Widespread transduction of astrocytes and neurons in the mouse central nervous system after systemic delivery of a self-complementary AAV-PHP.B vector

The astrocyte α1A-adrenoreceptor is a key component of the neuromodulatory system in mouse visual cortex

Noradrenaline (norepinephrine) is known to modulate many physiological functions and behaviors. In this study, we tested to what extent astrocytes, a type of glial cell, participate in noradrenergic signaling in mouse primary visual cortex (V1). Astrocytes are essential partners of neurons in the central nervous system. They are central to brain homeostasis, but also dynamically regulate neuronal activity, notably by relaying and regulating neuromodulator signaling. Indeed, astrocytes express receptors for multiple neuromodulators, including noradrenaline, but the extent to which astrocytes are involved in noradrenergic signaling remains unclear. To test whether astrocytes are involved in noradrenergic neuromodulation in mice, we employed both short hairpin RNA mediated knockdown as well as pharmacological manipulation of the major noradrenaline receptor in astrocytes, the α1A-adrenoreceptor. Using acute brain slices, we found that the astrocytic α1A-adrenoreceptor subtype contributes to the generation of large intracellular Ca2+ signals in visual cortex astrocytes, which are generally thought to underlie astrocyte function. To test if reduced α1A-adrenoreceptor signaling in astrocytes affected the function of neuronal circuits in V1, we used both patch-clamp and field potential recordings. These revealed that noradrenergic signaling through the astrocyte α1A-adrenoreceptor is important to not only modulate synaptic activity but also to regulate plasticity in V1, through the potentiation of synaptic responses in circuits involved in visual information processing.

A minimally invasive endovascular approach to the cerebellopontine angle cistern enables broad CNS biodistribution of scAAV9-CB-GFP

Neurological disorders pose a challenge for targeted therapy due to restricted access of therapeutic agents to the central nervous system (CNS). Current methods are limited by procedure-related risks, invasiveness, and insufficient CNS biodistribution. A novel percutaneous transvenous technology, currently in clinical trials for communicating hydrocephalus, offers a minimally invasive approach by providing endovascular access to the cerebrospinal fluid-filled cerebellopontine angle (CPA) cistern. We hypothesized that drug delivery to the CPA cistern could yield widespread CNS distribution. Using an ovine model, we compared the biodistribution of scAAV9-CB-GFP following CPA cistern infusion with previously reported cisterna magna (CM) administration. Targeting both the CPA cistern and CM in sheep, we employed a lumbar spine-inserted microcatheter under fluoroscopy. CPA delivery of AAV9 demonstrated biodistribution and transduction in the cerebral cortices, striatum, thalamus, midbrain, cerebellum, and spinal cord, with minor liver distribution comparable to CM. The favorable safety profile in humans with hydrocephalus suggests that percutaneous endovascular injection into the CPA could offer a clinically safer and minimally invasive delivery system for CNS gene and cell-based therapies.

Use of adeno-associated viruses for transgenic modulation of microglia structure and function: A review of technical considerations and challenges

Microglia play a central role in the etiology of many neuropathologies. Transgenic tools are a powerful experiment approach to gain reliable and specific control over microglia function. Adeno-associated virus (AAVs) vectors are already an indispensable tool in neuroscience research. Despite ubiquitous use of AAVs and substantial interest in the role of microglia in the study of central nervous system (CNS) function and disease, transduction of microglia using AAVs is seldom reported. This review explores the challenges and advancements made in using AAVs for expressing transgenes in microglia. First, we will examine the functional anatomy of the AAV capsid, which will serve as a basis for subsequent discussions of studies exploring the relationship between capsid mutations and microglia transduction efficacy. After outlining the functional anatomy of AAVs, we will consider the experimental evidence demonstrating AAV-mediated transduction of microglia and microglia-like cell lines followed by an examination of the most promising experimental approaches identified in the literature. Finally, technical limitations will be considered in future applications of AAV experimental approaches.

AAV-Mediated Restoration of Dystrophin-Dp71 in the Brain of Dp71-Null Mice: Molecular, Cellular and Behavioral Outcomes

A deficiency in the shortest dystrophin-gene product, Dp71, is a pivotal aggravating factor for intellectual disabilities in Duchenne muscular dystrophy (DMD). Recent advances in preclinical research have achieved some success in compensating both muscle and brain dysfunctions associated with DMD, notably using exon skipping strategies. However, this has not been studied for distal mutations in the DMD gene leading to Dp71 loss. In this study, we aimed to restore brain Dp71 expression in the Dp71-null transgenic mouse using an adeno-associated virus (AAV) administrated either by intracardiac injections at P4 (ICP4) or by bilateral intracerebroventricular (ICV) injections in adults. ICP4 delivery of the AAV9-Dp71 vector enabled the expression of 2 to 14% of brain Dp71, while ICV delivery enabled the overexpression of Dp71 in the hippocampus and cortex of adult mice, with anecdotal expression in the cerebellum. The restoration of Dp71 was mostly located in the glial endfeet that surround capillaries, and it was associated with partial localization of Dp71-associated proteins, α1-syntrophin and AQP4 water channels, suggesting proper restoration of a scaffold of proteins involved in blood-brain barrier function and water homeostasis. However, this did not result in significant improvements in behavioral disturbances displayed by Dp71-null mice. The potential and limitations of this AAV-mediated strategy are discussed. This proof-of-concept study identifies key molecular markers to estimate the efficiencies of Dp71 rescue strategies and opens new avenues for enhancing gene therapy targeting cognitive disorders associated with a subgroup of severely affected DMD patients.

