Oligodendrocyte-specific gene expression in mouse brain: use of a myelin-forming cell type-specific promoter in an adeno-associated virus

Preclinical studies in Krabbe disease: A model for the investigation of novel combination therapies for lysosomal storage diseases

Krabbe disease (KD) is a lysosomal storage disease (LSD) caused by mutations in the galc gene. There are over 50 monogenetic LSDs, which largely impede the normal development of children and often lead to premature death. At present, there are no cures for LSDs and the available treatments are generally insufficient, short acting, and not without co-morbidities or long-term side effects. The last 30 years have seen significant advances in our understanding of LSD pathology as well as treatment options. Two gene therapy-based clinical trials, NCT04693598 and NCT04771416, for KD were recently started based on those advances. This review will discuss how our knowledge of KD got to where it is today, focusing on preclinical investigations, and how what was discovered may prove beneficial for the treatment of other LSDs.

The use of viral vectors to promote repair after spinal cord injury

Spinal cord injury (SCI) is a devastating event that can permanently disrupt multiple modalities. Unfortunately, the combination of the inhibitory environment at a central nervous system (CNS) injury site and the diminished intrinsic capacity of adult axons for growth results in the failure for robust axonal regeneration, limiting the ability for repair. Delivering genetic material that can either positively or negatively modulate gene expression has the potential to counter the obstacles that hinder axon growth within the spinal cord after injury. A popular gene therapy method is to deliver the genetic material using viral vectors. There are considerations when deciding on a viral vector approach for a particular application, including the type of vector, as well as serotypes, and promoters. In this review, we will discuss some of the aspects to consider when utilizing a viral vector approach to as a therapy for SCI. Additionally, we will discuss some recent applications of gene therapy to target extrinsic and/or intrinsic barriers to promote axon regeneration after SCI in preclinical models. While still in early stages, this approach has potential to treat those living with SCI.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Selective targeting of striatal parvalbumin-expressing interneurons for transgene delivery

PV^Cre mice--> combined with AAV-FLEX vectors allowed efficient and specific targeting of PV⁺ interneurons in the striatum. However, diffusion of viral particles to the globus pallidus caused massive transduction of PV⁺ projection neurons and subsequent anterograde transport of the transgene product to the subthalamic nucleus and the substantia nigra pars reticulata. Different AAV serotypes (1 and 9) and promoters (CBA and human synapsin) were evaluated. The combination of AAV1, a moderate expression level (human synapsin promoter) and a precise adjustment of the stereotaxic coordinates in the anterior and dorsolateral part of the striatum were necessary to avoid transduction of PV⁺ GP projection neurons. Even in the absence of direct transduction due to diffusion of viral particles, GP PV⁺ projection neurons could be retrogradely transduced via their terminals present in the dorsal striatum. However, in the absence of diffusion, GP-Str PV⁺ projection neurons were poorly or not transduced suggesting that retrograde transduction did not significantly impair the selective targeting of striatal PV⁺ neurons. Finally, a prominent reduction of the number of striatal PV⁺ interneurons (about 50 %) was evidenced in the presence of the Cre recombinase suggesting that functional effects of AAV-mediated transgene expression in PV⁺ striatal interneurons in PV^Cre mice should be analyzed with caution.

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.

AAV Capsid-Promoter Interactions Determine CNS Cell-Selective Gene Expression In Vivo

Cell-selective gene expression comprises a critical element of many adeno-associated virus (AAV) vector-based gene therapies, and to date achieving this goal has focused on AAV capsid engineering, cell-specific promoters, or cell-specific enhancers. Recently, we discovered that the capsid of AAV9 exerts a differential influence on constitutive promoters of sufficient magnitude to alter cell type gene expression in the rat CNS. For AAV9 vectors chicken β-actin (CBA) promoter-driven gene expression exhibited a dominant neuronal gene expression in the rat striatum. Surprisingly, for otherwise identical AAV9 vectors, the truncated CBA hybrid (CBh) promoter shifted gene expression toward striatal oligodendrocytes. In contrast, AAV2 vector gene expression was restricted to striatal neurons, regardless of the constitutive promoter used. Furthermore, a six-glutamate residue insertion immediately after the VP2 start residue shifted CBA-driven cellular gene expression from neurons to oligodendrocytes. Conversely, a six-alanine insertion in the same AAV9 capsid region reversed the CBh-mediated oligodendrocyte expression back to neurons without changing AAV9 capsid access to oligodendrocytes. Given the preponderance of AAV9 in ongoing clinical trials and AAV capsid engineering, this AAV9 capsid-promoter interaction reveals a previously unknown novel contribution to cell-selective AAV-mediated gene expression in the CNS.

Enhancer-Driven Gene Expression (EDGE) enables the generation of cell type specific tools for the analysis of neural circuits

As in all circuits, fully understanding how neural circuits operate requires the ability to specifically manipulate individual circuit elements, i.e. particular neuronal cell types. While recent years saw the development of molecular genetic tools allowing one to control and monitor neuronal activity, progress is limited by the ability to express such transgenes specifically enough. This goal is complicated by the fact that we are only beginning to understand how many cell types exist in the mammalian brain. Obtaining neuronal cell type-specific expression requires co-opting the genetic machinery which specifies their striking diversity, typically done by making transgenic animals using promoters expressing in neurons. However, while the vast majority of genes express in the brain, they almost always express in multiple cell types, meaning native promoters are not specific enough. We have recently taken a new approach to increase the specificity of transgene expression based upon identifying the distal cis-regulatory genomic elements (i.e. enhancers) uniquely active in a brain region and combining them with a heterologous minimal promoter. Termed Enhancer-Driven Gene Expression (EDGE), it allows for the generation of transgenic animals targeting the cell types of any brain region with far greater specificity than can be obtained with native promoters. Moreover, their small size allows for the generation of cell-specific viral vectors, conceivably enabling circuit-specific manipulations to any species.

