Self-complementary adeno-associated virus serotype 2 vector: global distribution and broad dispersion of AAV-mediated transgene expression in mouse brain

Adeno-associated virus vector delivery to the brain: Technology advancements and clinical applications

Adeno-associated virus (AAV) vectors have emerged as a promising tool in the development of gene therapies for various neurological diseases, including Alzheimer's disease and Parkinson's disease. However, the blood-brain barrier (BBB) poses a significant challenge to successfully delivering AAV vectors to the brain. Strategies that can overcome the BBB to improve the AAV delivery efficiency to the brain are essential to successful brain-targeted gene therapy. This review provides an overview of existing strategies employed for AAV delivery to the brain, including direct intraparenchymal injection, intra-cerebral spinal fluid injection, intranasal delivery, and intravenous injection of BBB-permeable AAVs. Focused ultrasound has emerged as a promising technology for the noninvasive and spatially targeted delivery of AAV administered by intravenous injection. This review also summarizes each strategy's current preclinical and clinical applications in treating neurological diseases. Moreover, this review includes a detailed discussion of the recent advances in the emerging focused ultrasound-mediated AAV delivery. Understanding the state-of-the-art of these gene delivery approaches is critical for future technology development to fulfill the great promise of AAV in neurological disease treatment.

Adeno-associated virus as a delivery vector for gene therapy of human diseases

Adeno-associated virus (AAV) has emerged as a pivotal delivery tool in clinical gene therapy owing to its minimal pathogenicity and ability to establish long-term gene expression in different tissues. Recombinant AAV (rAAV) has been engineered for enhanced specificity and developed as a tool for treating various diseases. However, as rAAV is being more widely used as a therapy, the increased demand has created challenges for the existing manufacturing methods. Seven rAAV-based gene therapy products have received regulatory approval, but there continue to be concerns about safely using high-dose viral therapies in humans, including immune responses and adverse effects such as genotoxicity, hepatotoxicity, thrombotic microangiopathy, and neurotoxicity. In this review, we explore AAV biology with an emphasis on current vector engineering strategies and manufacturing technologies. We discuss how rAAVs are being employed in ongoing clinical trials for ocular, neurological, metabolic, hematological, neuromuscular, and cardiovascular diseases as well as cancers. We outline immune responses triggered by rAAV, address associated side effects, and discuss strategies to mitigate these reactions. We hope that discussing recent advancements and current challenges in the field will be a helpful guide for researchers and clinicians navigating the ever-evolving landscape of rAAV-based gene therapy.

Novel MECP2 gene therapy is effective in a multicenter study using two mouse models of Rett syndrome and is safe in non-human primates

The AAV9 gene therapy vector presented in this study is safe in mice and non-human primates and highly efficacious without causing overexpression toxicity, a major challenge for clinical translation of Rett syndrome gene therapy vectors to date. Our team designed a new truncated methyl-CpG-binding protein 2 (MECP2) promoter allowing widespread expression of MECP2 in mice and non-human primates after a single injection into the cerebrospinal fluid without causing overexpression symptoms up to 18 months after injection. Additionally, this new vector is highly efficacious at lower doses compared with previous constructs as demonstrated in extensive efficacy studies performed by two independent laboratories in two different Rett syndrome mouse models carrying either a knockout or one of the most frequent human mutations of Mecp2. Overall, data from this multicenter study highlight the efficacy and safety of this gene therapy construct, making it a promising candidate for first-in-human studies to treat Rett syndrome.

The gene therapy for corneal pathology with novel nonsense cystinosis mouse lines created by CRISPR Gene Editing

PURPOSE

Cystinosis is an autosomal recessive lysosomal storage disease (LSDs) caused by mutations in the gene encoding cystinosin (CTNS) that leads to cystine crystal accumulation in the lysosome that compromises cellular functions resulting in tissue damage and organ failure, especially in kidneys and eyes. However, the underlying molecular mechanism of its pathogenesis remains elusive. Two novel mice lines created via CRISPR are used to examine the pathogenesis of cystinosis in the kidney and cornea and the treatment efficacy of corneal pathology using self-complimentary Adeno-associated viral (scAAV-CTNS) vector.

METHODS

The CRISPR technique generated two novel cystinotic mouse lines, Ctnsis1 (an insertional mutation) and Ctnsis2 (a nonsense mutation). Immune histochemistry, renal functions test and HRT2 in vivo confocal microscopy were used to evaluate the age-related renal pathogenesis and treatment efficacy of the scAAV-CTNS virus in corneal pathology.

RESULTS

Both mutations lead to the production of truncated Ctns proteins. Ctnsis1 and Ctnsis 2 mice exhibit the characteristic of cystinotic corneal crystal phenotype at four-week-old. Treatment with the scAAV-CTNS viral vector decreased the corneal crystals in the treated mice cornea. Ctnsis 1 show renal abnormalities manifested by increased urine volume, reduced urine osmolality, and the loss of response to Desmopressin (dDAVP) at 22-month-old but Ctnsis2 don't manifest renal pathology up to 2 years of age.

CONCLUSIONS

Both Ctnsis1 and Ctnsis2 mice exhibit phenotypes resembling human intermediate nephropathic and ocular cystinosis, respectively. scAAV-CTNS viral vectors reduce the corneal cystine crystals and have a great potential as a therapeutic strategy for treating patients suffering from cystinosis.

Prospects for gene replacement therapies in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating and incurable neurodegenerative disease characterized by the progressive loss of upper and lower motor neurons. ALS causes death, usually within 2-5 years of diagnosis. Riluzole, the only drug currently approved in Europe for the treatment of this condition, offers only a modest benefit, increasing survival by 3 months on average. Recent advances in our understanding of causative or disease-modifying genetic variants and in the development of genetic therapy strategies present exciting new therapeutic opportunities for ALS. In addition, the approval of adeno-associated virus-mediated delivery of functional copies of the SMN1 gene to treat spinal muscular atrophy represents an important therapeutic milestone and demonstrates the potential of gene replacement therapies for motor neuron disorders. In this Review, we describe the current landscape of genetic therapies in ALS, highlighting achievements and critical challenges. In particular, we discuss opportunities for gene replacement therapy in subgroups of people with ALS, and we describe loss-of-function mutations that are known to contribute to the pathophysiology of ALS and could represent novel targets for gene replacement therapies.

