein tau and transactive response DNA-binding protein 43 kDa (TDP-43).

EXPERT OPINION

AAV9 will be more widely used based on current data, although other natural serotypes and recombineered vectors may also support or improve upon wide-scale expression. A peripheral-to-central gene delivery that can affect the entire CNS without having to inject the CNS is promising for basic functional experiments, and potentially for gene therapy. Systemic or intra-cerebrospinal fluid routes of AAV9 administration should be considered for spinal muscular atrophy, lysosomal storage diseases and amyotrophic lateral sclerosis, if more neuronal expression can be achieved in adults, or if glial expression can be exploited.

Adeno-associated virus (AAV) gene therapy for neurological disease

Diseases of the central nervous system (CNS) have provided enormous opportunities for the therapeutic application of viral vector gene transfer. Adeno-associated virus (AAV) has been the vector of choice in recent clinical trials of neurological disease, including Parkinson's and Alzheimer's disease, due to the safety, efficacy, and stability of AAV gene transfer to the CNS. This review highlights the strategies employed for improving direct and peripheral targeting of therapeutic vectors to CNS tissue, and considers the significance of cellular and tissue transduction specificity, transgene regulation, and other variables that influence achievement of successful therapeutic goals. This article is part of the Special Issue entitled 'New Targets and Approaches to the Treatment of Epilepsy'.

Corticospinal tract transduction: a comparison of seven adeno-associated viral vector serotypes and a non-integrating lentiviral vector

The corticospinal tract (CST) is extensively used as a model system for assessing potential therapies to enhance neuronal regeneration and functional recovery following spinal cord injury (SCI). However, efficient transduction of the CST is challenging and remains to be optimised. Recombinant adeno-associated viral (AAV) vectors and integration-deficient lentiviral vectors are promising therapeutic delivery systems for gene therapy to the central nervous system (CNS). In the present study the cellular tropism and transduction efficiency of seven AAV vector serotypes (AAV1, 2, 3, 4, 5, 6, 8) and an integration-deficient lentiviral vector were assessed for their ability to transduce corticospinal neurons (CSNs) following intracortical injection. AAV1 was identified as the optimal serotype for transducing cortical and CSNs with green fluorescent protein (GFP) expression detectable in fibres projecting through the dorsal CST (dCST) of the cervical spinal cord. In contrast, AAV3 and AAV4 demonstrated a low efficacy for transducing CNS cells and AAV8 presented a potential tropism for oligodendrocytes. Furthermore, it was shown that neither AAV nor lentiviral vectors generate a significant microglial response. The identification of AAV1 as the optimal serotype for transducing CSNs should facilitate the design of future gene therapy strategies targeting the CST for the treatment of SCI.

Gene therapy for Huntington's disease

Huntington's disease (HD) is a neurodegenerative disease for which there is no cure. Therapies that are efficacious in animal models have to date shown benefit for humans. One potential powerful approach is gene therapy. The ideal method of administration of gene therapy has been hotly debated and viral vectors have provided one method of long-term and wide-spread delivery to the brain. Trophic factors to protect cells from degeneration and RNAi to reduce mutant huntingtin (mHtt) protein expression are 2 main classes of compounds that demonstrate benefit in animal models. This review will examine some commonly used adeno-associated viral (AAV) vectors and discuss some therapies that hold promise for HD.

Viral-mediated overexpression of mutant huntingtin to model HD in various species

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder caused by an expansion of CAG repeats in the huntingtin (Htt) gene. Despite intensive efforts devoted to investigating the mechanisms of its pathogenesis, effective treatments for this devastating disease remain unavailable. The lack of suitable models recapitulating the entire spectrum of the degenerative process has severely hindered the identification and validation of therapeutic strategies. The discovery that the degeneration in HD is caused by a mutation in a single gene has offered new opportunities to develop experimental models of HD, ranging from in vitro models to transgenic primates. However, recent advances in viral-vector technology provide promising alternatives based on the direct transfer of genes to selected sub-regions of the brain. Rodent studies have shown that overexpression of mutant human Htt in the striatum using adeno-associated virus or lentivirus vectors induces progressive neurodegeneration, which resembles that seen in HD. This article highlights progress made in modeling HD using viral vector gene transfer. We describe data obtained with of this highly flexible approach for the targeted overexpression of a disease-causing gene. The ability to deliver mutant Htt to specific tissues has opened pathological processes to experimental analysis and allowed targeted therapeutic development in rodent and primate pre-clinical models.