Rationally Engineered AAV Capsids Improve Transduction and Volumetric Spread in the CNS

Adeno-associated virus (AAV) is the most common vector for clinical gene therapy of the CNS. This popularity originates from a high safety record and the longevity of transgene expression in neurons. Nevertheless, clinical efficacy for CNS indications is lacking, and one reason for this is the relatively limited spread and transduction efficacy in large regions of the human brain. Using rationally designed modifications of the capsid, novel AAV capsids have been generated that improve intracellular processing and result in increased transgene expression. Here, we sought to improve AAV-mediated neuronal transduction to minimize the existing limitations of CNS gene therapy. We investigated the efficacy of CNS transduction using a variety of tyrosine and threonine capsid mutants based on AAV2, AAV5, and AAV8 capsids, as well as AAV2 mutants incapable of binding heparan sulfate (HS). We found that mutating several tyrosine residues on the AAV2 capsid significantly enhanced neuronal transduction in the striatum and hippocampus, and the ablation of HS binding also increased the volumetric spread of the vector. Interestingly, the analogous tyrosine substitutions on AAV5 and AAV8 capsids did not improve the efficacy of these serotypes. Our results demonstrate that the efficacy of CNS gene transfer can be significantly improved with minor changes to the AAV capsid and that the effect is serotype specific.

Gene therapy targeting oligodendrocytes provides therapeutic benefit in a leukodystrophy model

Pelizaeus-Merzbacher-like disease or hypomyelinating leukodystrophy-2 is an autosomal recessively inherited leukodystrophy with childhood onset resulting from mutations in the gene encoding the gap junction protein connexin 47 (Cx47, encoded by GJC2). Cx47 is expressed specifically in oligodendrocytes and is crucial for gap junctional communication throughout the central nervous system. Previous studies confirmed that a cell autonomous loss-of-function mechanism underlies hypomyelinating leukodystrophy-2 and that transgenic oligodendrocyte-specific expression of another connexin, Cx32 (GJB1), can restore gap junctions in oligodendrocytes to achieve correction of the pathology in a disease model. To develop an oligodendrocyte-targeted gene therapy, we cloned the GJC2/Cx47 gene under the myelin basic protein promoter and used an adeno-associated viral vector (AAV.MBP.Cx47myc) to deliver the gene to postnatal Day 10 mice via a single intracerebral injection in the internal capsule area. Lasting Cx47 expression specifically in oligodendrocytes was detected in Cx47 single knockout and Cx32/Cx47 double knockout mice up to 12 weeks post-injection, including the corpus callosum and the internal capsule but also in more distant areas of the cerebrum and in the spinal cord. Application of this oligodendrocyte-targeted somatic gene therapy at postnatal Day 10 in groups of double knockout mice, a well characterized model of hypomyelinating leukodystrophy-2, resulted in significant improvement in motor performance and coordination at 1 month of age in treated compared to mock-treated mice, as well as prolonged survival. Furthermore, immunofluorescence and morphological analysis revealed improvement in demyelination, oligodendrocyte apoptosis, inflammation, and astrogliosis, all typical features of this leukodystrophy model in both brain and spinal cord. Functional dye transfer analysis confirmed the re-establishment of oligodendrocyte gap junctional connectivity in treated as opposed to untreated mice. These results provide a significant advance in the development of oligodendrocyte-cell specific gene therapy. Adeno-associated viral vectors can be used to target therapeutic expression of a myelin gene to oligodendrocytes. We show evidence for the first somatic gene therapy approach to treat hypomyelinating leukodystrophy-2 preclinically, providing a potential treatment for this and similar forms of leukodystrophies.

Gene therapy approaches in the non-human primate model of Parkinson's disease

The field of gene therapy has recently witnessed a number of major conceptual changes. Besides the traditional thinking that comprises the use of viral vectors for the delivery of a given therapeutic gene, a number of original approaches have been recently envisaged, focused on using vectors carrying genes to further modify basal ganglia circuits of interest. It is expected that these approaches will ultimately induce a therapeutic potential being sustained by gene-induced changes in brain circuits. Among others, at present, it is technically feasible to use viral vectors to (1) achieve a controlled release of neurotrophic factors, (2) conduct either a transient or permanent silencing of any given basal ganglia circuit of interest, (3) perform an in vivo cellular reprogramming by promoting the conversion of resident cells into dopaminergic-like neurons, and (4) improving levodopa efficacy over time by targeting aromatic L-amino acid decarboxylase. Furthermore, extensive research efforts based on viral vectors are currently ongoing in an attempt to better replicate the dopaminergic neurodegeneration phenomena inherent to the progressive intraneuronal aggregation of alpha-synuclein. Finally, a number of incoming strategies will soon emerge over the horizon, these being sustained by the underlying goal of promoting alpha-synuclein clearance, such as, for instance, gene therapy initiatives based on increasing the activity of glucocerebrosidase. To provide adequate proof-of-concept on safety and efficacy and to push forward true translational initiatives based on these different types of gene therapies before entering into clinical trials, the use of non-human primate models undoubtedly plays an instrumental role.

