Axonal transport of AAV9 in nonhuman primate brain

Efficient retinal ganglion cells transduction by retro-orbital venous sinus injection of AAV-PHP.eB in mature mice

Gene therapy is one of the strategies that may reduce or reverse progressive neurodegeneration in retinal neurodegenerative diseases. However, efficiently delivering transgenes to retinal ganglion cells (RGCs) remains hard to achieve. In this study, we innovatively investigated transduction efficiency of adeno-associated virus (AAV)-PHP.eB in murine RGCs by retro-orbital venous sinus injection. Five doses of AAV-PHP.eB-EGFP were retro-orbitally injected in venous sinus in adult C57/BL6J mice. Two weeks after administration, RGCs transduction efficiency was quantified by retinal flat-mounts and frozen section co-labeling with RGCs marker Rbpms. In addition, safety of this method was evaluated by RGCs survival rate and retinal morphology. To conform efficacy of this new method, AAV-PHP.eB-CNTF was administrated into mature mice through single retro-orbital venous injection after optic nerve crush injury to evaluate axonal elongation. Results indicated that AAV- PHP.eB readily crossed the blood-retina barrier and was able to transduce more than 90% of RGCs when total dose of virus reached 5 × 1010 vector genomes (vg). Moreover, this technique did not affect RGCs survival rate and retinal morphology. Furthermore, retro-orbital venous delivery of AAV-PHP.eB-CNTF effectively transduced RGCs, robustly promoted axonal regeneration after optic nerve crush injury. Thus, novel AAV-PHP.eB retro-orbital injection provides a minimally invasive and efficient route for transgene delivery in treatment of retinal neurodegenerative diseases.

Gene replacement therapies for inherited disorders of neurotransmission: Current progress in succinic semialdehyde dehydrogenase deficiency

Neurodevelopment is a highly organized and complex process involving lasting and often irreversible changes in the central nervous system. Inherited disorders of neurotransmission (IDNT) are a group of genetic disorders where neurotransmission is primarily affected, resulting in abnormal brain development from early life, manifest as neurodevelopmental disorders and other chronic conditions. In principle, IDNT (particularly those of monogenic causes) are amenable to gene replacement therapy via precise genetic correction. However, practical challenges for gene replacement therapy remain major hurdles for its translation from bench to bedside. We discuss key considerations for the development of gene replacement therapies for IDNT. As an example, we describe our ongoing work on gene replacement therapy for succinic semialdehyde dehydrogenase deficiency, a GABA catabolic disorder.

Development and characterization of a non-human primate model of disseminated synucleinopathy

Introduction

The presence of a widespread cortical synucleinopathy is the main neuropathological hallmark underlying clinical entities such as Parkinson's disease with dementia (PDD) and dementia with Lewy bodies (DLB). There currently is a pressing need for the development of non-human primate (NHPs) models of PDD and DLB to further overcome existing limitations in drug discovery.

Methods

Here we took advantage of a retrogradely-spreading adeno-associated viral vector serotype 9 coding for the alpha-synuclein A53T mutated gene (AAV9-SynA53T) to induce a widespread synucleinopathy of cortical and subcortical territories innervating the putamen. Four weeks post-AAV deliveries animals were sacrificed and a comprehensive biodistribution study was conducted, comprising the quantification of neurons expressing alpha-synuclein, rostrocaudal distribution and their specific location.

Results

Intraputaminal deliveries of AAV9-SynA53T lead to a disseminated synucleinopathy throughout ipsi- and contralateral cerebral cortices, together with transduced neurons located in the ipsilateral caudal intralaminar nuclei and in the substantia nigra pars compacta (leading to thalamostriatal and nigrostriatal projections, respectively). Cortical afferent systems were found to be the main contributors to putaminal afferents (superior frontal and precentral gyri in particular).

Discussion

Obtained data extends current models of synucleinopathies in NHPs, providing a reproducible platform enabling the adequate implementation of end-stage preclinical screening of new drugs targeting alpha-synuclein.

Gene therapy for neurotransmitter-related disorders

Inborn errors of neurotransmitter (NT) metabolism are a group of rare, heterogenous diseases with predominant neurological features, such as movement disorders, autonomic dysfunction, and developmental delay. Clinical overlap with other disorders has led to delayed diagnosis and treatment, and some conditions are refractory to oral pharmacotherapies. Gene therapies have been developed and translated to clinics for paediatric inborn errors of metabolism, with 38 interventional clinical trials ongoing to date. Furthermore, efforts in restoring dopamine synthesis and neurotransmission through viral gene therapy have been developed for Parkinson's disease. Along with the recent European Medicines Agency (EMA) and Medicines and Healthcare Products Regulatory Agency (MHRA) approval of an AAV2 gene supplementation therapy for AADC deficiency, promising efficacy and safety profiles can be achieved in this group of diseases. In this review, we present preclinical and clinical advances to address NT-related diseases, and summarise potential challenges that require careful considerations for NT gene therapy studies.