Natural Adeno-Associated Virus Serotypes and Engineered Adeno-Associated Virus Capsid Variants: Tropism Differences and Mechanistic Insights

Today, adeno-associated virus (AAV)-based vectors are arguably the most promising in vivo gene delivery vehicles for durable therapeutic gene expression. Advances in molecular engineering, high-throughput screening platforms, and computational techniques have resulted in a toolbox of capsid variants with enhanced performance over parental serotypes. Despite their considerable promise and emerging clinical success, there are still obstacles hindering their broader use, including limited transduction capabilities, tissue/cell type-specific tropism and penetration into tissues through anatomical barriers, off-target tissue biodistribution, intracellular degradation, immune recognition, and a lack of translatability from preclinical models to clinical settings. Here, we first describe the transduction mechanisms of natural AAV serotypes and explore the current understanding of the systemic and cellular hurdles to efficient transduction. We then outline progress in developing designer AAV capsid variants, highlighting the seminal discoveries of variants which can transduce the central nervous system upon systemic administration, and, to a lesser extent, discuss the targeting of the peripheral nervous system, eye, ear, lung, liver, heart, and skeletal muscle, emphasizing their tissue and cell specificity and translational promise. In particular, we dive deeper into the molecular mechanisms behind their enhanced properties, with a focus on their engagement with host cell receptors previously inaccessible to natural AAV serotypes. Finally, we summarize the main findings of our review and discuss future directions.

Adeno-associated Virus-Mediated Gene Delivery Across the Blood-Brain Barrier

Recombinant adeno-associated viruses (AAVs) have emerged as a popular tool for gene therapy in the central nervous system (CNS). Given the dense vasculature in the CNS, systemic administration is an appealing approach for achieving a broad distribution of AAV vectors across the CNS. However, the blood-brain barrier (BBB) is a major obstacle that blocks the entry of AAV vectors into the brain and spinal cord. Thus, there is a great need to develop novel AAV vector technology with enhanced BBB penetration. In this chapter, we briefly summarize AAV biology, possible mechanisms for AAV vectors to overcome the BBB and further engineering strategies, and current clinical trials using systemic AAV gene therapy for CNS diseases.

Gene therapy of adeno-associated virus (AAV) vectors in preclinical models of ischemic stroke

Stroke has been associated with devastating clinical outcomes, with current treatment strategies proving largely ineffective. Therefore, there is a need to explore alternative treatment options for addressing post-stroke functional deficits. Gene therapy utilizing adeno-associated viruses (AAVs) as a critical gene vector delivering genes to the central nervous system (CNS) gene delivery has emerged as a promising approach for treating various CNS diseases. This review aims to provide an overview of the biological characteristics of AAV vectors and the therapeutic advancements observed in preclinical models of ischemic stroke. The study further investigates the potential of manipulating AAV vectors in preclinical applications, emphasizing the challenges and prospects in the selection of viral vectors, drug delivery strategies, immune reactions, and clinical translation.

Molecular and cognitive signatures of ageing partially restored through synthetic delivery of IL2 to the brain

Cognitive decline is a common pathological outcome during aging, with an ill-defined molecular and cellular basis. In recent years, the concept of inflammaging, defined as a low-grade inflammation increasing with age, has emerged. Infiltrating T cells accumulate in the brain with age and may contribute to the amplification of inflammatory cascades and disruptions to the neurogenic niche observed with age. Recently, a small resident population of regulatory T cells has been identified in the brain, and the capacity of IL2-mediated expansion of this population to counter neuroinflammatory disease has been demonstrated. Here, we test a brain-specific IL2 delivery system for the prevention of neurological decline in aging mice. We identify the molecular hallmarks of aging in the brain glial compartments and identify partial restoration of this signature through IL2 treatment. At a behavioral level, brain IL2 delivery prevented the age-induced defect in spatial learning, without improving the general decline in motor skill or arousal. These results identify immune modulation as a potential path to preserving cognitive function for healthy aging.

Evaluating the transduction efficiency of systemically delivered AAV vectors in the rat nervous system

Gene delivery or manipulation with viral vectors is a frequently used tool in basic neuroscience studies. Adeno-associated viruses (AAV) are the most widely used vectors due to their relative safety and long-term efficacy without causing overt immunological complications. Many AAV serotypes have been discovered and engineered that preferentially transduce different populations of neurons. However, efficient targeting of peripheral neurons remains challenging for many researchers, and evaluation of peripheral neuron transduction with AAVs in rats is limited. Here, we aimed to test the efficiency of systemic AAVs to transduce peripheral neurons in rats. We administered AAV9-tdTomato, AAV-PHP.S-tdTomato, or AAV-retro-GFP systemically to neonatal rats via intraperitoneal injection. After 5 weeks, we evaluated expression patterns in peripheral sensory, motor, and autonomic neurons. No significant difference between the serotypes in the transduction of sensory neurons was noted, and all serotypes were more efficient in transducing NF200 + neurons compared to smaller CGRP + neurons. AAV-retro was more efficient at transducing motor neurons compared to other serotypes. Moreover, PHP.S was more efficient at transducing sympathetic neurons, and AAV-retro was more efficient at transducing parasympathetic neurons. These results indicate that specific AAV serotypes target peripheral neuron populations more efficiently than others in the neonatal rat.