Strategies for targeting primate neural circuits with viral vectors

Understanding how the brain works requires understanding how different types of neurons contribute to circuit function and organism behavior. Progress on this front has been accelerated by optogenetics and chemogenetics, which provide an unprecedented level of control over distinct neuronal types in small animals. In primates, however, targeting specific types of neurons with these tools remains challenging. In this review, we discuss existing and emerging strategies for directing genetic manipulations to targeted neurons in the adult primate central nervous system. We review the literature on viral vectors for gene delivery to neurons, focusing on adeno-associated viral vectors and lentiviral vectors, their tropism for different cell types, and prospects for new variants with improved efficacy and selectivity. We discuss two projection targeting approaches for probing neural circuits: anterograde projection targeting and retrograde transport of viral vectors. We conclude with an analysis of cell type-specific promoters and other nucleotide sequences that can be used in viral vectors to target neuronal types at the transcriptional level.

Gene delivery targeted to oligodendrocytes using a lentiviral vector

BACKGROUND

Most leukodystrophies result from mutations in genes expressed in oligodendrocytes that may cause autonomous loss of function of cell structural proteins. Therefore, effective gene delivery to oligodendrocytes is necessary to develop future treatments.

MATERIALS

To achieve this, we cloned a lentiviral vector in which the enhanced green fluorescent protein (EGFP) expression was driven by the oligodendrocyte specific 2,3-cyclic nucleotide 3-phosphodiesterase promoter. The vector was inserted into C57BL/6 neonatal mouse brain by combined intraventricular and parenchymal injections.

RESULTS

Assessment of EGFP expression revealed a widespread distribution, specifically in cells of the oligodendrocyte linage, starting from postnatal day 6 (P6) in the subventricular zone and spreading through migrating oligodendrocyte precursors. By P30, it was detectable throughout the brain and persisted for at least 3 months, showing an increase both in the number of expressing cells and in intensity over time. EGFP expression was restricted to oligodendrocyte linage cells. On average, 20.3 ± 2.56% of all oligodendrocytes in different central nervous system areas were EGFP-positive, with regional variations.

CONCLUSIONS

Lentiviral gene delivery using an oligodendrocyte-specific promoter may achieve widespread and long-lasting expression selectively in oligodendrocytes, offering a possibility for gene therapy in certain leukodystrophies, although the relatively low rates of oligodendrocyte transduction are a limitation that remains to be overcome.

Serotype-dependent transduction efficiencies of recombinant adeno-associated viral vectors in monkey neocortex

Viral vector-mediated expression of genes (e.g., coding for opsins and designer receptors) has grown increasingly popular. Cell-type specific expression is achieved by altering viral vector tropism through crosspackaging or by cell-specific promoters driving gene expression. Detailed information about transduction properties of most recombinant adeno-associated viral vector (rAAV) serotypes in macaque cortex is gradually becoming available. Here, we compare transduction efficiencies and expression patterns of reporter genes in two macaque neocortical areas employing different rAAV serotypes and promoters. A short version of the calmodulin-kinase-II (CaMKIIα0.4) promoter resulted in reporter gene expression in cortical neurons for all tested rAAVs, albeit with different efficiencies for spread: rAAV2/5>>rAAV2/7>rAAV2/8>rAAV2/9>>rAAV2/1 and proportion of transduced cells: rAAV2/1>rAAV2/5>rAAV2/7=rAAV2/9>rAAV2/8. In contrast to rodent studies, the cytomegalovirus (CMV) promoter appeared least efficient in macaque cortex. The human synapsin-1 promoter preceded by the CMV enhancer (enhSyn1) produced homogeneous reporter gene expression across all layers, while two variants of the CaMKIIα promoter resulted in different laminar transduction patterns and cell specificities. Finally, differences in expression patterns were observed when the same viral vector was injected in two neocortical areas. Our results corroborate previous findings that reporter-gene expression patterns and efficiency of rAAV transduction depend on serotype, promoter, cortical layer, and area.

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.

A new method to measure autophagy flux in the nervous system

A current need in the neuroscience field is a simple method to monitor autophagic activity in vivo in neurons. Until very recently, most reports have been based on correlative and static determinations of the expression levels of autophagy markers in the brain, generating conflicting interpretations. Autophagy is a fundamental process mediating the degradation of diverse cellular components, including organelles and protein aggregates at basal levels, whereas alterations in the process (i.e., autophagy impairment) operate as a pathological mechanism driving neurodegeneration in most prevalent diseases. We have recently described a new simple method to deliver and express an autophagy flux reporter through the peripheral and central nervous system of mice by the intracerebroventricular delivery of adeno-associated viruses (AAV) into newborn mice. We obtained a wide expression of a monomeric tandem mCherry-GFP-LC3 construct in neurons through the nervous system and demonstrated efficient and accurate measurements of LC3 flux after pharmacological stimulation of the pathway or in disease settings of axonal damage. Here we discuss the possible applications of this new method to assess autophagy activity in neurons in vivo.

Intracisternal delivery of AAV9 results in oligodendrocyte and motor neuron transduction in the whole central nervous system of cats

Systemic and intracerebrospinal fluid delivery of adeno-associated virus serotype 9 (AAV9) has been shown to achieve widespread gene delivery to the central nervous system (CNS). However, after systemic injection, the neurotropism of the vector has been reported to vary according to age at injection, with greater neuronal transduction in newborns and preferential glial cell tropism in adults. This difference has not yet been reported after cerebrospinal fluid (CSF) delivery. The present study analyzed both neuronal and glial cell transduction in the CNS of cats according to age of AAV9 CSF injection. In both newborns and young cats, administration of AAV9-GFP in the cisterna magna resulted in high levels of motor neurons (MNs) transduction from the cervical (84±5%) to the lumbar (99±1%) spinal cord, demonstrating that the remarkable tropism of AAV9 for MNs is not affected by age at CSF delivery. Surprisingly, numerous oligodendrocytes were also transduced in the brain and in the spinal cord white matter of young cats, but not of neonates, indicating that (i) age of CSF delivery influences the tropism of AAV9 for glial cells and (ii) AAV9 intracisternal delivery could be relevant for both the treatment of MN and demyelinating disorders.