Striatonigral distribution of a fluorescent reporter following intracerebral delivery of genome editors

Introduction: Targeted gene editing is proposed as a therapeutic approach for numerous disorders, including neurological diseases. As the brain is organized into neural networks, it is critical to understand how anatomically connected structures are affected by genome editing. For example, neurons in the substantia nigra pars compacta (SNpc) project to the striatum, and the striatum contains neurons that project to the substantia nigra pars reticulata (SNpr). Methods: Here, we report the effect of injecting genome editors into the striatum of Ai14 reporter mice, which have a LoxP-flanked stop cassette that prevents expression of the red fluorescent protein tdTomato. Two weeks following intracerebral delivery of either synthetic nanocapsules (NCs) containing CRISPR ribonucleoprotein targeting the tdTomato stop cassette or adeno-associated virus (AAV) vectors expressing Cre recombinase, the brains were collected, and the presence of tdTomato was assessed in both the striatum and SN. Results: TdTomato expression was observed at the injection site in both the NC- and AAV-treated groups and typically colocalized with the neuronal marker NeuN. In the SN, tdTomato-positive fibers were present in the pars reticulata, and SNpr area expressing tdTomato correlated with the size of the striatal genome edited area. Conclusion: These results demonstrate in vivo anterograde axonal transport of reporter gene protein products to the SNpr following neuronal genome editing in the striatum.

Biochemical Correction of GM2 Ganglioside Accumulation in AB-Variant GM2 Gangliosidosis

GM2 gangliosidosis is a group of genetic disorders that result in the accumulation of GM2 ganglioside (GM2) in brain cells, leading to progressive central nervous system (CNS) atrophy and premature death in patients. AB-variant GM2 gangliosidosis (ABGM2) arises from loss-of-function mutations in the GM2 activator protein (GM2AP), which is essential for the breakdown of GM2 in a key catabolic pathway required for CNS lipid homeostasis. In this study, we show that intrathecal delivery of self-complementary adeno-associated virus serotype-9 (scAAV9) harbouring a functional human GM2A transgene (scAAV9.hGM2A) can prevent GM2 accumulation in in GM2AP-deficient mice (Gm2a^-/- mice). Additionally, scAAV9.hGM2A efficiently distributes to all tested regions of the CNS within 14 weeks post-injection and remains detectable for the lifespan of these animals (up to 104 weeks). Remarkably, GM2AP expression from the transgene scales with increasing doses of scAAV9.hGM2A (0.5, 1.0 and 2.0 × 10¹¹ vector genomes (vg) per mouse), and this correlates with dose-dependent correction of GM2 accumulation in the brain. No severe adverse events were observed, and comorbidities in treated mice were comparable to those in disease-free cohorts. Lastly, all doses yielded corrective outcomes. These data indicate that scAAV9.hGM2A treatment is relatively non-toxic and tolerable, and biochemically corrects GM2 accumulation in the CNS-the main cause of morbidity and mortality in patients with ABGM2. Importantly, these results constitute proof-of-principle for treating ABGM2 with scAAV9.hGM2A by means of a single intrathecal administration and establish a foundation for future preclinical research.

Early postnatal administration of an AAV9 gene therapy is safe and efficacious in CLN3 disease

CLN3 disease, caused by biallelic mutations in the CLN3 gene, is a rare pediatric neurodegenerative disease that has no cure or disease modifying treatment. The development of effective treatments has been hindered by a lack of etiological knowledge, but gene replacement has emerged as a promising therapeutic platform for such disorders. Here, we utilize a mouse model of CLN3 disease to test the safety and efficacy of a cerebrospinal fluid-delivered AAV9 gene therapy with a study design optimized for translatability. In this model, postnatal day one administration of the gene therapy virus resulted in robust expression of human CLN3 throughout the CNS over the 24-month duration of the study. A range of histopathological and behavioral parameters were assayed, with the therapy consistently and persistently rescuing a number of hallmarks of disease while being safe and well-tolerated. Together, the results show great promise for translation of the therapy into the clinic, prompting the launch of a first-in-human clinical trial (NCT03770572).

AAV vectors applied to the treatment of CNS disorders: Clinical status and challenges

In recent years, adeno-associated virus (AAV) has become the most important vector for central nervous system (CNS) gene therapy. AAV has already shown promising results in the clinic, for several CNS diseases that cannot be treated with drugs, including neurodegenerative diseases, neuromuscular diseases, and lysosomal storage disorders. Currently, three of the four commercially available AAV-based drugs focus on neurological disorders, including Upstaza for aromatic l-amino acid decarboxylase deficiency, Luxturna for hereditary retinal dystrophy, and Zolgensma for spinal muscular atrophy. All these studies have provided paradigms for AAV-based therapeutic intervention platforms. AAV gene therapy, with its dual promise of targeting disease etiology and enabling 'long-term correction' of disease processes, has the advantages of immune privilege, high delivery efficiency, tissue specificity, and cell tropism in the CNS. Although AAV-based gene therapy has been shown to be effective in most CNS clinical trials, limitations have been observed in its clinical applications, which are often associated with side effects. In this review, we summarized the therapeutic progress, challenges, limitations, and solutions for AAV-based gene therapy in 14 types of CNS diseases. We focused on viral vector technologies, delivery routes, immunosuppression, and other relevant clinical factors. We also attempted to integrate several hurdles faced in clinical and preclinical studies with their solutions, to seek the best path forward for the application of AAV-based gene therapy in the context of CNS diseases. We hope that these thoughtful recommendations will contribute to the efficient translation of preclinical studies and wide application of clinical trials.