Gene Delivery of Manf to Beta-Cells of the Pancreatic Islets Protects NOD Mice from Type 1 Diabetes Development

In type 1 diabetes, dysfunctional glucose regulation occurs due to the death of insulin-producing beta-cells in the pancreatic islets. Initiation of this process is caused by the inheritance of an adaptive immune system that is predisposed to responding to beta-cell antigens, most notably to insulin itself, coupled with unknown environmental insults priming the autoimmune reaction. While autoimmunity is a primary driver in beta-cell death, there is growing evidence that cellular stress participates in the loss of beta-cells. In the beta-cell fragility model, partial loss of islet mass requires compensatory upregulation of insulin production in the remaining islets, driving a cellular stress capable of triggering apoptosis in the remaining cells. The Glis3-Manf axis has been identified as being pivotal to the relative fragility or robustness of stressed islets, potentially operating in both type 1 and type 2 diabetes. Here, we have used an AAV-based gene delivery system to enhance the expression of the anti-apoptotic protein Manf in the beta-cells of NOD mice. Gene delivery substantially lowered the rate of diabetes development in treated mice. Manf-treated mice demonstrated minimal insulitis and superior preservation of insulin production. Our results demonstrating the therapeutic potential of Manf delivery to enhance beta-cell robustness and avert clinical diabetes.

Tyrosine 7.43 is important for mu-opioid receptor downstream signaling pathways activated by fentanyl

G protein–coupled receptors can signal through both G proteins and ß-arrestin2. For the µ-opioid receptor (MOR), early experimental evidence from a single study suggested that G protein signaling mediates analgesia and sedation, whereas ß-arrestin signaling mediates respiratory depression and constipation. Then, receptor mutations were used to clarify which residues interact with ligands to selectively regulate signals in a ligand-specific manner. However, there is no systematic study on how to determine these residues and clarify the molecular mechanism of their influence on signal pathways. We have therefore used molecular docking to predict the amino acid sites that affect the binding of ligands and MOR. Then, the corresponding sites were mutated to determine the effect of the structural determinant of MOR on G_i/o protein and ß-arrestin pathways. The pharmacological and animal behavioral experiments in combination with molecular dynamics simulations were used to elucidate the molecular mechanism of key residues governing the signaling. Without affecting ligand binding to MOR, MOR^Y7.43A attenuated the activation of both G_i/o protein and ß-arrestin signaling pathways stimulated by fentanyl, whereas it did not change these two pathways stimulated by morphine. Likewise, the activation peak time of extracellular regulated protein kinases was significantly prolonged at MOR^Y7.43A compared with that at MOR^wildtype stimulated by fentanyl, but there was no difference stimulated by morphine. In addition, MOR^Y7.43A significantly enhanced analgesia by fentanyl but not by morphine in the mice behavioral experiment. Furthermore, the molecular dynamics simulations showed that H6 moves toward the cellular membrane. H6 of the fentanyl–Y7.43A system moved outward more than that in the morphine–Y7.43A system. Y7.43 mutation disrupted hydrophobic interactions between W6.48 and Y7.43 in the fentanyl–Y7.43A system but not in the morphine–Y7.43A system. Our results have disclosed novel mechanisms of Y7.43 mutation affecting MOR signaling pathways. Y7.43 mutation reduced the activation of the G_i/o protein pathway and blocked the ß-arrestin2 recruitment, increased the H6 outward movement of MOR, and disrupted hydrophobic interactions. This may be responsible for the enhanced fentanyl analgesia. These findings are conducive to designing new drugs from the perspective of ligand and receptor binding, and Y7.43 is also expected to be a key site to structure optimization of synthesized compounds.

A conformational switch controlling the toxicity of the prion protein

Prion infections cause conformational changes of the cellular prion protein (PrP^C) and lead to progressive neurological impairment. Here we show that toxic, prion-mimetic ligands induce an intramolecular R208-H140 hydrogen bond ('H-latch'), altering the flexibility of the α2-α3 and β2-α2 loops of PrP^C. Expression of a PrP^2Cys mutant mimicking the H-latch was constitutively toxic, whereas a PrP^R207A mutant unable to form the H-latch conferred resistance to prion infection. High-affinity ligands that prevented H-latch induction repressed prion-related neurodegeneration in organotypic cerebellar cultures. We then selected phage-displayed ligands binding wild-type PrP^C, but not PrP^2Cys. These binders depopulated H-latched conformers and conferred protection against prion toxicity. Finally, brain-specific expression of an antibody rationally designed to prevent H-latch formation prolonged the life of prion-infected mice despite unhampered prion propagation, confirming that the H-latch is an important reporter of prion neurotoxicity.