A next step in adeno-associated virus-mediated gene therapy for neurological diseases: regulation and targeting

Recombinant adeno-associated virus (rAAV) vectors mediating long term transgene expression are excellent gene therapy tools for chronic neurological diseases. While rAAV2 was the first serotype tested in the clinics, more efficient vectors derived from the rh10 serotype are currently being evaluated and other serotypes are likely to be tested in the near future. In addition, aside from the currently used stereotaxy-guided intraparenchymal delivery, new techniques for global brain transduction (by intravenous or intra-cerebrospinal injections) are very promising. Various strategies for therapeutic gene delivery to the central nervous system have been explored in human clinical trials in the past decade. Canavan disease, a genetic disease caused by an enzymatic deficiency, was the first to be approved. Three gene transfer paradigms for Parkinson's disease have been explored: converting L-dopa into dopamine through AADC gene delivery in the putamen; synthesizing GABA through GAD gene delivery in the overactive subthalamic nucleus and providing neurotrophic support through neurturin gene delivery in the nigro-striatal pathway. These pioneer clinical trials demonstrated the safety and tolerability of rAAV delivery in the human brain at moderate doses. Therapeutic effects however, were modest, emphasizing the need for higher doses of the therapeutic transgene product which could be achieved using more efficient vectors or expression cassettes. This will require re-addressing pharmacological aspects, with attention to which cases require either localized and cell-type specific expression or efficient brain-wide transgene expression, and when it is necessary to modulate or terminate the administration of transgene product. The ongoing development of targeted and regulated rAAV vectors is described.

Intraspinal AAV Injections Immediately Rostral to a Thoracic Spinal Cord Injury Site Efficiently Transduces Neurons in Spinal Cord and Brain

In the vast majority of studies utilizing adeno-associated virus (AAV) in central nervous system applications, including those published with spinal cord injury (SCI) models, AAV has been administered at the level of the cell body of neurons targeted for genetic modification, resulting in transduction of neurons in the vicinity of the injection site. However, as SCI interrupts many axon tracts, it may be more beneficial to transduce a diverse pool of supraspinal neurons. We determined if descending axons severed by SCI are capable of retrogradely transporting AAV to remotely transduce a variety of brain regions. Different AAV serotypes encoding the reporter green fluorescent protein (GFP) were injected into gray and white matter immediately rostral to a spinal transection site. This resulted in the transduction of thousands of neurons within the spinal cord and in multiple regions within the brainstem that project to spinal cord. In addition, we established that different serotypes had disparate regional specificity and that AAV5 transduced the most brain and spinal cord neurons. This is the first demonstration that retrograde transport of AAV by axons severed by SCI is an effective means to transduce a collection of supraspinal neurons. Thus, we identify a novel, minimally invasive means to transduce a variety of neuronal populations within both the spinal cord and the brain following SCI. This paradigm to broadly distribute viral vectors has the potential to be an important component of a combinatorial strategy to promote functional axonal regeneration.

Glial promoter selectivity following AAV-delivery to the immature brain

Recombinant adeno-associated virus (AAV) vectors are versatile tools for gene transfer to the central nervous system (CNS) and proof-of-concept studies in adult rodents have shown that the use of cell type-specific promoters is sufficient to target AAV-mediated transgene expression to glia. However, neurological disorders caused by glial pathology usually have an early onset. Therefore, modelling and treatment of these conditions require expanding the concept of targeted glial transgene expression by promoter selectivity for gene delivery to the immature CNS. Here, we have investigated the AAV-mediated green fluorescent protein (GFP) expression driven by the myelin basic protein (MBP) or glial fibrillary acidic protein (GFAP) promoters in the developing mouse brain. Generally, the extent of transgene expression after infusion at immature stages was widespread and higher than in adults. The GFAP promoter-driven GFP expression was found to be highly specific for astrocytes following vector infusion to the brain of neonates and adults. In contrast, the selectivity of the MBP promoter for oligodendrocytes was poor following neonatal AAV delivery, but excellent after vector injection at postnatal day 10. To extend these findings obtained in naïve mice to a disease model, we performed P10 infusions of AAV-MBP-GFP in aspartoacylase (ASPA)-deficient mouse mutants presenting with early onset oligodendrocyte pathology. Spread of GFP expression and selectivity for oligodendrocytes in ASPA-mutants was comparable with our observations in normal animals. Our data suggest that direct AAV infusion to the developing postnatal brain, utilising cellular promoters, results in targeted and long-term transgene expression in glia. This approach will be relevant for disease modelling and gene therapy for the treatment of glial pathology.

Myelin membrane assembly is driven by a phase transition of myelin basic proteins into a cohesive protein meshwork

Rapid conduction of nerve impulses requires coating of axons by myelin. To function as an electrical insulator, myelin is generated as a tightly packed, lipid-rich multilayered membrane sheath. Knowledge about the mechanisms that govern myelin membrane biogenesis is required to understand myelin disassembly as it occurs in diseases such as multiple sclerosis. Here, we show that myelin basic protein drives myelin biogenesis using weak forces arising from its inherent capacity to phase separate. The association of myelin basic protein molecules to the inner leaflet of the membrane bilayer induces a phase transition into a cohesive mesh-like protein network. The formation of this protein network shares features with amyloid fibril formation. The process is driven by phenylalanine-mediated hydrophobic and amyloid-like interactions that provide the molecular basis for protein extrusion and myelin membrane zippering. These findings uncover a physicochemical mechanism of how a cytosolic protein regulates the morphology of a complex membrane architecture. These results provide a key mechanism in myelin membrane biogenesis with implications for disabling demyelinating diseases of the central nervous system.