Current Strategies to Enhance Delivery of Drugs across the Blood–Brain Barrier

The blood–brain barrier (BBB) has shown to be a significant obstacle to brain medication delivery. The BBB in a healthy brain is a diffusion barrier that prevents most substances from passing from the blood to the brain; only tiny molecules can pass across the BBB. The BBB is disturbed in specific pathological illnesses such as stroke, diabetes, seizures, multiple sclerosis, Parkinson’s disease, and Alzheimer’s disease. The goal of this study is to offer a general overview of current brain medication delivery techniques and associated topics from the last five years. It is anticipated that this review will stimulate readers to look into new ways to deliver medications to the brain. Following an introduction of the construction and function of the BBB in both healthy and pathological conditions, this review revisits certain contested questions, such as whether nanoparticles may cross the BBB on their own and if medications are selectively delivered to the brain by deliberately targeted nanoparticles. Current non-nanoparticle options are also discussed, including drug delivery via the permeable BBB under pathological circumstances and the use of non-invasive approaches to improve brain medication absorption.

Propagation of tau and α-synuclein in the brain: therapeutic potential of the glymphatic system

Many neurodegenerative diseases, including Alzheimer’s disease and Parkinson’s disease, are characterised by the accumulation of misfolded protein deposits in the brain, leading to a progressive destabilisation of the neuronal network and neuronal death. Among the proteins that can abnormally accumulate are tau and α-synuclein, which can propagate in a prion-like manner and which upon aggregation, represent the most common intracellular proteinaceous lesions associated with neurodegeneration. For years it was thought that these intracellular proteins and their accumulation had no immediate relationship with extracellular homeostasis pathways such as the glymphatic clearance system; however, mounting evidence has now suggested that this is not the case. The involvement of the glymphatic system in neurodegenerative disease is yet to be fully defined; however, it is becoming increasingly clear that this pathway contributes to parenchymal solute clearance. Importantly, recent data show that proteins prone to intracellular accumulation are subject to glymphatic clearance, suggesting that this system plays a key role in many neurological disorders. In this review, we provide a background on the biology of tau and α-synuclein and discuss the latest findings on the cell-to-cell propagation mechanisms of these proteins. Importantly, we discuss recent data demonstrating that manipulation of the glymphatic system may have the potential to alleviate and reduce pathogenic accumulation of propagation-prone intracellular cytotoxic proteins. Furthermore, we will allude to the latest potential therapeutic opportunities targeting the glymphatic system that might have an impact as disease modifiers in neurodegenerative diseases.

Hyperosmolar blood-brain barrier opening using intra-arterial injection of hyperosmotic mannitol in mice under real-time MRI guidance

The blood-brain barrier (BBB) is the main obstacle to the effective delivery of therapeutic agents to the brain, compromising treatment efficacy for a variety of neurological disorders. Intra-arterial (IA) injection of hyperosmotic mannitol has been used to permeabilize the BBB and improve parenchymal entry of therapeutic agents following IA delivery in preclinical and clinical studies. However, the reproducibility of IA BBB manipulation is low and therapeutic outcomes are variable. We demonstrated that this variability could be highly reduced or eliminated when the procedure of osmotic BBB opening is performed under the guidance of interventional MRI. Studies have reported the utility and applicability of this technique in several species. Here we describe a protocol to open the BBB by IA injection of hyperosmotic mannitol under the guidance of MRI in mice. The procedures (from preoperative preparation to postoperative care) can be completed within ~1.5 h, and the skill level required is on par with the induction of middle cerebral artery occlusion in small animals. This MRI-guided BBB opening technique in mice can be utilized to study the biology of the BBB and improve the delivery of various therapeutic agents to the brain.

Treatment of adult metachromatic leukodystrophy model mice using intrathecal administration of type 9 AAV vector encoding arylsulfatase A

Metachromatic leukodystrophy (MLD) is a lysosomal storage disease caused by an arylsulfatase A (ARSA) deficiency and characterized by severe neurological symptoms resulting from demyelination within the central and peripheral nervous systems. We investigated the feasibility and efficacy of intrathecal administration of a type 9 adeno-associated viral vector encoding ARSA (AAV9/ARSA) for the treatment of 6-week-old MLD model mice, which are presymptomatic, and 1-year-old mice, which exhibit neurological abnormalities. Immunohistochemical analysis following AAV9/ARSA administration showed ARSA expression within the brain, with highest activities in the cerebellum and olfactory bulbs. In mice treated at 1 year, alcian blue staining and quantitative analysis revealed significant decreases in stored sulfatide. Behaviorally, mice treated at 1 year showed no improvement in their ability to traverse narrow balance beams as compared to untreated mice. By contrast, MLD mice treated at 6 weeks showed significant decreases in stored sulfatide throughout the entire brain and improved ability to traverse narrow balance beams. These findings suggest intrathecal administration of an AAV9/ARSA vector is a promising approach to treating genetic diseases of the central nervous system, including MLD, though it may be essential to begin therapy before the onset of neurological symptoms.

The role of gap junctions in cell death and neuromodulation in the retina

Vision altering diseases, such as glaucoma, diabetic retinopathy, age-related macular degeneration, myopia, retinal vascular disease, traumatic brain injuries and others cripple many lives and are projected to continue to cause anguish in the foreseeable future. Gap junctions serve as an emerging target for neuromodulation and possible regeneration as they directly connect healthy and/or diseased cells, thereby playing a crucial role in pathophysiology. Since they are permeable for macromolecules, able to cross the cellular barriers, they show duality in illness as a cause and as a therapeutic target. In this review, we take recent advancements in gap junction neuromodulation (pharmacological blockade, gene therapy, electrical and light stimulation) into account, to show the gap junction's role in neuronal cell death and the possible routes of rescuing neuronal and glial cells in the retina succeeding illness or injury.

Identification of peripheral oxytocin-expressing cells using systemically applied cell-type specific adeno-associated viral vector

Oxytocin is primarily synthesised in the brain and is widely known for its role in lactation and parturition after being released into the blood from the posterior pituitary gland. Nevertheless, peripheral tissues have also been reported to express oxytocin. Using systemic injection of a recombinant adeno-associated virus vector, we investigated the expression of the green fluorescent protein Venus under the control of the oxytocin promoter in the gastrointestinal tract, pancreas and testes of adult rats. Here, we confirm that the vector infects oxytocin neurones of the enteric nervous system in ganglia of the myenteric and submucosal plexuses. Venus was detected in 25%-60% of the ganglia in the myenteric and submucosal plexuses identified by co-staining with the neuronal marker PGP9.5. Oxytocin expression was also detected in the islets of Langerhans in the pancreas and the Leydig cells of the testes. Our data illustrate that peripheral administration of the viral vector represents a powerful method for selectively labelling oxytocin-producing cells outside the brain.

Is Viral Vector Gene Delivery More Effective Using Biomaterials?