Supervised Learning in Neural Networks: Feedback-Network-Free Implementation and Biological Plausibility

The well-known backpropagation learning algorithm is probably the most popular learning algorithm in artificial neural networks. It has been widely used in various applications of deep learning. The backpropagation algorithm requires a separate feedback network to back propagate errors. This feedback network must have the same topology and connection strengths (weights) as the feed-forward network. In this article, we propose a new learning algorithm that is mathematically equivalent to the backpropagation algorithm but does not require a feedback network. The elimination of the feedback network makes the implementation of the new algorithm much simpler. The elimination of the feedback network also significantly increases biological plausibility for biological neural networks to learn using the new algorithm by means of some retrograde regulatory mechanisms that may exist in neurons. This new algorithm also eliminates the need for two-phase adaptation (feed-forward phase and feedback phase). Hence, neurons can adapt asynchronously and concurrently in a way analogous to that of biological neurons.

Routes of administration for adeno-associated viruses carrying gene therapies for brain diseases

Gene therapy is a powerful tool to treat various central nervous system (CNS) diseases ranging from monogenetic diseases to neurodegenerative disorders. Adeno-associated viruses (AAVs) have been widely used as the delivery vehicles for CNS gene therapies due to their safety, CNS tropism, and long-term therapeutic effect. However, several factors, including their ability to cross the blood-brain barrier, the efficiency of transduction, their immunotoxicity, loading capacity, the choice of serotype, and peripheral off-target effects should be carefully considered when designing an optimal AAV delivery strategy for a specific disease. In addition, distinct routes of administration may affect the efficiency and safety of AAV-delivered gene therapies. In this review, we summarize different administration routes of gene therapies delivered by AAVs to the brain in mice and rats. Updated knowledge regarding AAV-delivered gene therapies may facilitate the selection from various administration routes for specific disease models in future research.

Rhesus macaque versus rat divergence in the corticospinal projectome

We used viral intersectional tools to map the entire projectome of corticospinal neurons associated with fine distal forelimb control in Fischer 344 rats and rhesus macaques. In rats, we found an extraordinarily diverse set of collateral projections from corticospinal neurons to 23 different brain and spinal regions. Remarkably, the vast weighting of this "motor" projection was to sensory systems in both the brain and spinal cord, confirmed by optogenetic and transsynaptic viral intersectional tools. In contrast, rhesus macaques exhibited far heavier and narrower weighting of corticospinal outputs toward spinal and brainstem motor systems. Thus, corticospinal systems in macaques primarily constitute a final output system for fine motor control, whereas this projection in rats exerts a multi-modal integrative role that accesses far broader CNS regions. Unique structural-functional correlations can be achieved by mapping and quantifying a single neuronal system's total axonal output and its relative weighting across CNS targets.

Organoid transduction using recombinant adeno-associated viral vectors: Challenges and opportunities

Cellular 3D structures, for example, organoids, are an excellent model for studying and developing treatments for various diseases, including hereditary ones. Therefore, they are increasingly being used in biomedical research. From the point of view of safety and efficacy, recombinant adeno-associated viral (rAAV) vectors are currently most in demand for the delivery of various transgenes for gene replacement therapy or other applications. The delivery of transgenes using rAAV vectors to various types of organoids is an urgent task, however, it is associated with a number of problems that are discussed in this review. Cellular heterogeneity and specifics of cultivation of 3D structures determine the complexity of rAAV delivery and are sometimes associated with low transduction efficiency. This review surveys the main ways to solve emerging problems and increase the efficiency of transgene delivery using rAAVs to organoids. A clear understanding of the stage of development of the organoid, its cellular composition and the presence of surface receptors will allow obtaining high levels of organoid transduction with existing rAAV vectors.

Improving Gene Therapy for Angelman Syndrome with Secreted Human UBE3A

The rare genetic neurodevelopmental disease Angelman syndrome (AS) is caused by the loss of function of UBE3A, a ubiquitin ligase. The disease results in a lifetime of severe symptoms, including intellectual disability and motor impairments for which there are no effective treatments. One avenue of treatment for AS is the use of gene therapy to reintroduce a functional copy of the UBE3A gene. Our group had previously shown that recombinant adeno-associated virus (rAAV) expressing mouse Ube3a could rescue deficits in a mouse model of AS. Here, we expand on this work and show that this approach could be successfully replicated in a second AS model using the human UBE3A gene. Furthermore, we address the challenge of limited vector distribution in the brain by developing a novel modified form of UBE3A. This modified protein, termed STUB, was designed with a secretion signal and a cell-penetrating peptide. This allowed transduced cells to act as factories for the production of UBE3A protein that could be taken up by neighboring non-transduced cells, thus increasing the number of neurons receiving the therapeutic protein. Combining this construct with intracerebroventricular injections to maximize rAAV distribution within the brain, we demonstrate that this novel approach improves the recovery of behavioral and electrophysiological deficits in the AS rat model. More importantly, a comparison of rAAV-STUB to a rAAV expressing the normal human UBE3A gene showed that STUB was a more effective therapeutic. These data suggest that rAAV-STUB is a new potential approach for the treatment of AS.