Noninvasive and targeted gene delivery into the brain using microbubble-facilitated focused ultrasound

Recombinant adeno-associated viral (rAAV) vectors are potentially powerful tools for gene therapy of CNS diseases, but their penetration into brain parenchyma is severely limited by the blood-brain barrier (BBB) and current delivery relies on invasive stereotactic injection. Here we evaluate the local, targeted delivery of rAAV vectors into the brains of mice by noninvasive, reversible, microbubble-facilitated focused ultrasound (FUS), resulting in BBB opening that can be monitored and controlled by magnetic resonance imaging (MRI). Using this method, we found that IV-administered AAV2-GFP (green fluorescence protein) with a low viral vector titer (1×10(9) vg/g) can successfully penetrate the BBB-opened brain regions to express GFP. We show that MRI monitoring of BBB-opening could serve as an indicator of the scale and distribution of AAV transduction. Transduction peaked at 3 weeks and neurons and astrocytes were affected. This novel, noninvasive delivery approach could significantly broaden the application of AAV-viral-vector-based genes for treatment of CNS diseases.

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.

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'.

Efficient gene delivery and selective transduction of glial cells in the mammalian brain by AAV serotypes isolated from nonhuman primates

Adeno-associated viral (AAV) vectors have become the primary delivery agent for somatic gene transfer into the central nervous system (CNS). To date, AAV-mediated gene delivery to the CNS is based on serotypes 1-9, with efficient gene transfer to neurons only-selective and widespread transduction of glial cells have not been observed. Recently, additional endogenous AAVs have been isolated from nonhuman primate tissues. In this study, transduction obtained with AAV serotypes bb2, cy5, rh20, rh39, and rh43 was compared to that obtained with AAV8, another nonhuman primate isolate previously shown to perform well in mammalian brain. Titer-matched vectors encoding the enhanced green fluorescent protein (EGFP) reporter, driven by the constitutive CAG promoter, were injected into the hippocampus, striatum, or substantia nigra (SN) of adult rats. More widespread neuronal transduction was observed following infusion of cy5, rh20, and rh39 than observed with AAV8. Of interest, preferential transduction of astrocytes was observed with rh43. To optimize glial transduction, vector stocks driven by cell-specific promoters were generated-widespread and targeted transduction of astrocytes and oligodendrocytes was observed using rh43 and AAV8, driven by the glial fibrillary acidic protein (GFAP) and myelin basic protein (MBP) promoters, expanding the utility of AAV for modeling and treating diseases involving glial cell pathology.

Transcriptional targeting to brain cells: Engineering cell type-specific promoter containing cassettes for enhanced transgene expression

Transcriptional targeting using a mammalian cellular promoter to restrict transgene expression to target cells is often desirable for gene therapy. This strategy is, however, hindered by relatively weak activity of some cellular promoters, which may lead to low levels of gene expression, thus declining therapeutic efficacy. Here we outline the advances accomplished in the area of transcriptional targeting to brain cells, with a particular focus on engineering gene cassettes to augment cell type-specific expression. Among the effective approaches that improve gene expression while retaining promoter specificity are promoter engineering to change authentic sequences of a cellular promoter and the combined use of a native cellular promoter and other cis-acting elements. Success in achieving high level and sustained transgene expression only in the cell types of interest would be of importance in allowing gene therapy to have its impact on patient treatment.

Preferential labeling of inhibitory and excitatory cortical neurons by endogenous tropism of adeno-associated virus and lentivirus vectors

Despite increasingly widespread use of recombinant adeno-associated virus (AAV) and lentiviral (LV) vectors for transduction of neurons in a wide range of brain structures and species, the diversity of cell types within a given brain structure is rarely considered. For example, the ability of a vector to transduce neurons within a brain structure is often assumed to indicate that all neuron types within the structure are transduced. We have characterized the transduction of mouse somatosensory cortical neuron types by recombinant AAV pseudotyped with serotype 1 capsid (rAAV2/1) and by recombinant lentivirus pseudotyped with the vesicular stomatitis virus (VSV) glycoprotein. Both vectors used human synapsin (hSyn) promoter driving DsRed-Express. We demonstrate that high titer rAAV2/1-hSyn efficiently transduces both cortical excitatory and inhibitory neuronal populations, but use of lower titers exposes a strong preference for transduction of cortical inhibitory neurons and layer 5 pyramidal neurons. In contrast, we find that VSV-G-LV-hSyn principally labels excitatory cortical neurons at the highest viral titer generated. These findings demonstrate that endogenous tropism of rAAV2/1 and VSV-G-LV can be used to obtain preferential gene expression in mouse somatosensory cortical inhibitory and excitatory neuron populations, respectively.

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.

Tropism and toxicity of adeno-associated viral vector serotypes 1, 2, 5, 6, 7, 8, and 9 in rat neurons and glia in vitro

Recombinant adeno-associated viral (rAAV) vectors are frequently used for gene delivery to the central nervous system and are capable of transducing neurons and glia in vitro. In this study, seven serotypes of a rAAV vector expressing green fluorescent protein (GFP) were characterized for tropism and toxicity in primary cortical cells derived from embryonic rat brain. At 2 days after transduction, serotypes 1 and 5 through 8 expressed GFP predominately in glia, but by 6 days post-transduction expression was neuronal except for AAV5. AAV2 and 9 produced minimal GFP expression. Using cell viability assays, toxicity was observed at higher multiplicities of infection (MOI) for all serotypes except AAV2 and 9. The toxicity of AAV1 and 5-8 affected mostly glia as indicated by a loss of glial-marker immunoreactivity. A frameshift mutation in the GFP gene reduced overall toxicity for serotypes 1, 5 and 6, but not 7 and 8 suggesting that the toxicity was not solely due to the overexpression of GFP. Collectively, a differential tropism and toxicity was observed among the AAV serotypes on primary cortical cultures with an overall preferential glial transduction and toxicity.