Gene delivery has been extensively investigated for introducing foreign genetic material into cells to promote expression of therapeutic proteins or to silence relevant genes. This approach can regulate genetic or epigenetic disorders, offering an attractive alternative to pharmacological therapy or invasive protein delivery options. However, the exciting potential of viral gene therapy has yet to be fully realized, with a number of clinical trials failing to deliver optimal therapeutic outcomes. Reasons for this include difficulty in achieving localized delivery, and subsequently lower efficacy at the target site, as well as poor or inconsistent transduction efficiency. Thus, ongoing efforts are focused on improving local viral delivery and enhancing its efficiency. Recently, biomaterials have been exploited as an option for more controlled, targeted and programmable gene delivery. There is a growing body of literature demonstrating the efficacy of biomaterials and their potential advantages over other delivery strategies. This review explores current limitations of gene delivery and the progress of biomaterial-mediated gene delivery. The combination of biomaterials and gene vectors holds the potential to surmount major challenges, including the uncontrolled release of viral vectors with random delivery duration, poorly localized viral delivery with associated off-target effects, limited viral tropism, and immune safety concerns.

CRISPR-READI: Efficient Generation of Knockin Mice by CRISPR RNP Electroporation and AAV Donor Infection

Genetically engineered mouse models harboring large sequence insertions or modifications are critical for a wide range of applications including endogenous gene tagging, conditional knockout, site-specific transgene insertion, and gene replacement; however, existing methods to generate such animals remain laborious and costly. To address this, we developed an approach called CRISPR-READI (CRISPR RNP electroporation and AAV donor infection), combining adeno-associated virus (AAV)-mediated HDR donor delivery with Cas9/sgRNA RNP electroporation to engineer large site-specific modifications in the mouse genome with high efficiency and throughput. We successfully targeted a 774 bp fluorescent reporter, a 2.1 kb CreERT2 driver, and a 3.3 kb expression cassette into endogenous loci in both embryos and live mice. CRISPR-READI is applicable to most widely used knockin schemes requiring donor lengths within the 4.9 kb AAV packaging capacity. Altogether, CRISPR-READI is an efficient, high-throughput, microinjection-free approach for sophisticated mouse genome engineering with potential applications in other mammalian species.

Volume and Infusion Rate Dynamics of Intraparenchymal Central Nervous System Infusion in a Large Animal Model

Thalamic infusion of adeno-associated viral (AAV) vectors has been shown to have therapeutic effects in neuronopathic lysosomal storage diseases. Preclinical studies in sheep model of Tay-Sachs disease demonstrated that bilateral thalamic injections of AAV gene therapy are required for maximal benefit. Translation of thalamic injection to patients carries risks in that (1) it has never been done in humans, and (2) dosing scale-up based on brain weight from animals to humans requires injection of larger volumes. To increase the safety margin of this infusion, a flexible cannula was selected to enable simultaneous bilateral thalamic infusion in infants while monitoring by imaging and/or to enable awake infusions for injection of large volumes at low infusion rates. In this study, we tested various infusion volumes (200-800 μL) and rates (0.5-5 μL/min) to determine the maximum tolerated combination of injection parameters. Animals were followed for ∼1 month postinjection with magnetic resonance imaging (MRI) performed at 14 and 28 days. T1-weighted MRI was used to quantify thalamic damage followed by histopathological assessment of the brain. Trends in data show that infusion volumes of 800 μL (2 × the volume required in sheep based on thalamic size) resulted in larger lesions than lower volumes, where the long infusion times (between 13 and 26 h) could have contributed to the generation of larger lesions. The target volume (400 μL, projected to be sufficient to cover most of the sheep thalamus) created the smallest lesion size. Cannula placement alone did result in damage, but this is likely associated with an inherent limitation of its use in a small brain due to the length of the distal rigid portion and lack of stable fixation. An injection rate of 5 μL/min at a volume ∼1/3 of the thalamus (400-600 μL) appears to be well tolerated in sheep both clinically and histopathologically.

Gene replacement ameliorates deficits in mouse and human models of cyclin-dependent kinase-like 5 disorder

Cyclin-dependent kinase-like 5 disorder is a severe neurodevelopmental disorder caused by mutations in the X-linked cyclin-dependent kinase-like 5 (CDKL5) gene. It predominantly affects females who typically present with severe early epileptic encephalopathy, global developmental delay, motor dysfunction, autistic features and sleep disturbances. To develop a gene replacement therapy, we initially characterized the human CDKL5 transcript isoforms expressed in the brain, neuroblastoma cell lines, primary astrocytes and embryonic stem cell-derived cortical interneurons. We found that the isoform 1 and to a lesser extent the isoform 2 were expressed in human brain, and both neuronal and glial cell types. These isoforms were subsequently cloned into recombinant adeno-associated viral (AAV) vector genome and high-titre viral vectors were produced. Intrajugular delivery of green fluorescence protein via AAV vector serotype PHP.B in adult wild-type male mice transduced neurons and astrocytes throughout the brain more efficiently than serotype 9. Cdkl5 knockout male mice treated with isoform 1 via intrajugular injection at age 28–30 days exhibited significant behavioural improvements compared to green fluorescence protein-treated controls (1012 vg per animal, n = 10 per group) with PHP.B vectors. Brain expression of the isoform 1 transgene was more abundant in hindbrain than forebrain and midbrain. Transgene brain expression was sporadic at the cellular level and most prominent in hippocampal neurons and cerebellar Purkinje cells. Correction of postsynaptic density protein 95 cerebellar misexpression, a major fine cerebellar structural abnormality in Cdkl5 knockout mice, was found in regions of high transgene expression within the cerebellum. AAV vector serotype DJ efficiently transduced CDKL5-mutant human induced pluripotent stem cell-derived neural progenitors, which were subsequently differentiated into mature neurons. When treating CDKL5-mutant neurons, isoform 1 expression led to an increased density of synaptic puncta, while isoform 2 ameliorated the calcium signalling defect compared to green fluorescence protein control, implying distinct functions of these isoforms in neurons. This study provides the first evidence that gene therapy mediated by AAV vectors can be used for treating CDKL5 disorder.

Safe and stable noninvasive focal gene delivery to the mammalian brain following focused ultrasound

OBJECTIVE

Surgical infusion of gene therapy vectors has provided opportunities for biological manipulation of specific brain circuits in both animal models and human patients. Transient focal opening of the blood-brain barrier (BBB) by MR-guided focused ultrasound (MRgFUS) raises the possibility of noninvasive CNS gene therapy to target precise brain regions. However, variable efficiency and short follow-up of studies to date, along with recent suggestions of the potential for immune reactions following MRgFUS BBB disruption, all raise questions regarding the viability of this approach for clinical translation. The objective of the current study was to evaluate the efficiency, safety, and long-term stability of MRgFUS-mediated noninvasive gene therapy in the mammalian brain.