Astrocyte-targeted gene delivery of interleukin 2 specifically increases brain-resident regulatory T cell numbers and protects against pathological neuroinflammation

The ability of immune-modulating biologics to prevent and reverse pathology has transformed recent clinical practice. Full utility in the neuroinflammation space, however, requires identification of both effective targets for local immune modulation and a delivery system capable of crossing the blood-brain barrier. The recent identification and characterization of a small population of regulatory T (Treg) cells resident in the brain presents one such potential therapeutic target. Here, we identified brain interleukin 2 (IL-2) levels as a limiting factor for brain-resident Treg cells. We developed a gene-delivery approach for astrocytes, with a small-molecule on-switch to allow temporal control, and enhanced production in reactive astrocytes to spatially direct delivery to inflammatory sites. Mice with brain-specific IL-2 delivery were protected in traumatic brain injury, stroke and multiple sclerosis models, without impacting the peripheral immune system. These results validate brain-specific IL-2 gene delivery as effective protection against neuroinflammation, and provide a versatile platform for delivery of diverse biologics to neuroinflammatory patients.

AAV-mediated delivery of an anti-BACE1 VHH alleviates pathology in an Alzheimer's disease model

Single domain antibodies (VHHs) are potentially disruptive therapeutics, with important biological value for treatment of several diseases, including neurological disorders. However, VHHs have not been widely used in the central nervous system (CNS), largely because of their restricted blood-brain barrier (BBB) penetration. Here, we propose a gene transfer strategy based on BBB-crossing Adeno-associated virus (AAV)-based vectors to deliver VHH directly into the CNS. As a proof-of-concept, we explored the potential of AAV-delivered VHH to inhibit BACE1, a well-characterized target in Alzheimer's disease. First, we generated a panel of VHHs targeting BACE1, one of which, VHH-B9, shows high selectivity for BACE1 and efficacy in lowering BACE1 activity in vitro. We further demonstrate that a single systemic dose of AAV-VHH-B9 produces positive long-term (12 months plus) effects on amyloid load, neuroinflammation, synaptic function, and cognitive performance, in the App^NL-G-F Alzheimer's disease mouse model. These results constitute a novel therapeutic approach forneurodegenerative diseases, which is applicable to a range of CNS disease targets.

AAV capsid variants with brain-wide transgene expression and decreased liver targeting after intravenous delivery in mouse and marmoset

Genetic intervention is increasingly being explored as a therapeutic option for debilitating disorders of the central nervous system. The safety and efficacy of gene therapies rely upon expressing a transgene in affected cells while minimizing off-target expression. Here we show organ-specific targeting of adeno-associated virus (AAV) capsids after intravenous delivery, which we achieved by employing a Cre-transgenic-based screening platform and sequential engineering of AAV-PHP.eB between the surface-exposed AA452 and AA460 of VP3. From this selection, we identified capsid variants that were enriched in the brain and targeted away from the liver in C57BL/6J mice. This tropism extends to marmoset (Callithrix jacchus), enabling robust, non-invasive gene delivery to the marmoset brain after intravenous administration. Notably, the capsids identified result in distinct transgene expression profiles within the brain, with one exhibiting high specificity to neurons. The ability to cross the blood-brain barrier with neuronal specificity in rodents and non-human primates enables new avenues for basic research and therapeutic possibilities unattainable with naturally occurring serotypes.

Pilot Study Assessing the Impact of Intrathecal Administration of Variants AAV-PHP.B and AAV-PHP.eB on Brain Transduction in Adult Rhesus Macaques

Adeno-associated virus (AAV) vectors are increasingly used as an effective and safe approach to deliver genetic material to the central nervous system (CNS). The AAV9-derived variants, AAV-PHP. B and AAV-PHP.eB, reportedly broadly transduce cells throughout the CNS compared to the original serotype 9, AAV9. As non-human primate data are scarce, we here evaluated the CNS transduction efficiencies after lumbar intrathecal bolus delivery of identical doses of either AAV-PHP. B:CAG-EGFP or AAV-PHP. eB:CAG-EGFP in rhesus macaque monkeys. AAV-PHP.eB achieved a more efficient and widespread CNS transduction compared to AAV-PHP.B. We report a strong neuronal and oligodendroglial tropism for both variants in the putamen and in the hippocampus. This proof-of-concept experiment highlights the potential value of intrathecal infusions of AAV-PHP.eB to distribute genetic material in the CNS with cell-type specificity and introduces a new opportunity to model brain diseases in rhesus macaque monkeys and further develop gene therapies targeting the CNS in humans.