Motor neuron inhibition-based gene therapy for spasticity

Spasticity is a condition resulting from excess motor neuron excitation, leading to involuntary muscle contraction in response to increased velocity of movement, for which there is currently no cure. Existing symptomatic therapies face a variety of limitations. The extent of relief that can be delivered by ablative techniques such as rhizotomy is limited by the potential for sensory denervation. Pharmacological approaches, including intrathecal baclofen, can be undermined by tolerance. One potential new approach to the treatment of spasticity is the control of neuromuscular overactivity through the delivery of genes capable of inducing synaptic inhibition. A variety of experiments in cell culture and animal models have demonstrated the ability of neural gene transfer to inhibit neuronal activity and suppress transmission. Similarly, enthusiasm for the application of gene therapy to neurodegenerative diseases of motor neurons has led to the development of a variety of strategies for motor neuron gene delivery. In this review, we discuss the limitations of existing spasticity therapies, the feasibility of motor neuron inhibition as a gene-based treatment for spasticity, potential inhibitory transgene candidates, strategies for control of transgene expression, and applicable motor neuron gene targeting strategies. Finally, we discuss future directions and the potential for gene-based motor neuron inhibition in therapeutic clinical trials to serve as an effective treatment modality for spasticity, either in conjunction with or as a replacement for presently available therapies.

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.

Lentiviral transduction of murine oligodendrocytes in vivo

Lentiviral vectors are used widely to direct efficient gene transfer in vivo. We examined cell-specific expression in adult murine white matter after stereotaxic microinjection of four lentiviral constructs. We synthesized vesicular stomatitis virus glycoprotein (VSV-G) pseudotyped lentiviruses with combinations of two promoters, cytomegalovirus (CMV) or myelin basic protein (MBP), and two reporter sequences, cytosolic enhanced green fluorescent protein (eGFP) or a plasma membrane-targeted eGFP (human lymphocyte-specific protein tyrosine kinase [Lck]-eGFP). For all constructs, intracerebral injections to lateral corpus callosum resulted in widespread GFP expression in forebrain white matter glial cells. Intense cellular GFP fluorescence was observed within 3 days after injection and lasted for at least 28 days. The CMV promoter directed GFP expression in multiple glial cell types, whereas the MBP promoter targeted GFP specifically to oligodendrocytes. Expression of the membrane-targeted Lck-eGFP construct distinctly labeled individual myelinating processes of oligodendrocytes. Lentiviral constructs expressing eGFP or Lck-eGFP under the MBP promoter provide excellent visualization of oligodendrocyte morphology in intact white matter, and may prove valuable for delivering additional genes of interest to oligodendrocytes in vivo.

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.

Effects of AAV-2-mediated aspartoacylase gene transfer in the tremor rat model of Canavan disease

The tremor rat is a spontaneous epilepsy model with a seizure phenotype caused by a deletion in the aspartoacylase (ASPA) gene. The absence of ASPA expression in these animals results in undetectable levels of enzyme activity and the accumulation of the substrate N-acetyl-aspartate (NAA) in brain, leading to generalized myelin vacuolation and severe motor and cognitive impairment. In support of human gene therapy for CD, recombinant adeno-associated viral vector (AAV-2) expressing ASPA was stereotactically delivered to the tremor rat brain and effects on the mutant phenotype were measured. AAV-ASPA gene transfer resulted in elevated aspartoacylase bioactivity compared to untreated mutant animals and elicited a significant decrease in the pathologically elevated whole-brain NAA levels. Assessment of motor function via quantitative rotorod testing demonstrated that rats injected with AAV-ASPA significantly improved on tests of balance and coordinated locomotion compared to animals receiving control vectors. This study provides evidence that AAV-2-mediated aspartoacylase gene transfer to the brain improves biochemical and behavioral deficits in tremor rat mutants (tm/tm) and supports the rationale of human gene transfer for Canavan disease.

Recombinant AAV viral vectors pseudotyped with viral capsids from serotypes 1, 2, and 5 display differential efficiency and cell tropism after delivery to different regions of the central nervous system

Recombinant adeno-associated virus 2 (rAAV2) has been shown to deliver genes to neurons effectively in the brain, retina, and spinal cord. The characterization of new AAV serotypes has revealed that they have different patterns of transduction in diverse tissues. We have investigated the tropism and transduction frequency in the central nervous system (CNS) of three different rAAV vector serotypes. The vectors contained AAV2 terminal repeats flanking a green fluorescent protein expression cassette under the control of the synthetic CBA promoter, in AAV1, AAV2, or AAV5 capsids, producing the pseudotypes rAAV2/1, rAAV2/2, and rAAV2/5. Rats were injected with rAAV2/1, rAAV2/2, or rAAV2/5 into selected regions of the CNS, including the hippocampus (HPC), substantia nigra (SN), striatum, globus pallidus, and spinal cord. In all regions injected, the three vectors transduced neurons almost exclusively. All three vectors transduced the SN pars compacta with high efficiency, but rAAV2/1 and rAAV2/5 also transduced the pars reticulata. Moreover, rAAV2/1 showed widespread distribution throughout the entire midbrain. In the HPC, rAAV2/1 and rAAV2/5 targeted the pyramidal cell layers in the CA1-CA3 regions, whereas AAV2/2 primarily transduced the hilar region of the dentate gyrus. In general, rAAV2/1 and rAAV2/5 exhibited higher transduction frequencies than rAAV2/2 in all regions injected, although the differences were marginal in some regions. Retrograde transport of rAAV1 and rAAV5 was also observed in particular CNS areas. These results suggest that vectors based on distinct AAV serotypes can be chosen for specific applications in the nervous system.