METHODS

Focused ultrasound under the control of MRI, in combination with microbubbles consisting of albumin-coated gas microspheres, was applied to rat striatum, followed by intravenous infusion of an adeno-associated virus serotype 1/2 (AAV1/2) vector expressing green fluorescent protein (GFP) as a marker. Following recovery, animals were followed from several hours up to 15 months. Immunostaining for GFP quantified transduction efficiency and stability of expression. Quantification of neuronal markers was used to determine histological safety over time, while inflammatory markers were examined for evidence of immune responses.

RESULTS

Transitory disruption of the BBB by MRgFUS resulted in efficient delivery of the AAV1/2 vector to the targeted rodent striatum, with 50%-75% of striatal neurons transduced on average. GFP transgene expression appeared to be stable over extended periods of time, from 2 weeks to 6 months, with evidence of ongoing stable expression as long as 16 months in a smaller cohort of animals. No evidence of substantial toxicity, tissue injury, or neuronal loss was observed. While transient inflammation from BBB disruption alone was noted for the first few days, consistent with prior observations, no evidence of brain inflammation was observed from 2 weeks to 6 months following MRgFUS BBB opening, despite delivery of a virus and expression of a foreign protein in target neurons.

CONCLUSIONS

This study demonstrates that transitory BBB disruption using MRgFUS can be a safe and efficient method for site-specific delivery of viral vectors to the brain, raising the potential for noninvasive focal human gene therapy for neurological disorders.

Intravenous Infusion of AAV for Widespread Gene Delivery to the Nervous System

The central nervous system (CNS) is a fascinating and intricate set of biological structures that we have yet to fully understand. Studying the in vivo function of the CNS and finding novel methods for treating neurological disorders have been particularly challenging. One difficulty is correcting genetic disorders afflicting the CNS in a targeted manner. Recombinant adeno-associated viruses (rAAVs) have emerged as promising therapeutic tools for treating genetic defects of the CNS, due to their excellent safety profile and ability to cross the blood-brain barrier (BBB). While stereotactic injection of AAV is promising for localized gene delivery, it is less desirable for some applications because of the technique's invasiveness and limited intraparenchymal spread. Alternatively, intravascular administration can achieve widespread delivery of rAAV to the CNS. In this chapter, we will discuss the prevalent routes of administration to deliver rAAV to the CNS via intravenous (IV) injection in mice. We will highlight key considerations for using rAAV, and the advantages and disadvantages of each administration method. We will also briefly discuss intravenous delivery in larger animal models, factors that may impact experimental interpretations, and outlooks for clinical translation.

Transvascular Delivery of Hydrophobically Modified siRNAs: Gene Silencing in the Rat Brain upon Disruption of the Blood-Brain Barrier

Effective transvascular delivery of therapeutic oligonucleotides to the brain presents a major hurdle to the development of gene silencing technologies for treatment of genetically defined neurological disorders. Distribution to the brain after systemic administrations is hampered by the low permeability of the blood-brain barrier (BBB) and the rapid clearance kinetics of these drugs from the blood. Here we show that transient osmotic disruption of the BBB enables transvascular delivery of hydrophobically modified small interfering RNA (hsiRNA) to the rat brain. Intracarotid administration of 25% mannitol and hsiRNA conjugated to phosphocholine-docosahexanoic acid (PC-DHA) resulted in broad ipsilateral distribution of PC-DHA-hsiRNAs in the brain. PC-DHA conjugation enables hsiRNA retention in the parenchyma proximal to the brain vasculature and enabled active internalization by neurons and astrocytes. Moreover, transvascular delivery of PC-DHA-hsiRNAs effected Htt mRNA silencing in the striatum (55%), hippocampus (51%), somatosensory cortex (52%), motor cortex (37%), and thalamus (33%) 1 week after administration. Aside from mild gliosis induced by osmotic disruption of the BBB, transvascular delivery of PC-DHA-hsiRNAs was not associated with neurotoxicity. Together, these findings provide proof-of-concept that temporary disruption of the BBB is an effective strategy for the delivery of therapeutic oligonucleotides to the brain.

Physical positioning markedly enhances brain transduction after intrathecal AAV9 infusion

Several neurological disorders may benefit from gene therapy. However, even when using the lead vector candidate for intrathecal administration, adeno-associated virus serotype 9 (AAV9), the strength and distribution of gene transfer to the brain are inconsistent. On the basis of preliminary observations that standard intrathecal AAV9 infusions predominantly drive reporter gene expression in brain regions where gravity might cause cerebrospinal fluid to settle, we tested the hypothesis that counteracting vector "settling" through animal positioning would enhance vector delivery to the brain. When rats are either inverted in the Trendelenburg position or continuously rotated after intrathecal AAV9 infusion, we find (i) a significant 15-fold increase in the number of transduced neurons, (ii) a marked increase in gene delivery to cortical regions, and (iii) superior animal-to-animal consistency of gene expression. Entorhinal, prefrontal, frontal, parietal, hippocampal, limbic, and basal forebrain neurons are extensively transduced: 95% of transduced cells are neurons, and greater than 70% are excitatory. These findings provide a novel and simple method for broad gene delivery to the cortex and are of substantial relevance to translational programs for neurological disorders, including Alzheimer's disease and related dementias, stroke, and traumatic brain injury.

Targeting Root Cause by Systemic scAAV9-hIDS Gene Delivery: Functional Correction and Reversal of Severe MPS II in Mice

No treatment is available to address the neurological need and reversibility of MPS II. We developed a scAAV9-hIDS vector to deliver the human iduronate-2-sulfatase gene and test it in mouse model. We treated MPS II mice at different disease stages with an intravenous injection of scAAV9-mCMV-hIDS at different doses. The treatments led to rapid and persistent restoration of IDS activity and the reduction of glycosaminoglycans (GAG) throughout the CNS and somatic tissues in all cohorts. Importantly, the vector treatment at up to age 6 months improved behavior performance in the Morris water maze and normalized the survival. Notably, vector treatment at age 9 months also resulted in persistent rIDS expression and GAG clearance in MPS II mice, and the majority of these animals survived within the normal range of lifespan. Notably, the vector delivery did not result in any observable adverse events or detectable systemic toxicity in any treated animal groups. We believe that we have developed a safe and effective gene therapy for treating MPS II, which led to recent IND approval for a phase 1/2 clinical trial in MPS II patients, further supporting the extended potential of the demonstrated systemic rAAV9 gene delivery platform for broad disease targets.