Highly efficient neuronal gene knockout in vivo by CRISPR-Cas9 via neonatal intracerebroventricular injection of AAV in mice

CRISPR-Cas systems have emerged as a powerful tool to generate genetic models for studying normal and diseased central nervous system (CNS). Targeted gene disruption at specific loci has been demonstrated successfully in non-dividing neurons. Despite its simplicity, high specificity and low cost, the efficiency of CRISPR-mediated knockout in vivo can be substantially impacted by many parameters. Here, we used CRISPR-Cas9 to disrupt the neuronal-specific gene, NeuN, and optimized key parameters to achieve effective gene knockout broadly in the CNS in postnatal mice. Three cell lines and two primary neuron cultures were used to validate the disruption of NeuN by single-guide RNAs (sgRNA) harboring distinct spacers and scaffold sequences. This triage identified an optimal sgRNA design with the highest NeuN disruption in in vitro and in vivo systems. To enhance CRISPR efficiency, AAV-PHP.B, a vector with superior neuronal transduction, was used to deliver this sgRNA in Cas9 mice via neonatal intracerebroventricular (ICV) injection. This approach resulted in 99.4% biallelic indels rate in the transduced cells, leading to greater than 70% reduction of total NeuN proteins in the cortex, hippocampus and spinal cord. This work contributes to the optimization of CRISPR-mediated knockout and will be beneficial for fundamental and preclinical research.

E2F4-Based Gene Therapy Mitigates the Phenotype of the Alzheimer's Disease Mouse Model 5xFAD

After decades of unfruitful work, no effective therapies are available for Alzheimer's disease (AD), likely due to its complex etiology that requires a multifactorial therapeutic approach. We have recently shown using transgenic mice that E2 factor 4 (E2F4), a transcription factor that regulates cell quiescence and tissue homeostasis, and controls gene networks affected in AD, represents a good candidate for a multifactorial targeting of AD. Here we show that the expression of a dominant negative form of human E2F4 (hE2F4DN), unable to become phosphorylated in a Thr-conserved motif known to modulate E2F4 activity, is an effective and safe AD multifactorial therapeutic agent. Neuronal expression of hE2F4DN in homozygous 5xFAD (h5xFAD) mice after systemic administration of an AAV.PHP.B-hSyn1.hE2F4DN vector reduced the production and accumulation of Aβ in the hippocampus, attenuated reactive astrocytosis and microgliosis, abolished neuronal tetraploidization, and prevented cognitive impairment evaluated by Y-maze and Morris water maze, without triggering side effects. This treatment also reversed other alterations observed in h5xFAD mice such as paw-clasping behavior and body weight loss. Our results indicate that E2F4DN-based gene therapy is a promising therapeutic approach against AD.

Toward Development of Neuron Specific Transduction After Systemic Delivery of Viral Vectors

Widespread transduction of the CNS with a single, non-invasive systemic injection of adeno-associated virus is now possible due to the creation of blood-brain barrier-permeable capsids. However, as these capsids are mutants of AAV9, they do not have specific neuronal tropism. Therefore, it is necessary to use genetic tools to restrict expression of the transgene to neuronal tissues. Here we compare the strength and specificity of two neuron-specific promoters, human synapsin 1 and mouse calmodulin/calcium dependent kinase II, to the ubiquitous CAG promoter. Administration of a high titer of virus is necessary for widespread CNS transduction. We observed the neuron-specific promoters drive comparable overall expression in the brain to the CAG promoter. Furthermore, the neuron-specific promoters confer significantly less transgene expression in peripheral tissues compared with the CAG promoter. Future experiments will utilize these delivery platforms to over-express the Alzheimer-associated pathological proteins amyloid-beta and tau to create mouse models without transgenesis.

Challenges and Opportunities of Targeting Astrocytes to Halt Neurodegenerative Disorders

Neurodegenerative diseases are a heterogeneous group of disorders whose incidence is likely to duplicate in the next 30 years along with the progressive aging of the western population. Non-cell-specific therapeutics or therapeutics designed to tackle aberrant pathways within neurons failed to slow down or halt neurodegeneration. Yet, in the last few years, our knowledge of the importance of glial cells to maintain the central nervous system homeostasis in health conditions has increased exponentially, along with our awareness of their fundamental and multifaced role in pathological conditions. Among glial cells, astrocytes emerge as promising therapeutic targets in various neurodegenerative disorders. In this review, we present the latest evidence showing the astonishing level of specialization that astrocytes display to fulfill the demands of their neuronal partners as well as their plasticity upon injury. Then, we discuss the controversies that fuel the current debate on these cells. We tackle evidence of a potential beneficial effect of cell therapy, achieved by transplanting astrocytes or their precursors. Afterwards, we introduce the different strategies proposed to modulate astrocyte functions in neurodegeneration, ranging from lifestyle changes to environmental cues. Finally, we discuss the challenges and the recent advancements to develop astrocyte-specific delivery systems.

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.

A Confocal Microscopic Study of Gene Transfer into the Mesencephalic Tegmentum of Juvenile Chum Salmon, Oncorhynchus keta, Using Mouse Adeno-Associated Viral Vectors