Adeno-Associated Viral Vectors as Versatile Tools for Neurological Disorders: Focus on Delivery Routes and Therapeutic Perspectives

It is without doubt that the gene therapy field is currently in the spotlight for the development of new therapeutics targeting unmet medical needs. Thus, considering the gene therapy scenario, neurological diseases in general and neurodegenerative disorders in particular are emerging as the most appealing choices for new therapeutic arrivals intended to slow down, stop, or even revert the natural progressive course that characterizes most of these devastating neurodegenerative processes. Since an extensive coverage of all available literature is not feasible in practical terms, here emphasis was made in providing some advice to beginners in the field with a narrow focus on elucidating the best delivery route available for fulfilling any given AAV-based therapeutic approach. Furthermore, it is worth nothing that the number of ongoing clinical trials is increasing at a breath-taking speed. Accordingly, a landscape view of preclinical and clinical initiatives is also provided here in an attempt to best illustrate what is ongoing in this quickly expanding field.

Characterization of the GfaABC1D Promoter to Selectively Target Astrocytes in the Rhesus Macaque Brain

BACKGROUND

The study of astrocytic functions in non-human primates (NHPs) has been hampered by the lack of genetic tools to selectively target astrocytes. Viral vectors with selective and efficient transduction of astrocytes could be a potent tool to express marker proteins, modulators, or sensors in NHP astrocytes, but the availability of thoroughly characterized astrocytic selective promoter sequences to use in these species remains extremely limited.

NEW METHOD

We describe the specificity and efficiency of an astrocyte-specific promoter, GfaABC1D in the brain of the rhesus macaque, with emphasis in basal ganglia regions. AAV5-pZac2.1-GfaABC1D-tdTomato was locally injected into the globus pallidus external segment (GPe) and putamen. The extent, efficiency, and specificity of transduction was analyzed with immunohistochemistry at the light and electron microscope levels.

RESULTS

The GfaABC1D promoter directed the expression of tdTomato in an astrocyte-specific manner in directly or indirectly targeted regions (including both segments of the globus pallidus, putamen, subthalamic nucleus and cortex).

COMPARISON WITH EXISTING METHODS

Due to its small size, the GfaABC1D promoter is advantageous over other previously used glial fibrillary acidic protein-based promoter sequences, facilitating its use to drive expression of various transgenes in adeno-associated viruses (AAV) or other viral vectors.

CONCLUSION

GfaABC1D is an efficient promoter that selectively targets astrocytes in the monkey basal ganglia and expands the viral vector toolbox to study astrocytic functions in non-human primates.

Feasibility of Targeted Delivery of AAV5-GFP into the Cerebellum of Nonhuman Primates Following a Single Convection-Enhanced Delivery Infusion

In this study, we built upon our previous work to demonstrate the distribution and transport of AAV5-green fluorescent protein (GFP) following a single convection-enhanced delivery infusion into the nonhuman primate cerebellum, with no untoward side effects noted. Dosing under magnetic resonance imaging guidance revealed a sixfold larger volume of distribution compared with the volume of infusion, with no evidence of reflux underscoring the convective properties of the cerebellum and step design of the cannula. Postmortem tissue analysis, 4 weeks post-adeno-associated viral (AAV) delivery, revealed the robust presence of the transgene in situ, with GFP detection in secondary regions not directly targeted by the infusion, denoting distal transport of the vector. Irrespective of tropism, a twofold larger area of transgene expression was found and was corroborated against the presence of contrast on T1-weighted images. Different levels of transduction were detected between animals, which were negatively correlated with the level of antibody titer against the GFP construct, whereby the higher the antibody titer, the lower the level of transgene expression. These findings support the use of the posterior fossa as a potential target site for direct delivery of gene-based therapeutics for cerebellar diseases.

Scientific and Regulatory Policy Committee Points to Consider: Nonclinical Research and Development of In Vivo Gene Therapy Products, Emphasizing Adeno-Associated Virus Vectors

Sequencing of the human genome and numerous advances in molecular techniques have launched the era of genetic medicine. Increasingly precise technologies for genetic modification, manufacturing, and administration of pharmaceutical-grade biologics have proved the viability of in vivo gene therapy (GTx) as a therapeutic modality as shown in several thousand clinical trials and recent approval of several GTx products for treating rare diseases and cancers. In recognition of the rapidly advancing knowledge in this field, the regulatory landscape has evolved considerably to maintain appropriate monitoring of safety concerns associated with this modality. Nonetheless, GTx safety assessment remains complex and is designed on a case-by-case basis that is determined by the disease indication and product attributes. This article describes our current understanding of fundamental biological principles and possible procedures (emphasizing those related to toxicology and toxicologic pathology) needed to support research and development of in vivo GTx products. This article is not intended to provide comprehensive guidance on all GTx modalities but instead provides an overview relevant to in vivo GTx generally by utilizing recombinant adeno-associated virus-based GTx-the most common in vivo GTx platform-to exemplify the main points to be considered in nonclinical research and development of GTx products.

A direct interareal feedback-to-feedforward circuit in primate visual cortex

The mammalian sensory neocortex consists of hierarchically organized areas reciprocally connected via feedforward (FF) and feedback (FB) circuits. Several theories of hierarchical computation ascribe the bulk of the computational work of the cortex to looped FF-FB circuits between pairs of cortical areas. However, whether such corticocortical loops exist remains unclear. In higher mammals, individual FF-projection neurons send afferents almost exclusively to a single higher-level area. However, it is unclear whether FB-projection neurons show similar area-specificity, and whether they influence FF-projection neurons directly or indirectly. Using viral-mediated monosynaptic circuit tracing in macaque primary visual cortex (V1), we show that V1 neurons sending FF projections to area V2 receive monosynaptic FB inputs from V2, but not other V1-projecting areas. We also find monosynaptic FB-to-FB neuron contacts as a second motif of FB connectivity. Our results support the existence of FF-FB loops in primate cortex, and suggest that FB can rapidly and selectively influence the activity of incoming FF signals.