Considerations for the use of virally delivered genetic tools for in-vivo circuit analysis and behavior in mutant mice: a practical guide to optogenetics

Optogenetics was the method of the year in 2010 according to Nature Neuroscience. Since then, this method has become widespread, the use of virally delivered genetic tools has extended to other fields such as pharmacogenetics, and optogenetic techniques have become frequently applied in genetically manipulated animals for in-vivo circuit analysis and behavioral studies. However, several issues should be taken into consideration when planning such experiments. We aimed to summarize the critical points concerning optogenetic manipulation of a specific brain area in mutant mice. First, the appropriate vector should be chosen to allow optimal optogenetic manipulation. Adeno-associated viral vectors are the most common carriers with different available serotypes. Light-sensitive channels are available in many forms, and the expression of the delivered genetic material can be influenced in many ways. Second, selecting the adequate stimulation protocol is also essential. The pattern, intensity, and timing could be determinative parameters. Third, the mutant strain might have a phenotype that influences the observed behavior. In conclusion, detailed preliminary experiments and numerous control groups are required to choose the best vector and stimulation protocol and to ensure that the mutant animals do not have a specific phenotype that can influence the examined behavior.

Viral Strategies for Targeting the Central and Peripheral Nervous Systems

Recombinant viruses allow for targeted transgene expression in specific cell populations throughout the nervous system. The adeno-associated virus (AAV) is among the most commonly used viruses for neuroscience research. Recombinant AAVs (rAAVs) are highly versatile and can package most cargo composed of desired genes within the capsid's ∼5-kb carrying capacity. Numerous regulatory elements and intersectional strategies have been validated in rAAVs to enable cell type–specific expression. rAAVs can be delivered to specific neuronal populations or globally throughout the animal. The AAV capsids have natural cell type or tissue tropism and trafficking that can be modified for increased specificity. Here, we describe recently engineered AAV capsids and associated cargo that have extended the utility of AAVs in targeting molecularly defined neurons throughout the nervous system, which will further facilitate neuronal circuit interrogation and discovery.

Lentiviral Vectors and Adeno-Associated Virus Vectors: Useful Tools for Gene Transfer in Pain Research

Pain, especially chronic pain, has always been a heated point in both basic and clinical researches since it puts heavy burdens on both individuals and the whole society. A better understanding of the role of biological molecules and various ionic channels involved in pain can shed light on the mechanism under pain and advocate the development of pain management. Using viral vectors to transfer specific genes at targeted sites is a promising method for both research and clinical applications. Lentiviral vectors and adeno-associated virus (AAV) vectors which allow stable and long-term expression of transgene in non-dividing cells are widely applied in pain research. In this review, we thoroughly outline the structure, category, advantages and disadvantages and the delivery methods of lentiviral and AAV vectors. The methods through which lentiviral and AAV vectors are delivered to targeted sites are closely related with the sites, level and period of transgene expression. Focus is placed on the various delivery methods applied to deliver vectors to spinal cord and dorsal root ganglion both of which play important roles in primary nociception. Our goal is to provide insight into the features of these two viral vectors and which administration approach can be chosen for different pain researches. Anat Rec, 301:825-836, 2018. © 2017 The Authors. The Anatomical Record published by Wiley Periodicals, Inc. on behalf of American Association of Anatomists.

Silencing Genes in the Heart

Silencing of cardiac genes by RNA interference (RNAi) has developed into a powerful new method to treat cardiac diseases. Small interfering (si)RNAs are the inducers of RNAi, but cultured primary cardiomyocytes and heart are highly resistant to siRNA transfection. This can be overcome by delivery of small hairpin (sh)RNAs or artificial microRNA (amiRNAs) by cardiotropic adeno-associated virus (AAV) vectors. Here we describe as example of the silencing of a cardiac gene, the generation and cloning of shRNA, and amiRNAs directed against the cardiac protein phospholamban. We further describe the generation of AAV shuttle plasmids with self complementary vector genomes, the production of AAV vectors in roller bottles, and their purification via iodixanol gradient centrifugation and concentration with filter systems. Finally we describe the preparation of primary neonatal rat cardiomyocytes (PNRC), the transduction of PNRC with AAV vectors, and the maintenance of the transduced cell culture.

A novel animal model for neuroinflammation and white matter degeneration

Small interference RNA has been widely used to suppress gene expression. Three different short hairpin RNAs (shRNAs) against dopamine D1 receptor (Drd1), driven by mouse U6 promoter in self-complementary AAV8 vector (scAAV8), were used to silence mouse striatal Drd1 expression. Transduction of mouse striatum with all three scAAV8-D1shRNA viruses, but not the control scAAV8 virus, causes extensive neuroinflammation, demyelination, and axon degeneration. RNA interference is known to be coupled to the innate immune system as a host cell defense against virus infection. Activation of the innate immune system may play a causal role in the development of neuroinflammation and white matter degeneration, providing a novel animal model for multiple sclerosis (MS) and other neuroinflammatory diseases.

Viral vectors for therapy of neurologic diseases

Neurological disorders - disorders of the brain, spine and associated nerves - are a leading contributor to global disease burden with a shockingly large associated economic cost. Various treatment approaches - pharmaceutical medication, device-based therapy, physiotherapy, surgical intervention, among others - have been explored to alleviate the resulting extent of human suffering. In recent years, gene therapy using viral vectors - encoding a therapeutic gene or inhibitory RNA into a "gutted" viral capsid and supplying it to the nervous system - has emerged as a clinically viable option for therapy of brain disorders. In this Review, we provide an overview of the current state and advances in the field of viral vector-mediated gene therapy for neurological disorders. Vector tools and delivery methods have evolved considerably over recent years, with the goal of providing greater and safer genetic access to the central nervous system. Better etiological understanding of brain disorders has concurrently led to identification of improved therapeutic targets. We focus on the vector technology, as well as preclinical and clinical progress made thus far for brain cancer and various neurodegenerative and neurometabolic disorders, and point out the challenges and limitations that accompany this new medical modality. Finally, we explore the directions that neurological gene therapy is likely to evolve towards in the future. This article is part of the Special Issue entitled "Beyond small molecules for neurological disorders".