To date, data on the presence of adenoviral receptors in fish are very limited. In the present work, we used mouse recombinant adeno-associated viral vectors (rAAV) with a calcium indicator of the latest generation GCaMP6m that are usually applied for the dorsal hippocampus of mice but were not previously used for gene delivery into fish brain. The aim of our work was to study the feasibility of transduction of rAAV in the mouse hippocampus into brain cells of juvenile chum salmon and subsequent determination of the phenotype of rAAV-labeled cells by confocal laser scanning microscopy (CLSM). Delivery of the gene in vivo was carried out by intracranial injection of a GCaMP6m-GFP-containing vector directly into the mesencephalic tegmentum region of juvenile (one-year-old) chum salmon, Oncorhynchus keta. AAV incorporation into brain cells of the juvenile chum salmon was assessed at 1 week after a single injection of the vector. AAV expression in various areas of the thalamus, pretectum, posterior-tuberal region, postcommissural region, medial and lateral regions of the tegmentum, and mesencephalic reticular formation of juvenile O. keta was evaluated using CLSM followed by immunohistochemical analysis of the localization of the neuron-specific calcium binding protein HuCD in combination with nuclear staining with DAPI. The results of the analysis showed partial colocalization of cells expressing GCaMP6m-GFP with red fluorescent HuCD protein. Thus, cells of the thalamus, posterior tuberal region, mesencephalic tegmentum, cells of the accessory visual system, mesencephalic reticular formation, hypothalamus, and postcommissural region of the mesencephalon of juvenile chum salmon expressing GCaMP6m-GFP were attributed to the neuron-specific line of chum salmon brain cells, which indicates the ability of hippocampal mammal rAAV to integrate into neurons of the central nervous system of fish with subsequent expression of viral proteins, which obviously indicates the neuronal expression of a mammalian adenoviral receptor homolog by juvenile chum salmon neurons.

Improvement of HSV-1 based amplicon vectors for a safe and long-lasting gene therapy in non-replicating cells

A key factor for developing gene therapy strategies for neurological disorders is the availability of suitable vectors. Currently, the most advanced are adeno-associated vectors that, while being safe and ensuring long-lasting transgene expression, have a very limited cargo capacity. In contrast, herpes simplex virus-based amplicon vectors can host huge amounts of foreign DNA, but concerns exist about their safety and ability to express transgenes long-term. We aimed at modulating and prolonging amplicon-induced transgene expression kinetics in vivo using different promoters and preventing transgene silencing. To pursue the latter, we deleted bacterial DNA sequences derived from vector construction and shielded the transgene cassette using AT-rich and insulator-like sequences (SAm technology). We employed luciferase and GFP as reporter genes. To determine transgene expression kinetics, we injected vectors in the hippocampus of mice that were longitudinally scanned for bioluminescence for 6 months. To evaluate safety, we analyzed multiple markers of damage and performed patch clamp electrophysiology experiments. All vectors proved safe, and we managed to modulate the duration of transgene expression, up to obtaining a stable, long-lasting expression using the SAm technology. Therefore, these amplicon vectors represent a flexible, efficient, and safe tool for gene delivery in the brain.

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.

Adeno-associated virus (AAV)-based gene therapy for glioblastoma

Glioblastoma (GBM) is the most common and malignant Grade IV primary craniocerebral tumor caused by glial cell carcinogenesis with an extremely poor median survival of 12–18 months. The current standard treatments for GBM, including surgical resection followed by chemotherapy and radiotherapy, fail to substantially prolong survival outcomes. Adeno-associated virus (AAV)-mediated gene therapy has recently attracted considerable interest because of its relatively low cytotoxicity, poor immunogenicity, broad tissue tropism, and long-term stable transgene expression. Furthermore, a range of gene therapy trials using AAV as vehicles are being investigated to thwart deadly GBM in mice models. At present, AAV is delivered to the brain by local injection, intracerebroventricular (ICV) injection, or systematic injection to treat experimental GBM mice model. In this review, we summarized the experimental trials of AAV-based gene therapy as GBM treatment and compared the advantages and disadvantages of different AAV injection approaches. We systematically introduced the prospect of the systematic injection of AAV as an approach for AAV-based gene therapy for GBM.

AAV9-mediated gene delivery of MCT1 to oligodendrocytes does not provide a therapeutic benefit in a mouse model of ALS

Oligodendrocyte dysfunction has been implicated in the pathophysiology of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder characterized by progressive motor neuron loss. The failure of trophic support provided by oligodendrocytes is associated with a concomitant reduction in oligodendroglial monocarboxylate transporter 1 (MCT1) expression and is detrimental for the long-term survival of motor neuron axons. Therefore, we established an adeno-associated virus 9 (AAV9)-based platform by which MCT1 was targeted mostly to white matter oligodendrocytes to investigate whether this approach could provide a therapeutic benefit in the SOD1^G93A mouse model of ALS. Despite good oligodendrocyte transduction and AAV-mediated MCT1 transgene expression, the disease outcome of SOD1^G93A mice was not altered. Our study further increases our current understanding about the complex nature of oligodendrocyte pathology in ALS and provides valuable insights into the future development of therapeutic strategies to efficiently modulate these cells.

Crossing the blood-brain barrier with AAV vectors

Central nervous system (CNS) diseases are some of the most difficult to treat because the blood-brain barrier (BBB) almost entirely limits the passage of many therapeutic drugs into the CNS. Gene therapy based on the adeno-associated virus (AAV) vector has the potential to overcome this problem. For example, an AAV serotype AAV9 has been widely studied for its ability to cross the BBB to transduce astrocytes, but its efficiency is limited. The emergence of AAV directed evolution technology provides a solution, and the variants derived from AAV9 directed evolution have been shown to have significantly higher crossing efficiency than AAV9. However, the mechanisms by which AAV crosses the BBB are still unclear. In this review, we focus on recent advances in crossing the blood-brain barrier with AAV vectors. We first review the AAV serotypes that can be applied to treating CNS diseases. Recent progress in possible AAV crossing the BBB and transduction mechanisms are then summarized. Finally, the methods to improve the AAV transduction efficiency are discussed.