Evaluation and optimization of the administration of recombinant adeno-associated viral vectors (serotypes 2/1, 2/2, 2/rh8, 2/9, and 2/rh10) by convection-enhanced delivery to the striatum

Convection-enhanced delivery (CED) of recombinant adeno-associated virus (rAAV) vectors is a promising approach for delivery of therapeutic transgenes to the brain. In this study we have systematically examined vector dosing in vivo. Infusions of rAAV serotypes 2/1, 2/2, 2/rh8, 2/9, and 2/rh10 expressing an enhanced green fluorescent protein reporter gene were undertaken into the striatum of rats and pigs using CED. Vector distribution, as defined by the volume of distribution and number of transduced cells following each infusion, was determined using stereological methods. Immunohistochemistry was used to determine the transductional tropism of serotypes and to evaluate for the presence of immune cell infiltration into the brain. Vector distribution was highly variable between serotypes. Infusion rate had no significant effect on vector distribution or the occurrence of tissue damage. For serotypes 2/1, 2/2 and 2/rh10, as the vector concentration was increased beyond 10(12) vg/ml, no increase in vector distribution was observed. In contrast, for serotypes 2/rh8 and 2/9, retrograde axonal transport was observed above this threshold concentration. Cell transduction was principally neuronal for all serotypes and was associated with a low-level immune response. In planning clinical trials it is critical that these observations are considered in order to achieve optimal vector dosing.

Adeno-associated viral gene delivery in neurodegenerative disease

The advent of viral gene therapy technology has contributed greatly to the study of a variety of medical conditions, and there is increasing promise for clinical translation of gene therapy into human treatments. Adeno-associated viral (AAV) vectors provide one of the more promising approaches to gene delivery, and have been used extensively over the last 20 years. Derived from nonpathogenic parvoviruses, these vectors allow for stable and robust expression of desired transgenes in vitro and in vivo. AAV vectors efficiently and stably transduce neurons, with some strains targeting neurons exclusively in the brain. Thus, AAV vectors are particularly useful for neurodegenerative diseases, which have led to numerous preclinical studies and several human trials of gene therapy in patients with Parkinson's disease, Alzheimer's disease, and pediatric neurogenetic disorders. Here, we describe an efficient and reliable method for the production and purification of AAV serotype 2 vectors for both in vitro and in vivo applications.

Treatment of focal brain ischemia with viral vector-mediated gene transfer

Promoting functional recovery after ischemic brain injury has emerged as a potential approach for the treatment of ischemic stroke. An ideal restorative approach to enhance long-term functional recovery is to promote postischemic angiogenesis and neurogenesis. This chapter describes a system using adeno-associated viral (AAV) vector-mediated vascular endothelial growth factor (VEGF) gene transfer into the ischemic brain. The methods described here for construction, production, and purification of AAV vector expressing VEGF gene can also be applied to producing AAV vectors expressing other genes. This chapter also illustrates the methods to produce mouse middle cerebral artery occlusion (MCAO), injection of viral vector into the mouse brain, and standard assays for determining the success of brain ischemia and gene transfer.

Early or late-stage anti-N-terminal Huntingtin intrabody gene therapy reduces pathological features in B6.HDR6/1 mice

Huntington disease (HD) is a progressive neurodegenerative disease caused by an expansion of a polyglutamine sequence in mutant huntingtin (mhtt) that produces abnormal folding and aggregation that results in the formation of nuclear and cytoplasmic neuronal inclusion bodies. Although the precise role of mhtt aggregates in the pathogenesis is unclear, attempts to reduce accumulated mhtt protein have ameliorated the phenotype in multiple cellular and in vivo HD models. Here, we provide critical results on intracranial delivery of a single-chain Fv intrabody, C4, which targets the first 17 amino acids of the htt protein, a region of httExon1 that is increasingly being recognized as pivotal. To assess long-term efficacy and safety issues, we used adenoassociated viral vectors (AAV2/1) to deliver intrabody genes to the striatum of inbred B6.HDR6/1 mice. Treatment initiation at various stages of the disease showed that early treatment preserved the largest number of cells without nuclear aggregates and that the accumulation of aggregated material could be delayed by several months. Even when intrabody treatment was not initiated until the clinical disease stage, significant, albeit smaller, effects were seen. These data indicate that neuronal intrabodies against critical N-terminal epitopes can be safely and effectively delivered using AAV2/1 to delay the aggregation phenotype during a sustained period in this HD model, even when delivery is initiated after disease onset.