Trafficking of adeno-associated virus vectors across a model of the blood-brain barrier; a comparative study of transcytosis and transduction using primary human brain endothelial cells

Developing therapies for central nervous system (CNS) diseases is exceedingly difficult because of the blood-brain barrier (BBB). Notably, emerging technologies may provide promising new options for the treatment of CNS disorders. Adeno-associated virus serotype 9 (AAV9) has been shown to transduce cells in the CNS following intravascular administration in rodents, cats, pigs, and non-human primates. These results suggest that AAV9 is capable of crossing the BBB. However, mechanisms that govern AAV9 transendothelial trafficking at the BBB remain unknown. Furthermore, possibilities that AAV9 may transduce brain endothelial cells or affect BBB integrity still require investigation. Using primary human brain microvascular endothelial cells as a model of the human BBB, we performed transduction and transendothelial trafficking assays comparing AAV9 to AAV2, a serotype that does not cross the BBB or transduce endothelial cells effectively in vivo. Results of our in vitro studies indicate that AAV9 penetrates brain microvascular endothelial cells barriers more effectively than AAV2, but has reduced transduction efficiency. In addition, our data suggest that (i) AAV9 penetrates endothelial barriers through an active, cell-mediated process, and (ii) AAV9 fails to disrupt indicators of BBB integrity such as transendothelial electrical resistance, tight junction protein expression/localization, and inflammatory activation status. Overall, this report shows how human brain endothelial cells configured in BBB models can be utilized for evaluating transendothelial movement and transduction kinetics of various AAV capsids. Importantly, the use of a human in vitro BBB model can provide import insight into the possible effects that candidate AVV gene therapy vectors may have on the status of BBB integrity. Read the Editorial Highlight for this article on page 192.

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.

A GLP-Compliant Toxicology and Biodistribution Study: Systemic Delivery of an rAAV9 Vector for the Treatment of Mucopolysaccharidosis IIIB

No treatment is currently available for mucopolysaccharidosis (MPS) IIIB, a neuropathic lysosomal storage disease due to defect in α-N-acetylglucosaminidase (NAGLU). In preparation for a clinical trial, we performed an IND-enabling GLP-toxicology study to assess systemic rAAV9-CMV-hNAGLU gene delivery in WT C57BL/6 mice at 1 × 10(14) vg/kg and 2 × 10(14) vg/kg (n = 30/group, M:F = 1:1), and non-GLP testing in MPS IIIB mice at 2 × 10(14) vg/kg. Importantly, no adverse clinical signs or chronic toxicity were observed through the 6 month study duration. The rAAV9-mediated rNAGLU expression was rapid and persistent in virtually all tested CNS and somatic tissues. However, acute liver toxicity occurred in 33% (5/15) WT males in the 2 × 10(14) vg/kg cohort, which was dose-dependent, sex-associated, and genotype-specific, likely due to hepatic rNAGLU overexpression. Interestingly, a significant dose response was observed only in the brain and spinal cord, whereas in the liver at 24 weeks postinfection (pi), NAGLU activity was reduced to endogenous levels in the high dose cohort but remained at supranormal levels in the low dose group. The possibility of rAAV9 germline transmission appears to be minimal. The vector delivery resulted in transient T-cell responses and characteristic acute antibody responses to both AAV9 and rNAGLU in all rAAV9-treated animals, with no detectable impacts on tissue transgene expression. This study demonstrates a generally safe and effective profile, and may have identified the upper dosing limit of rAAV9-CMV-hNAGLU via systemic delivery for the treatment of MPS IIIB.

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.

Long-term glycemic control with hepatic insulin gene therapy in streptozotocin-diabetic mice

BACKGROUND

Insulin self-administration is burdensome and can produce dangerous hypoglycemia. Insulin gene therapy may improve and simplify the treatment of diabetes mellitus. In rats, metabolically responsive hepatic insulin gene therapy (HIGT) delivered by adenovirus normalizes random blood sugars but with a limited duration. To prolong glycemic control, we delivered a metabolically regulated insulin transgene by adeno-associated virus (AAV).

METHODS

We administered increasing doses of self-complementary (SC), pseudotyped AAV8 expressing the (GlRE)3 BP1-2xfur insulin transgene to streptozotocin-diabetic CD-1 mice, and monitored blood sugar and body weight. We also compared responses to intraperitoneal glucose and chow withdrawal, assessed for viral genomes in liver by Southern blotting, and measured hepatic glycogen.

RESULTS

Glucose lowering required the combination of SC genomes and AAV capsid pseudotyping. HIGT controlled glycemia in diabetic mice (DM) for > 1 year. However, glycemic responses were variable. Approximately 30% of mice succumbed to hypoglycemia, and approximately 30% of mice again became hyperglycemic. During an intraperitoneal glucose tolerance test, blood sugars declined to normal within 180 min in HIGT-treated DM compared to 90 min in control mice. Hypoglycemia was common among HIGT-treated mice during a 24-h fast. However, HIGT mice lost less weight than either diabetic or nondiabetic controls as a result of increased water intake. HIGT treatment reduced the hepatic glycogen content of fed mice.

CONCLUSIONS

Our studies demonstrate the possibility for long-term glycemic correction following AAV-mediated HIGT in mice. However, the dose-response relationship is irregular, and metabolic responsiveness may be less than that observed in rats.

Self-Complementary Adeno-Associated Virus Vectors Improve Transduction Efficiency of Corneal Endothelial Cells

Transplantation of a donor cornea to restore vision is the most frequently performed transplantation in the world. Corneal endothelial cells (CEC) are crucial for the outcome of a graft as they maintain corneal transparency and avoid graft failure due to corneal opaqueness. Given the characteristic of being a monolayer and in direct contact with culture medium during cultivation in eye banks, CEC are specifically suitable for gene therapeutic approaches prior to transplantation. Recombinant adeno-associated virus 2 (rAAV2) vectors represent a promising tool for gene therapy of CEC. However, high vector titers are needed to achieve sufficient gene expression. One of the rate-limiting steps for transgene expression is the conversion of single-stranded (ss-) DNA vector genome into double-stranded (ds-) DNA. This step can be bypassed by using self-complementary (sc-) AAV2 vectors. Aim of this study was to compare for the first time transduction efficiencies of ss- and scAAV2 vectors in CEC. For this purpose AAV2 vectors containing enhanced green fluorescent protein (GFP) as transgene were used. Both in CEC and in donor corneas, transduction with scAAV2 resulted in significantly higher transgene expression compared to ssAAV2. The difference in transduction efficiency decreased with increasing vector titer. In most cases, only half the vector titer of scAAV2 was required for equal or higher gene expression rates than those of ssAAV2. In human donor corneas, GFP expression was 64.7±11.3% (scAAV) and 38.0±8.6% (ssAAV) (p<0.001), respectively. Furthermore, transduced cells maintained their viability and showed regular morphology. Working together with regulatory authorities, a translation of AAV2 vector-mediated gene therapy to achieve a temporary protection of corneal allografts during cultivation and transplantation could therefore become more realistic.