Identification of oligodendrocytes in experimental disease models

The ability to identify oligodendrocytes in culture, in fixed tissue, and in vivo using unique markers is a requisite step to understanding their responses in any damage, recovery, or developmental process. Their nuclei are readily seen in histological preparations of healthy white and gray matter, and their cell bodies can be reliably identified with a variety of immunocytochemical markers. However, there is little consensus regarding optimal methods to assess oligodendrocyte survival or morphology under experimental injury conditions. We review common approaches for histological and immunocytochemical identification of these cells. Transgenic and viral methods for cell type-selective transfer of genes encoding fluorescent proteins offer promising new approaches for manipulating and visualizing oligodendrocytes in models of health and disease.

Adeno-associated virus terminal repeat (TR) mutant generates self-complementary vectors to overcome the rate-limiting step to transduction in vivo

An important limitation of recombinant adeno-associated virus (rAAV) vector efficiency is the requirement of hostcell-mediated synthesis of double-stranded DNA from the single-stranded genome. We have bypassed this step in a specialized self-complementary rAAV (scAAV) vector, by utilizing the tendency of AAV to package DNA dimers when the replicating genome is half the length of the wild type (wt). To produce these vectors efficiently, we have deleted the terminal resolution site (trs) from one rAAV TR, preventing the initiation of replication at the mutated end. These constructs generate single-stranded, inverted repeat genomes, with a wt TR at each end, and a mutated TR in the middle. After uncoating, the viral DNA folds through intramolecular base pairing within the mutant TR, which then proceeds through the genome to form a double-stranded molecule. We have used the scAAV to investigate barriers to rAAV transduction in the mouse liver, muscle and brain. In each tissue, scAAV was characterized by faster onset of gene expression and higher transduction efficiency. This study confirms earlier predictions that complementary-strand DNA synthesis is the primary barrier to rAAV-2 transduction. The scAAV is unaffected by this barrier, and provides an extremely efficient vector for gene transfer into many types of cells in vivo.

Are animal models useful for understanding the pathophysiology of lysosomal storage disease?

Spontaneously occurring genetic lysosomal storage diseases are as rare in other mammalian species as in man. However, the advent of gene targeting technology has revolutionized the state of animal models of genetic diseases. Nearly all lysosomal storage diseases known in man have been duplicated in the mouse. The technology now allows, not only complete inactivation of endogenous genes, but also the introduction of essentially any type of mutation. These animal models can overcome many of the limitations inherent in studies of human patients--rarity of the disease, extremely complex genetic background and logistical and ethical constraints in the design and execution of experiments with human subjects. For example, genetic manipulations of germ cells or cross-breeding experiments between two mutants are readily feasible with animal models. Two major areas of the utility of animal models are the clarification of the pathophysiology/pathogenetic mechanism of disease and the exploration of therapeutic approaches. Examples of experiments using animal models of lysosomal storage disease are presented, primarily from studies undertaken in our own laboratory.

CONCLUSION

Animal models have proved invaluable in extending our knowledge of the lysosomal storage diseases and exploring potential therapies.

Globoid cell leukodystrophy (Krabbe's disease): update

The classic globoid cell leukodystrophy (Krabbe's disease) is caused by genetic defects in a lysosomal enzyme, galactosylceramidase. It is one of the two classic genetic leukodystrophies, together with metachromatic leukodystrophy. The mode of inheritance is autosomal recessive. Typically, the disease occurs among infants and takes a rapidly fatal course, but rarer late-onset forms also exist. Clinical manifestations are exclusively neurologic with prominent white-matter signs. The pathology is unique, consisting of a rapid and nearly complete disappearance of myelin and myelin-forming cells--the oligodendrocytes in the central nervous system and the Schwann cells in the peripheral nervous system, reactive astroytic gliosis, and infiltration of the unique and often multinucleated macrophages ("globoid cells") that contain strongly periodic acid-Schiff (PAS)-positive materials. A normally insignificant but highly cytotoxic metabolite, galactosylsphingosine (psychosine), is also a substrate of galactosylceramidase and is considered to play a critical role in the pathogenesis. The galactosylceramidase gene has been cloned, and a large number of disease-causing mutations have been identified. Equivalent genetic galactosylceramidase deficiency occurs in several mammalian species, such as mouse, dog, and monkey. Recently, deficiency of one of the sphingolipid activator proteins, saposin A, was demonstrated to cause a late-onset, slowly progressive globoid cell leukodystrophy at least in the mouse, with all of the phenotypic consequences of impaired degradation of galactosylceramidase substrates. Human globoid cell leukodystrophy owing to saposin A deficiency might be anticipated and should be suspected in human patients with a late-onset leukodystrophy with normal galactosylceramidase activity when other possibilities are also excluded. The only serious attempt at treating human patients is bone marrow transplantation, which can provide significant alleviation of symptoms, particularly in those patients with later-onset, more slowly progressive globoid cell leukodystrophy.