Rescue of a dystrophin-like protein by exon skipping in vivo restores GABAA-receptor clustering in the hippocampus of the mdx mouse

Dystrophin, the cytoskeletal protein whose defect is responsible for Duchenne muscular dystrophy (DMD), is normally expressed in both muscles and brain. Genetic loss of brain dystrophin in the mdx mouse model of DMD reduces the capacity for type A gamma-aminobutyric acid (GABA(A))-receptor clustering in central inhibitory synapses, which is thought to be a main molecular defect leading to brain and cognitive alterations in this syndrome. U7 small nuclear RNAs modified to encode antisense sequences and expressed from recombinant adeno-associated viral (rAAV) vectors have proven efficient after intramuscular injection to induce skipping of the mutated exon 23 and rescue expression of a functional dystrophin-like product in muscle tissues of mdx mice in vivo. Here, we report that intrahippocampal injection of a single dose of rAAV2/1-U7 can rescue substantial levels of brain dystrophin expression (15-25%) in mdx mice for months. This is sufficient to completely restore GABA(A)-receptor clustering in pyramidal and dendritic layers of CA1 hippocampus, suggesting exon-skipping strategies offer the prospect to investigate and correct both brain and muscle alterations in DMD. This provides new evidence that in the adult brain dystrophin is critical for the control of GABA(A)-receptor clustering, which may have an important role in activity-dependent synaptic plasticity in hippocampal circuits.

Herpes simplex virus type 1/adeno-associated virus hybrid vectors

Herpes simplex virus type 1 (HSV-1) amplicons can accommodate foreign DNA of any size up to 150 kbp and, therefore, allow extensive combinations of genetic elements. Genomic sequences as well as cDNA, large transcriptional regulatory sequences for cell type-specific expression, multiple transgenes, and genetic elements from other viruses to create hybrid vectors may be inserted in a modular fashion. Hybrid amplicons use genetic elements from HSV-1 that allow replication and packaging of the vector DNA into HSV-1 virions, and genetic elements from other viruses that either direct integration of transgene sequences into the host genome or allow episomal maintenance of the vector. Thus, the advantages of the HSV-1 amplicon system, including large transgene capacity, broad host range, strong nuclear localization, and availability of helper virus-free packaging systems are retained and combined with those of heterologous viral elements that confer genetic stability to the vector DNA. Adeno-associated virus (AAV) has the unique capability of integrating its genome into a specific site, designated AAVS1, on human chromosome 19. The AAV rep gene and the inverted terminal repeats (ITRs) that flank the AAV genome are sufficient for this process. HSV-1 amplicons have thus been designed that contain the rep gene and a transgene cassette flanked by AAV ITRs. These HSV/AAV hybrid vectors direct site-specific integration of transgene sequences into AAVS1 and support long-term transgene expression.

Dystrophins, utrophins, and associated scaffolding complexes: role in mammalian brain and implications for therapeutic strategies

Two decades of molecular, cellular, and functional studies considerably increased our understanding of dystrophins function and unveiled the complex etiology of the cognitive deficits in Duchenne muscular dystrophy (DMD), which involves altered expression of several dystrophin-gene products in brain. Dystrophins are normally part of critical cytoskeleton-associated membrane-bound molecular scaffolds involved in the clustering of receptors, ion channels, and signaling proteins that contribute to synapse physiology and blood-brain barrier function. The utrophin gene also drives brain expression of several paralogs proteins, which cellular expression and biological roles remain to be elucidated. Here we review the structural and functional properties of dystrophins and utrophins in brain, the consequences of dystrophins loss-of-function as revealed by numerous studies in mouse models of DMD, and we discuss future challenges and putative therapeutic strategies that may compensate for the cognitive impairment in DMD based on experimental manipulation of dystrophins and/or utrophins brain expression.

Abeta-directed single-chain antibody delivery via a serotype-1 AAV vector improves learning behavior and pathology in Alzheimer's disease mice

Alzheimer's disease (AD) is a progressive dementing disorder characterized by age-related amyloid-beta (Abeta) deposition, neurofibrillary tangles, and synapse and neuronal loss. It is widely recognized that Abeta is a principal pathogenic mediator of AD. Our goal was to develop an immunotherapeutic approach, which would specifically lead to the clearance and/or neutralization of Abeta in the triple transgenic mouse model (3xTg-AD). These mice develop the amyloid and tangle pathologies and synaptic dysfunction reminiscent of human AD. Using a human single-chain variable fragment (scFv) antibody phage display library, a novel scFv antibody specific to Abeta was isolated, its activity characterized in vitro, and its open reading frame subsequently cloned into a recombinant adeno-associated virus (rAAV) vector. Three-month-old 3xTg-AD mice were intrahippocampally infused with serotype-1 rAAV vectors encoding Abeta-scFv or a control vector using convection-enhanced delivery (CED). Mice receiving rAAV1-Abeta-scFv harbored lower levels of insoluble Abeta and hyperphosphorylated tau, and exhibited improved cognitive function as measured by the Morris Water Maze (MWM) spatial memory task. These results underscore the potential of gene-based passive vaccination for AD, and provide further rationale for the development of Abeta-targeting strategies for this debilitating disease.