Adeno-Associated Viral Vectors as Versatile Tools for Parkinson's Research, Both for Disease Modeling Purposes and for Therapeutic Uses

It is without any doubt that precision medicine therapeutic strategies targeting neurodegenerative disorders are currently witnessing the spectacular rise of newly designed approaches based on the use of viral vectors as Trojan horses for the controlled release of a given genetic payload. Among the different types of viral vectors, adeno-associated viruses (AAVs) rank as the ones most commonly used for the purposes of either disease modeling or for therapeutic strategies. Here, we reviewed the current literature dealing with the use of AAVs within the field of Parkinson’s disease with the aim to provide neuroscientists with the advice and background required when facing a choice on which AAV might be best suited for addressing a given experimental challenge. Accordingly, here we will be summarizing some insights on different AAV serotypes, and which would be the most appropriate AAV delivery route. Next, the use of AAVs for modeling synucleinopathies is highlighted, providing potential readers with a landscape view of ongoing pre-clinical and clinical initiatives pushing forward AAV-based therapeutic approaches for Parkinson’s disease and related synucleinopathies.

Retrograde Axonal Transport Property of Adeno-Associated Virus and Its Possible Application in Future

Gene therapy has become a treatment method for many diseases. Adeno-associated virus (AAV) is one of the most common virus vectors, is also widely used in the gene therapy field. During the past 2 decades, the retrograde axonal transportability of AAV has been discovered and utilized. Many studies have worked on the retrograde axonal transportability of AAV, and more and more people are interested in this field. This review described the current application, influence factors, and mechanism of retrograde axonal transportability of AAV and predicted its potential use in disease treatment in near future.

Persistent repression of tau in the brain using engineered zinc finger protein transcription factors

Neuronal tau reduction confers resilience against β-amyloid and tau-related neurotoxicity in vitro and in vivo. Here, we introduce a novel translational approach to lower expression of the tau gene MAPT at the transcriptional level using gene-silencing zinc finger protein transcription factors (ZFP-TFs). Following a single administration of adeno-associated virus (AAV), either locally into the hippocampus or intravenously to enable whole-brain transduction, we selectively reduced tau messenger RNA and protein by 50 to 80% out to 11 months, the longest time point studied. Sustained tau lowering was achieved without detectable off-target effects, overt histopathological changes, or molecular alterations. Tau reduction with AAV ZFP-TFs was able to rescue neuronal damage around amyloid plaques in a mouse model of Alzheimer's disease (APP/PS1 line). The highly specific, durable, and controlled knockdown of endogenous tau makes AAV-delivered ZFP-TFs a promising approach for the treatment of tau-related human brain diseases.

The Challenge of Gene Therapy for Neurological Diseases: Strategies and Tools to Achieve Efficient Delivery to the Central Nervous System

For more than 10 years, gene therapy for neurological diseases has experienced intensive research growth and more recently therapeutic interventions for multiple indications. Beneficial results in several phase 1/2 clinical studies, together with improved vector technology have advanced gene therapy for the central nervous system (CNS) in a new era of development. Although most initial strategies have focused on orphan genetic diseases, such as lysosomal storage diseases, more complex and widespread conditions like Alzheimer's disease, Parkinson's disease, epilepsy, or chronic pain are increasingly targeted for gene therapy. Increasing numbers of applications and patients to be treated will require improvement and simplification of gene therapy protocols to make them accessible to the largest number of affected people. Although vectors and manufacturing are a major field of academic research and industrial development, there is a growing need to improve, standardize, and simplify delivery methods. Delivery is the major issue for CNS therapies in general, and particularly for gene therapy. The blood-brain barrier restricts the passage of vectors; strategies to bypass this obstacle are a central focus of research. In this study, we present the different ways that can be used to deliver gene therapy products to the CNS. We focus on results obtained in large animals that have allowed the transfer of protocols to human patients and have resulted in the generation of clinical data. We discuss the different routes of administration, their advantages, and their limitations. We describe techniques, equipment, and protocols and how they should be selected for safe delivery and improved efficiency for the next generation of gene therapy trials for CNS diseases.

Intracranial delivery of AAV9 gene therapy partially prevents retinal degeneration and visual deficits in CLN6-Batten disease mice

Batten disease is a family of rare, fatal, neuropediatric diseases presenting with memory/learning decline, blindness, and loss of motor function. Recently, we reported the use of an AAV9-mediated gene therapy that prevents disease progression in a mouse model of CLN6-Batten disease (Cln6 ^nclf ), restoring lifespans in treated animals. Despite the success of our viral-mediated gene therapy, the dosing strategy was optimized for delivery to the brain parenchyma and may limit the therapeutic potential to other disease-relevant tissues, such as the eye. Here, we examine whether cerebrospinal fluid (CSF) delivery of scAAV9.CB.CLN6 is sufficient to ameliorate visual deficits in Cln6 ^nclf mice. We show that intracerebroventricular (i.c.v.) delivery of scAAV9.CB.CLN6 completely prevents hallmark Batten disease pathology in the visual processing centers of the brain, preserving neurons of the superior colliculus, thalamus, and cerebral cortex. Importantly, i.c.v.-delivered scAAV9.CB.CLN6 also expresses in many cells throughout the central retina, preserving many photoreceptors typically lost in Cln6 ^nclf mice. Lastly, scAAV9.CB.CLN6 treatment partially preserved visual acuity in Cln6 ^nclf mice as measured by optokinetic response. Taken together, we report the first instance of CSF-delivered viral gene reaching and rescuing pathology in both the brain parenchyma and retinal neurons, thereby partially slowing visual deterioration.