AAV9-mediated central nervous system-targeted gene delivery via cisterna magna route in mice

Current barriers to the use of adeno-associated virus serotype 9 (AAV9) in clinical trials for treating neurological disorders are its high expression in many off-target tissues such as liver and heart, and lack of cell specificity within the central nervous system (CNS) when using ubiquitous promoters such as human cytomegalovirus (CMV) or chicken-β-actin hybrid (CAG). To enhance targeting the transgene expression in CNS cells, self-complementary (sc) AAV9 vectors, scAAV9-GFP vectors carrying neuronal Hb9 and synapsin 1, and nonspecific CMV and CAG promoters were constructed. We demonstrate that synapsin 1 and Hb9 promoters exclusively targeted neurons in vitro, although their strengths were up to 10-fold lower than that of CMV. In vivo analyses of mouse tissue after scAAV9-GFP vector delivery via the cisterna magna revealed a significant advantage of synapsin 1 promoter over both Hb9 variants in targeting neurons throughout the brain, since Hb9 promoters were driving gene expression mainly within the motor-related areas of the brain stem. In summary, this study demonstrates that cisterna magna administration is a safe alternative to intracranial or intracerebroventricular vector delivery route using scAAV9, and introduces a novel utility of the Hb9 promoter for the targeted gene expression for both in vivo and in vitro applications.

Controlling AAV Tropism in the Nervous System with Natural and Engineered Capsids

More than one hundred naturally occurring variants of adeno-associated virus (AAV) have been identified, and this library has been further expanded by an array of techniques for modification of the viral capsid. AAV capsid variants possess unique antigenic profiles and demonstrate distinct cellular tropisms driven by differences in receptor binding. AAV capsids can be chemically modified to alter tropism, can be produced as hybrid vectors that combine the properties of multiple serotypes, and can carry peptide insertions that introduce novel receptor-binding activity. Furthermore, directed evolution of shuffled genome libraries can identify engineered variants with unique properties, and rational modification of the viral capsid can alter tropism, reduce blockage by neutralizing antibodies, or enhance transduction efficiency. This large number of AAV variants and engineered capsids provides a varied toolkit for gene delivery to the CNS and retina, with specialized vectors available for many applications, but selecting a capsid variant from the array of available vectors can be difficult. This chapter describes the unique properties of a range of AAV variants and engineered capsids, and provides a guide for selecting the appropriate vector for specific applications in the CNS and retina.

Super-resolution imaging of nuclear import of adeno-associated virus in live cells

Adeno-associated virus (AAV) has been developed as a promising human gene therapy vector. Particularly, recombinant AAV vector (rAAV) achieves its transduction of host cells by crossing at least three physiological barriers including plasma membrane, endosomal membrane, and nuclear envelope (NE). So far, the AAV transduction mechanism has not been explored thoroughly at the single viral particle level. In this study, we employed high-speed super-resolution single-point edge-excitation sub-diffraction (SPEED) microscopy to map the events of single rAAV2 particles infecting live human cells with an unprecedented spatiotemporal resolution of 9–12 nm and 2–20 ms. Data reveal that rAAV2 particles are imported through nuclear pore complexes (NPCs) rather than nuclear membrane budding into the nucleus. Moreover, approximately 17% of the rAAV2 molecules starting from the cytoplasm successfully transverse the NPCs to reach the nucleoplasm, revealing that the NPCs act as a strict selective step for AAV delivery. This study lastly suggests a new pathway to improve AAV vectors for human gene therapy.

Catalytic immunoglobulin gene delivery in a mouse model of Alzheimer's disease: prophylactic and therapeutic applications

Accumulation of amyloid beta-peptide (Aβ) in the brain is hypothesized to be a causal event leading to dementia in Alzheimer's disease (AD). Aβ vaccination removes Aβ deposits from the brain. Aβ immunotherapy, however, may cause T cell- and/or Fc-receptor-mediated brain inflammation and relocate parenchymal Aβ deposits to blood vessels leading to cerebral hemorrhages. Because catalytic antibodies do not form stable immune complexes and Aβ fragments produced by catalytic antibodies are less likely to form aggregates, Aβ-specific catalytic antibodies may have safer therapeutic profiles than reversibly-binding anti-Aβ antibodies. Additionally, catalytic antibodies may remove Aβ more efficiently than binding antibodies because a single catalytic antibody can hydrolyze thousands of Aβ molecules. We previously isolated Aβ-specific catalytic antibody, IgVL5D3, with strong Aβ-hydrolyzing activity. Here, we evaluated the prophylactic and therapeutic efficacy of brain-targeted IgVL5D3 gene delivery via recombinant adeno-associated virus serotype 9 (rAAV9) in an AD mouse model. One single injection of rAAV9-IgVL5D3 into the right ventricle of AD model mice yielded widespread, high expression of IgVL5D3 in the unilateral hemisphere. IgVL5D3 expression was readily detectable in the contralateral hemisphere but to a much lesser extent. IgVL5D3 expression was also confirmed in the cerebrospinal fluid. Prophylactic and therapeutic injection of rAAV9-IgVL5D3 reduced Aβ load in the ipsilateral hippocampus of AD model mice. No evidence of hemorrhages, increased vascular amyloid deposits, increased proinflammatory cytokines, or infiltrating T-cells in the brains was found in the experimental animals. AAV9-mediated anti-Aβ catalytic antibody brain delivery can be prophylactic and therapeutic options for AD.

Efficient central nervous system AAVrh10-mediated intrathecal gene transfer in adult and neonate rats

Intracerebral administration of recombinant adeno-associated vector (AAV) has been performed in several clinical trials. However, delivery into the brain requires multiple injections and is not efficient to target the spinal cord, thus limiting its applications. To assess widespread and less invasive strategies, we tested intravenous (IV) or intrathecal (that is, in the cerebrospinal fluid (CSF)) delivery of a rAAVrh10-egfp vector in adult and neonate rats and studied the effect of the age at injection on neurotropism. IV delivery is more efficient in neonates and targets predominantly Purkinje cells of the cerebellum and sensory neurons of the spinal cord and dorsal root ganglia. A single intra-CSF administration of AAVrh10, single strand or oversized self-complementary, is efficient for the targeting of neurons in the cerebral hemispheres, cerebellum, brainstem and spinal cord. Green fluorescent protein (GFP) expression is more widespread in neonates when compared with adults. More than 50% of motor neurons express GFP in the three segments of the spinal cord in neonates and in the cervical and thoracic regions in adults. Neurons are almost exclusively transduced in neonates, whereas neurons, astrocytes and rare oligodendrocytes are targeted in adults. These results expand the possible routes of delivery of AAVrh10, a serotype that has shown efficacy and safety in clinical trials concerning neurodegenerative diseases.