CNS Transduction Benefits of AAV-PHP.eB over AAV9 Are Dependent on Administration Route and Mouse Strain

Adeno-associated viral (AAV) vectors are attractive tools for central nervous system (CNS) gene therapy because some vectors can cross the blood-brain barrier (BBB), allowing them to be used as minimally invasive treatments. A novel AAV vector recently evolved in vivo, AAV-PHP.eB, has been reported to cross the BBB more effectively than the existing gold standard AAV9, but not under all conditions. Here, we compared the efficacy of single-stranded AAV-PHP.eB and AAV9 in targeting mouse CNS and peripheral tissues after administration via various routes, in two different mouse strains (C57BL/6J and B6C3), and after packaging AAV-PHP.eB with a self-complementary genome. We found that AAV-PHP.eB produced higher CNS transduction than AAV9 after intravenous injection, but only in C57BL/6J and not in B6C3 mice. AAV-PHP.eB and AAV9 produced similar CNS transduction when the administration route did not require the vectors to cross the BBB. Packaging AAV-PHP.eB with a self-complementary genome increased overall CNS transduction, but at the expense of strong neuronal tropism. AAV-PHP.eB resulted in less transduction of liver tissue than AAV9 under all conditions. Taken together, these results suggest the potential for AAV-PHP.eB as a vector for CNS gene therapy applications, but consideration will be required for translation beyond mouse models.

Small Alphaherpesvirus Latency-Associated Promoters Drive Efficient and Long-Term Transgene Expression in the CNS

Recombinant adeno-associated viruses (rAAVs) are used as gene therapy vectors to treat central nervous system (CNS) diseases. Despite their safety and broad tropism, important issues need to be corrected such as the limited payload capacity and the lack of small gene promoters providing long-term, pan-neuronal transgene expression in the CNS. Commonly used gene promoters are relatively large and can be repressed a few months after CNS transduction, risking the long-term performance of single-dose gene therapy applications. We used a whole-CNS screening approach based on systemic delivery of AAV-PHP.eB, iDisco+ tissue-clearing and light-sheet microscopy to identify three small latency-associated promoters (LAPs) from the herpesvirus pseudorabies virus (PRV). These promoters are LAP1 (404 bp), LAP2 (498 bp), and LAP1_2 (880 bp). They drive chronic transcription of the virus-encoded latency-associated transcript (LAT) during productive and latent phases of PRV infection. We observed stable, pan-neuronal transgene transcription and translation from AAV-LAPs in the CNS for 6 months post AAV transduction. In several CNS areas, the number of cells expressing the transgene was higher for LAP2 than the large conventional EF1α promoter (1,264 bp). Our data suggest that the LAPs are suitable candidates for viral vector-based CNS gene therapies requiring chronic transgene expression after one-time viral-vector administration.

Multiplexed Cre-dependent selection yields systemic AAVs for targeting distinct brain cell types

Recombinant adeno-associated viruses (rAAVs) are efficient gene delivery vectors via intravenous delivery; however, natural serotypes display a finite set of tropisms. To expand their utility, we evolved AAV capsids to efficiently transduce specific cell types in adult mouse brains. Building upon our Cre-recombination-based AAV targeted evolution (CREATE) platform, we developed Multiplexed-CREATE (M-CREATE) to identify variants of interest in a given selection landscape through multiple positive and negative selection criteria. M-CREATE incorporates next-generation sequencing, synthetic library generation and a dedicated analysis pipeline. We have identified capsid variants that can transduce the central nervous system broadly, exhibit bias toward vascular cells and astrocytes, target neurons with greater specificity or cross the blood-brain barrier across diverse murine strains. Collectively, the M-CREATE methodology accelerates the discovery of capsids for use in neuroscience and gene-therapy applications.

Distinct Mechanisms for Visual and Motor-Related Astrocyte Responses in Mouse Visual Cortex

Astrocytes are a major cell type in the mammalian nervous system, are in close proximity to neurons, and show rich Ca²⁺ activity thought to mediate cellular outputs. Astrocytes show activity linked to sensory [1, 2] and motor [3, 4] events, reflecting local neural activity and brain-wide neuromodulatory inputs. Sensory responses are highly variable [5, 6, 7, 8, 9, 10], which may reflect interactions between distinct input types [6, 7, 9]. However, the diversity of inputs generating astrocyte activity, particularly during sensory stimulation and behavior, is not fully understood [11, 12]. Using a combination of Ca²⁺ imaging, a treadmill assay, and visual stimulation, we examined the properties of astrocyte activity in mouse visual cortex associated with motor or sensory events. Consistent with previous work, motor activity activated astrocytes across the cortex with little specificity, reflecting a diffuse neuromodulatory mechanism. In contrast, moving visual stimuli generated specific activity patterns that reflected the stimulus' trajectory within the visual field, precisely as one would predict if astrocytes reported local neural activity. Visual responses depended strongly on behavioral state, with astrocytes showing high amplitude Ca²⁺ transients during locomotion and little activity during stillness. Furthermore, the amplitudes of visual responses were highly correlated with pupil size, suggesting a role of arousal. Interestingly, while depletion of cortical noradrenaline abolished locomotor responses, visual responses were only reduced in amplitude and their spatiotemporal organization remained intact, suggesting two distinct types of inputs underlie visual responses. We conclude that cortical astrocytes integrate local sensory information and behavioral state, suggesting a role in information processing.