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

Recombinant adeno-associated virus serotype 2 (rAAV2) vectors have shown promise as therapeutic agents for neurologic disorders. However, intracerebral administration of this vector leads to preferential transduction of neurons and a restricted region of transgene expression. The recently developed rAAV vectors based upon nonserotype 2 viruses have the potential to overcome these limitations. Therefore, we directly compared a rAAV type 1 to a type 2 vector in the murine brain. The vectors were engineered to carry identical genomes (AAV2 terminal repeat elements flanking an enhanced green fluorescent protein expression cassette) and were administered by stereotaxic-guided intracerebral injection. We found that the rAAV1 vector (rAAV1-GFP) had a 13- to 35-fold greater transduction efficiency than that of the rAAV2 vector (rAAV2-GFP). Also, rAAV1-transduced cells were observed at a greater distance from the injection site than rAAV2-transduced cells. Neurons were the predominant cell type transduced by both vector types. However, in contrast to rAAV2-GFP, rAAV1-GFP was capable of transducing glial and ependymal cells. Thus, rAAV1-based vectors have biologic properties within the brain distinct from that of rAAV2. These differences might be capitalized upon to develop novel gene transfer strategies for neurologic disorders.

Promoters and regulatory elements that improve adeno-associated virus transgene expression in the brain

Since the first demonstration of central nervous system (CNS) transduction with recombinant adeno-associated virus, improvements in vector production and promoter strength have lead to dramatic increases in the number of cells transduced and the level of expression within each cell. The improvements in promoter strength have resulted from a move away from the original cytomegalovirus (CMV) promoter toward the use of hybrid CMV-based promoters and constitutive cellular promoters. This review summarizes and compares different promoters and regulatory elements that have been used with rAAV as a reference toward achieving a high level of rAAV-mediated transgene expression in the CNS.

Adeno-associated virus-mediated gene transfer to the neonatal brain

For many metabolic diseases, early treatment is necessary to prevent irreversible developmental damage. This is particularly true for childhood diseases that affect the central nervous system (CNS). The development of effective techniques for gene transfer to the neonatal brain would provide a new set of therapeutic options for many of these disorders. Vectors based on adeno-associated virus (AAV) have shown promise as agents for neonatal CNS transduction. In preclinical animal models, a single treatment with AAV vectors at birth has been shown to produce persistent CNS expression of transduced genes into adulthood. Transduction of the neonatal brain has been accomplished by a variety of methods, including direct intraparenchymal injection, intraventricular infusion, and intravenous administration. Of these methods, intraparenchymal injection provides the highest levels of localized activity, while intraventricular infusion results in a more widespread distribution of activity when performed in the neonate. Here we describe a method for direct, intraparenchymal injection of AAV into the neonatal brain. This technique provides a method for investigators to evaluate the effects of in vivo expression of exogenous genes on the process of early brain development.

Recombinant adeno-associated virus vector design and gene expression in the mammalian brain

Efficiency and stability of recombinant adeno-associated virus (rAAV)-mediated gene expression within the mammalian brain are determined by several factors. These include the dose of infectious particles, the purity of the vector stock, the serotype of rAAV, the route of administration, and the intrinsic properties, most notably the rAAV receptor density, of the targeted area. Furthermore, the choice of appropriate regulatory elements in rAAV vector design is of fundamental importance to achieve high-level sustained in vivo transcription and translation. This review summarizes the characteristics of various transcriptional and posttranscriptional regulatory elements, and highlights their influence on the expression performance of rAAV vectors in the mammalian brain.

Intracranial injection of recombinant adeno-associated virus improves cognitive function in a murine model of mucopolysaccharidosis type VII

Mucopolysaccharidosis type VII (MPS VII) is a lysosomal storage disease caused by the lack of beta-glucuronidase (GUSB) activity. GUSB deficiency leads to the progressive accumulation of undegraded glycosaminoglycans (GAGs) in cells of most tissues, including the brain, and is associated with mental retardation. Reduction of lysosomal storage in the central nervous system and prevention of cognitive dysfunction may require intracranial delivery of a therapeutic agent during the newborn period that provides a continuous source of GUSB. Therefore, we injected recombinant adeno-associated virus encoding human GUSB into both the anterior cortex and the hippocampus of newborn MPS VII mice. Total GUSB activity in the brain approached normal levels by 18 weeks. Although GUSB activity was concentrated near the injection sites, lysosomal distension was reduced in most areas of the brain. In addition to histopathologic evidence of GAG reduction, the previously undescribed accumulation of GM2 and GM3 gangliosides in the brain was also prevented. Furthermore, GUSB expression and reduced lysosomal distension correlated with improvements in cognitive function as measured in the Morris Water Maze test. These findings indicate that localized overexpression of GUSB has positive effects on the pathology and cognitive function and does not have overt toxicity.

Adeno-associated virus vectors: activity and applications in the CNS

Transgenic strategies are useful for functional studies and they may also lead to novel therapies. Controlling transgene expression in defined cell populations over time is increasingly important for both functional and gene therapy experiments. The adeno-associated virus (AAV) vector may provide sufficient spatio-temporal control of gene expression for these purposes. This paper reviews in vivo somatic gene transfer methodology using AAV. Advantageous features of this system include neuronal gene expression that is: (1) efficient; (2) long-lived; and (3) non-toxic. Thus, AAV-mediated gene transfer is a good method for functional genomic research. From characterizing vector activity in the brain using different combinations of promoters and transgenes in the mid to late 1990s, researchers continue to discover novel uses of AAV for both basic and clinical neuroscience.

Gene transfer into hypothalamic organotypic cultures using an adeno-associated virus vector

Organotypic cultures of rat hypothalamic slice cultures were successfully transduced using adeno-associated viral vectors. Using nuclear-targeted Lac-Z as the reporter gene, transduction was found to be very effective, occurring in as high as 89% of a specific cell type, the oxytocin neurons, present in the cultured explants. These transduction levels were not accompanied by any deleterious effects in the cultured cells 7 days after transduction. Such an in vitro approach should be valuable for the study of cell-specific gene expression in neurons in the central nervous system for which there are no homologous (surrogate) cell lines.