AAV9 Gene Therapy Increases Lifespan and Treats Pathological and Behavioral Abnormalities in a Mouse Model of CLN8-Batten Disease

CLN8 disease is a rare form of neuronal ceroid lipofuscinosis caused by biallelic mutations in the CLN8 gene, which encodes a transmembrane endoplasmic reticulum protein involved in trafficking of lysosomal enzymes. CLN8 disease patients present with myoclonus, tonic-clonic seizures, and progressive declines in cognitive and motor function, with many cases resulting in premature death early in life. There are currently no treatments that can cure the disease or substantially slow disease progression. Using a mouse model of CLN8 disease, we tested the safety and efficacy of an intracerebroventricularly (i.c.v.) delivered self-complementary adeno-associated virus serotype 9 (scAAV9) gene therapy vector driving expression of human CLN8. A single neonatal injection was safe and well tolerated, resulting in robust transgene expression throughout the CNS from 4 to 24 months, reducing histopathological and behavioral hallmarks of the disease and restoring lifespan from 10 months in untreated animals to beyond 24 months of age in treated animals. While it is unclear whether some of these behavioral improvements relate to preserved visual function, improvements in learning/memory, or other central or peripheral benefits, these results demonstrate, by far, the most successful degree of rescue reported in an animal model of CLN8 disease, and they support further development of gene therapy for this disorder.

Gene therapy for neurodegenerative disorders: advances, insights and prospects

Gene therapy is rapidly emerging as a powerful therapeutic strategy for a wide range of neurodegenerative disorders, including Alzheimer's disease (AD), Parkinson's disease (PD) and Huntington's disease (HD). Some early clinical trials have failed to achieve satisfactory therapeutic effects. Efforts to enhance effectiveness are now concentrating on three major fields: identification of new vectors, novel therapeutic targets, and reliable of delivery routes for transgenes. These approaches are being assessed closely in preclinical and clinical trials, which may ultimately provide powerful treatments for patients. Here, we discuss advances and challenges of gene therapy for neurodegenerative disorders, highlighting promising technologies, targets, and future prospects.

Cerebral Organoids: A Human Model for AAV Capsid Selection and Therapeutic Transgene Efficacy in the Brain

The development of gene therapies for central nervous system disorders is challenging because it is difficult to translate preclinical data from current in vitro and in vivo models to the clinic. Therefore, we developed induced pluripotent stem cell (iPSC)-derived cerebral organoids as a model for recombinant adeno-associated virus (rAAV) capsid selection and for testing efficacy of AAV-based gene therapy in a human context. Cerebral organoids are physiological 3D structures that better recapitulate the human brain compared with 2D cell lines. To validate the model, we compared the transduction efficiency and distribution of two commonly used AAV serotypes (rAAV5 and rAAV9). In cerebral organoids, transduction with rAAV5 led to higher levels of vector DNA, transgenic mRNA, and protein expression as compared with rAAV9. The superior transduction of rAAV5 was replicated in iPSC-derived neuronal cells. Furthermore, rAAV5-mediated delivery of a human sequence-specific engineered microRNA to cerebral organoids led to a lower expression of its target ataxin-3. Our studies provide a new tool for selecting and deselecting AAV serotypes, and for demonstrating therapeutic efficacy of transgenes in a human context. Implementing cerebral organoids during gene therapy development could reduce the usage of animal models and improve translation to the clinic.

Intra-CSF AAV9 and AAVrh10 Administration in Nonhuman Primates: Promising Routes and Vectors for Which Neurological Diseases?

The identification of the most efficient method for whole central nervous system targeting that is translatable to humans and the safest route of adeno-associated virus (AAV) administration is a major concern for future applications in clinics. Additionally, as many AAV serotypes were identified for gene introduction into the brain and the spinal cord, another key to human gene-therapy success is to determine the most efficient serotype. In this study, we compared lumbar intrathecal administration through catheter implantation and intracerebroventricular administration in the cynomolgus macaque. We also evaluated and compared two AAV serotypes that are currently used in clinical trials: AAV9 and AAVrh10. We demonstrated that AAV9 lumbar intrathecal delivery using a catheter achieved consistent transgene expression in the motor neurons of the spinal cord and in the neurons/glial cells of several brain regions, whereas AAV9 intracerebroventricular delivery led to a consistent transgene expression in the brain. In contrast, AAVrh10 lumbar intrathecal delivery led to rare motor neuron targeting. Finally, we found that AAV9 efficiently targets respiratory and skeletal muscles after injection into the cerebrospinal fluid (CSF), which represents an outstanding new property that can be useful for the treatment of diseases affecting both the central nervous system and muscle.