Intracisternal delivery of NFκB-inducible scAAV2/9 reveals locoregional neuroinflammation induced by systemic kainic acid treatment

We have previously demonstrated disease-dependent gene delivery in the brain using an AAV vector responding to NFκB activation as a probe for inflammatory responses. This vector, injected focally in the parenchyma prior to a systemic kainic acid (KA) injection mediated inducible transgene expression in the hippocampus but not in the cerebellum, regions, respectively, known to be affected or not by the pathology. However, such a focal approach relies on previous knowledge of the model parameters and does not allow to predict the whole brain response to the disease. Global brain gene delivery would allow to predict the regional distribution of the pathology as well as to deliver therapeutic factors in all affected brain regions. We show that self-complementary AAV2/9 (scAAV2/9) delivery in the adult rat cisterna magna allows a widespread but not homogenous transduction of the brain. Indeed, superficial regions, i.e., cortex, hippocampus, and cerebellum were more efficiently transduced than deeper regions, such as striatum, and substantia nigra. These data suggest that viral particles penetration from the cerebrospinal fluid (CSF) into the brain is a limiting factor. Interestingly, AAV2/9-2YF a rationally designed capsid mutant (affecting surface tyrosines) increased gene transfer efficiency approximately fivefold. Neurons, astrocytes, and oligodendrocytes, but not microglia, were transduced in varying proportions depending on the brain region and the type of capsid. Finally, after a single intracisternal injection of scAAV2/9-2YF using the NFκB-inducible promoter, KA treatment induced transgene expression in the hippocampus and cortex but not in the cerebellum, corresponding to the expression of the CD11b marker of microglial activation. These data support the use of disease-inducible vectors administered in the cisterna magna as a tool to characterize the brain pathology in systemic drug-induced or transgenic disease models. However, further improvements are required to enhance viral particles penetration into the brain.

Biology of adeno-associated viral vectors in the central nervous system

Gene therapy is a promising approach for treating a spectrum of neurological and neurodegenerative disorders by delivering corrective genes to the central nervous system (CNS). In particular, adeno-associated viruses (AAVs) have emerged as promising tools for clinical gene transfer in a broad range of genetic disorders with neurological manifestations. In the current review, we have attempted to bridge our understanding of the biology of different AAV strains with their transduction profiles, cellular tropisms, and transport mechanisms within the CNS. Continued efforts to dissect AAV-host interactions within the brain are likely to aid in the development of improved vectors for CNS-directed gene transfer applications in the clinic.

Comparison of Endovascular and Intraventricular Gene Therapy With Adeno-Associated Virus-α-L-Iduronidase for Hurler Disease

BACKGROUND

Hurler disease (mucopolysaccharidosis type I [MPS-I]) is an inherited metabolic disorder characterized by deficiency of the lysosomal enzyme α-L-iduronidase (IDUA). Currently, the only therapies for MPS-I, enzyme replacement and hematopoietic stem cell transplantation, are generally ineffective for central nervous system manifestations.

OBJECTIVE

To test whether brain-targeted gene therapy with recombinant adeno-associated virus (rAAV5)-IDUA vectors in an MPS-I transgenic mouse model would reverse the pathological hallmarks.

METHODS

Gene therapy approaches were compared using intraventricular or endovascular delivery with a marker (rAAV5-green fluorescent protein) or therapeutic (rAAV5-IDUA) vector. To improve the efficiency of brain delivery, we tested different applications of hyperosmolar mannitol to disrupt the blood-brain barrier or ependymal-brain interface.

RESULTS

Intraventricular delivery of 1 × 10 viral particles of rAAV5-IDUA with systemic 5 g/kg mannitol co-administration resulted in IDUA expression throughout the brain, with global enzyme activity >200% of the baseline level in age-matched, wild-type mice. Endovascular delivery of 1 × 10 viral particles of rAAV5-IDUA to the carotid artery with 29.1% mannitol blood-brain barrier disruption resulted in mainly ipsilateral brain IDUA expression and ipsilateral brain enzyme activity 42% of that in wild-type mice. Quantitative assays for glycosaminoglycans showed a significant decrease in both hemispheres after intraventricular delivery and in the ipsilateral hemisphere after endovascular delivery compared with untreated MPS-I mice. Immunohistochemistry for ganglioside GM3, another disease marker, showed reversal of neuronal inclusions in areas with IDUA co-expression in both delivery methods.

CONCLUSION

Physiologically relevant biochemical correction is possible with neurosurgical or endovascular gene therapy approaches for MPS-I. Intraventricular or endovascular delivery of rAAV5-IDUA was effective in reversing brain pathology, but in the latter method, effects were limited to the ipsilateral hemisphere.

Supraspinal gene transfer by intrathecal adeno-associated virus serotype 5

We report the pattern of transgene expression across brain regions after intrathecal delivery of adeno-associated virus serotype 5 (AAV5). Labeling in hindbrain appeared to be primarily neuronal, and was detected in sensory nuclei of medulla, pontine nuclei, and all layers of cerebellar cortex. Expression in midbrain was minimal, and generally limited to isolated neurons and astrocytes in the cerebral peduncles. GFP immunoreactivity (-ir) in thalamus was most prominent in medial geniculate nucleus, and otherwise limited to posterior nuclei of the dorsal and lateral margins. Labeling was also observed in neurons and astrocytes of the hippocampal formation and amygdaloid complex. In the hippocampal formation, GFP-ir was found in neuronal cell bodies of the rostral ventral portion, but was largely restricted to fiber-like staining in the molecular layer of dentate gyrus and stratum lacunosum-moleculare of the rostral dorsal region. GFP-ir was seen in neurons and astroglia throughout caudal cortex, whereas in rostral regions of neocortex it was limited to isolated neurons and non-neuronal cells. Labeling was also present in olfactory bulb. These results demonstrate that intrathecal delivery of AAV5 vector leads to transgene expression in discrete CNS regions throughout the rostro-caudal extent of the neuraxis. A caudal-to-rostral gradient of decreasing GFP-ir was present in choroid plexus and Purkinje cells, suggesting that spread of virus through cerebrospinal fluid plays a role in the resulting transduction pattern. Other factors contributing to the observed expression pattern likely include variations in cell-surface receptors and inter-parenchymal space.