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Astrocytes of moving mice display robust retinotopic responses to visual stimuli

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Sensory responses are distinguishable from responses to locomotion

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Vision-driven responses are correlated to arousal

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Vision- and arousal-driven responses are differentially regulated by noradrenaline

Adeno-associated virus-based Alzheimer's disease mouse models and potential new therapeutic avenues

Alzheimer's disease (AD) is a highly prevalent neurodegenerative condition that presents with cognitive decline. The current understanding of underlying disease mechanisms remains incomplete. Genetically modified mouse models have been instrumental in deciphering pathomechanisms in AD. While these models were typically generated by classical transgenesis and genome editing, the use of adeno-associated viruses (AAVs) to model and investigate AD in mice, as well as to develop novel gene-therapy approaches, is emerging. Here, we reviewed literature that used AAVs to study and model AD and discuss potential gene therapy strategies. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

Basic and Clinical Application of Adeno-Associated Virus-Mediated Genome Editing

Traditional gene therapy (gene replacement) has made a breakthrough in treating inherited diseases. Adeno-associated virus (AAV) has emerged as a highly promising vector with innate ability, boosting the development of gene replacement and gene targeting. With the recent advance of engineered nucleases that work efficiently in human cells, AAV mediated-genome editing with nucleases has raised hopes for in situ gene therapy of inherited and non-inherited diseases. Here, the applications of AAV-mediated genome editing are highlighted, and the prospect of AAV and nucleases that will render extension of such success in clinical gene therapy is discussed.

Systemic AAV vectors for widespread and targeted gene delivery in rodents

We recently developed adeno-associated virus (AAV) capsids to facilitate efficient and noninvasive gene transfer to the central and peripheral nervous systems. However, a detailed protocol for generating and systemically delivering novel AAV variants was not previously available. In this protocol, we describe how to produce and intravenously administer AAVs to adult mice to specifically label and/or genetically manipulate cells in the nervous system and organs, including the heart. The procedure comprises three separate stages: AAV production, intravenous delivery, and evaluation of transgene expression. The protocol spans 8 d, excluding the time required to assess gene expression, and can be readily adopted by researchers with basic molecular biology, cell culture, and animal work experience. We provide guidelines for experimental design and choice of the capsid, cargo, and viral dose appropriate for the experimental aims. The procedures outlined here are adaptable to diverse biomedical applications, from anatomical and functional mapping to gene expression, silencing, and editing.

CRISPR to the Rescue: Advances in Gene Editing for the FMR1 Gene

Gene-editing using Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) is promising as a potential therapeutic strategy for many genetic disorders. CRISPR-based therapies are already being assessed in clinical trials, and evaluation of this technology in Fragile X syndrome has been performed by a number of groups. The findings from these studies and the advancement of CRISPR-based technologies are insightful as the field continues towards treatments and cures of Fragile X-Associated Disorders (FXADs). In this review, we summarize reports using CRISPR-editing strategies to target Fragile X syndrome (FXS) molecular dysregulation, and highlight how differences in FXS and Fragile X-associated Tremor/Ataxia Syndrome (FXTAS) might alter treatment strategies for each syndrome. We discuss the various modifications and evolutions of the CRISPR toolkit that expand its therapeutic potential, and other considerations for moving these strategies from bench to bedside. The rapidly growing field of CRISPR therapeutics is providing a myriad of approaches to target a gene, pathway, or transcript for modification. As cures for FXADs have remained elusive, CRISPR opens new avenues to pursue.

Neuro-Immuno-Gene- and Genome-Editing-Therapy for Alzheimer's Disease: Are We There Yet?

Alzheimer's disease (AD) is a highly complex neurodegenerative disorder and the current treatment strategies are largely ineffective thereby leading to irreversible and progressive cognitive decline in AD patients. AD continues to defy successful treatment despite significant advancements in the field of molecular medicine. Repeatedly, early promising preclinical and clinical results have catapulted into devastating setbacks leading to multi-billion dollar losses not only to the top pharmaceutical companies but also to the AD patients and their families. Thus, it is very timely to review the progress in the emerging fields of gene therapy and stem cell-based precision medicine. Here, we have made sincere efforts to feature the ongoing progress especially in the field of AD gene therapy and stem cell-based regenerative medicine. Further, we also provide highlights in elucidating the molecular mechanisms underlying AD pathogenesis and describe novel AD therapeutic targets and strategies for the new drug discovery. We hope that the quantum leap in the scientific advancements and improved funding will bolster novel concepts that will propel the momentum toward a trajectory leading to a robust AD patient-specific next generation precision medicine with improved cognitive function and excellent life quality.