Gene Therapy Corrects Brain and Behavioral Pathologies in CLN6-Batten Disease

CLN6-Batten disease, a form of neuronal ceroid lipofuscinosis is a rare lysosomal storage disorder presenting with gradual declines in motor, visual, and cognitive abilities and early death by 12–15 years of age. We developed a self-complementary adeno-associated virus serotype 9 (scAAV9) vector expressing the human CLN6 gene under the control of a chicken β-actin (CB) hybrid promoter. Intrathecal delivery of scAAV9.CB.hCLN6 into the cerebrospinal fluid (CSF) of the lumbar spinal cord of 4-year-old non-human primates was safe, well tolerated, and led to efficient targeting throughout the brain and spinal cord. A single intracerebroventricular (i.c.v.) injection at post-natal day 1 in Cln6 mutant mice delivered scAAV9.CB.CLN6 directly into the CSF, and it prevented or drastically reduced all of the pathological hallmarks of Batten disease. Moreover, there were significant improvements in motor performance, learning and memory deficits, and survival in treated Cln6 mutant mice, extending survival from 15 months of age (untreated) to beyond 21 months of age (treated). Additionally, many parameters were similar to wild-type counterparts throughout the lifespan of the treated mice.

Emerging Technologies for Delivery of Biotherapeutics and Gene Therapy Across the Blood-Brain Barrier

Antibody, immuno- and gene therapies developed for neurological indications face a delivery challenge posed by various anatomical and physiological barriers within the central nervous system (CNS); most notably, the blood–brain barrier (BBB). Emerging delivery technologies for biotherapeutics have focused on trans-cellular pathways across the BBB utilizing receptor-mediated transcytosis (RMT). ‘Traditionally’ targeted RMT receptors, transferrin receptor (TfR) and insulin receptor (IR), are ubiquitously expressed and pose numerous translational challenges during development, including species differences and safety risks. Recent advances in antibody engineering technologies and discoveries of RMT targets and BBB-crossing antibodies that are more BBB-selective have combined to create a new preclinical pipeline of BBB-crossing biotherapeutics with improved efficacy and safety. Novel BBB-selective RMT targets and carrier antibodies have exposed additional opportunities for re-targeting gene delivery vectors or nanocarriers for more efficient brain delivery. Emergence and refinement of core technologies of genetic engineering and editing as well as biomanufacturing of viral vectors and cell-derived products have de-risked the path to the development of systemic gene therapy approaches for the CNS. In particular, brain-tropic viral vectors and extracellular vesicles have recently expanded the repertoire of brain delivery strategies for biotherapeutics. Whereas protein biotherapeutics and bispecific antibodies enabled for BBB transcytosis are rapidly heading towards clinical trials, systemic gene therapy approaches for CNS will likely remain in research phase for the foreseeable future. The promise and limitations of these emerging cross-BBB delivery technologies are further discussed in this article.

Overcoming the Blood–Brain Barrier. Challenges and Tricks for CNS Drug Delivery

Treatment of certain central nervous system disorders, including different types of cerebral malignancies, is limited by traditional oral or systemic administrations of therapeutic drugs due to possible serious side effects and/or lack of the brain penetration and, therefore, the efficacy of the drugs is diminished. During the last decade, several new technologies were developed to overcome barrier properties of cerebral capillaries. This review gives a short overview of the structural elements and anatomical features of the blood–brain barrier. The various in vitro (static and dynamic), in vivo (microdialysis), and in situ (brain perfusion) blood–brain barrier models are also presented. The drug formulations and administration options to deliver molecules effectively to the central nervous system (CNS) are presented. Nanocarriers, nanoparticles (lipid, polymeric, magnetic, gold, and carbon based nanoparticles, dendrimers, etc.), viral and peptid vectors and shuttles, sonoporation and microbubbles are briefly shown. The modulation of receptors and efflux transporters in the cell membrane can also be an effective approach to enhance brain exposure to therapeutic compounds. Intranasal administration is a noninvasive delivery route to bypass the blood–brain barrier, while direct brain administration is an invasive mode to target the brain region with therapeutic drug concentrations locally. Nowadays, both technological and mechanistic tools are available to assist in overcoming the blood–brain barrier. With these techniques more effective and even safer drugs can be developed for the treatment of devastating brain disorders.

Gene Therapy for Neurodegenerative Diseases

Gene therapy has the potential to provide therapeutic benefit to millions of people with neurodegenerative diseases through several means, including direct correction of pathogenic mechanisms, neuroprotection, neurorestoration, and symptom control. Therapeutic efficacy is therefore dependent on knowledge of the disease pathogenesis and the required temporal and spatial specificity of gene expression. An additional critical challenge is achieving the most complete transduction of the target structure while avoiding leakage into neighboring regions or perivascular spaces. The gene therapy field has recently entered a new technological era, in which interventional MRI-guided convection-enhanced delivery (iMRI-CED) is the gold standard for verifying accurate vector delivery in real time. The availability of this advanced neurosurgical technique may accelerate the translation of the promising preclinical therapeutics under development for neurodegenerative disorders, including Parkinson's, Huntington's, and Alzheimer's diseases.

Adeno-associated virus-mediated delivery of CRISPR-Cas9 for genome editing in the central nervous system

The emergence of CRISPR-Cas9 as a powerful genome editing tool has led to several studies exploring its potential to treat neurological disorders. Cas9 and its sgRNA can be readily engineered to target any gene and can be multiplexed to target several genes at once. Furthermore, the use of adeno-associated virus (AAV) to deliver with Cas9 and its sgRNA is a promising therapeutic combination with strong potential to reach the clinic. Here we discuss how Cas9 editing has been utilized for gene insertion, knockout, and deletion in vivo for applications in the central nervous system (CNS). Furthermore, we highlight major challenges that remain for AAV-Cas9-sgRNA clinical translation.

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

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

Engineered viral vectors for functional interrogation, deconvolution, and manipulation of neural circuits

Optimization of traditional replication-competent viral tracers has granted access to immediate synaptic partners of target neuronal populations, enabling the dissection of complex brain circuits into functional neural pathways. The excessive virulence of most conventional tracers, however, impedes their utility in revealing and genetically perturbing cellular function on long time scales. As a promising alternative, the natural capacity of adeno-associated viral (AAV) vectors to safely mediate persistent and robust gene expression has stimulated strong interest in adapting them for sparse neuronal labeling and physiological studies. Furthermore, increasingly refined engineering strategies have yielded novel AAV variants with enhanced target specificity, transduction, and retrograde trafficking in the CNS. These potent vectors offer new opportunities for characterizing the identity and connectivity of single neurons within immense networks and modulating their activity via robust delivery of functional genetic tools.

MR-guided parenchymal delivery of adeno-associated viral vector serotype 5 in non-human primate brain

The present study was designed to characterize transduction of non-human primate brain and spinal cord with AAV5 viral vector after parenchymal delivery. AAV5-CAG-GFP (1 × 10¹³ vector genomes per milliliter (vg ml⁻¹)) was bilaterally infused either into putamen, thalamus or with the combination left putamen and right thalamus. Robust expression of GFP was seen throughout infusion sites and also in other distal nuclei. Interestingly, thalamic infusion of AAV5 resulted in the transduction of the entire corticospinal axis, indicating transport of AAV5 over long distances. Regardless of site of injection, AAV5 transduced both neurons and astrocytes equally. Our data demonstrate that AAV5 is a very powerful vector for the central nervous system and has potential for treatment of a wide range of neurological pathologies with cortical, subcortical and/or spinal cord affection.

AAV Vector-Mediated Gene Delivery to Substantia Nigra Dopamine Neurons: Implications for Gene Therapy and Disease Models

Gene delivery using adeno-associated virus (AAV) vectors is a widely used method to transduce neurons in the brain, especially due to its safety, efficacy, and long-lasting expression. In addition, by varying AAV serotype, promotor, and titer, it is possible to affect the cell specificity of expression or the expression levels of the protein of interest. Dopamine neurons in the substantia nigra projecting to the striatum, comprising the nigrostriatal pathway, are involved in movement control and degenerate in Parkinson’s disease. AAV-based gene targeting to the projection area of these neurons in the striatum has been studied extensively to induce the production of neurotrophic factors for disease-modifying therapies for Parkinson’s disease. Much less emphasis has been put on AAV-based gene therapy targeting dopamine neurons in substantia nigra. We will review the literature related to targeting striatum and/or substantia nigra dopamine neurons using AAVs in order to express neuroprotective and neurorestorative molecules, as well as produce animal disease models of Parkinson’s disease. We discuss difficulties in targeting substantia nigra dopamine neurons and their vulnerability to stress in general. Therefore, choosing a proper control for experimental work is not trivial. Since the axons along the nigrostriatal tract are the first to degenerate in Parkinson’s disease, the location to deliver the therapy must be carefully considered. We also review studies using AAV-α-synuclein (α-syn) to target substantia nigra dopamine neurons to produce an α-syn overexpression disease model in rats. Though these studies are able to produce mild dopamine system degeneration in the striatum and substantia nigra and some behavioural effects, there are studies pointing to the toxicity of AAV-carrying green fluorescent protein (GFP), which is often used as a control. Therefore, we discuss the potential difficulties in overexpressing proteins in general in the substantia nigra.

Survival of corticostriatal neurons by Rho/Rho-kinase signaling pathway

Developing cortical neurons undergo a number of sequential developmental events including neuronal survival/apoptosis, and the molecular mechanism underlying each characteristic process has been studied in detail. However, the survival pathway of cortical neurons at mature stages remains largely uninvestigated. We herein focused on mature corticostriatal neurons because of their important roles in various higher brain functions and the spectrum of neurological and neuropsychiatric disorders. The small GTPase Rho is known to control diverse and essential cellular functions through some effector molecules, including Rho-kinase, during neural development. In the present study, we investigated the role of Rho signaling through Rho-kinase in the survival of corticostriatal neurons. We performed the conditional expression of Clostridium botulinum C3 ADP-ribosyltransferase (C3 transferase) or dominant-negative form for Rho-kinase (Rho-K DN), a well-known inhibitor of Rho or Rho-kinase, respectively, in corticostriatal neurons using a dual viral vector approach combining a neuron-specific retrograde gene transfer lentiviral vector and an adeno-associated virus vector. C3 transferase markedly decreased the number of corticostriatal neurons, which was attributed to caspase-3-dependent enhanced apoptosis. In addition, Rho-K DN produced phenotypic defects similar to those caused by C3 transferase. These results indicate that the Rho/Rho-kinase signaling pathway plays a crucial role in the survival of corticostriatal neurons.