Adeno-associated virus structural biology as a tool in vector development

Delivery of CRISPR/Cas9 system by AAV as vectors for gene therapy

The adeno-associated virus (AAV) is a defective single-stranded DNA virus with the simplest structure reported to date. It constitutes a capsid protein and single-stranded DNA. With its high transduction efficiency, low immunogenicity, and tissue specificity, it is the most widely used and promising gene therapy vector. The clustered regularly interspaced short palindromic sequence (CRISPR)/CRISPR-associated protein 9 (Cas9) gene editing system is an emerging technology that utilizes cas9 nuclease to specifically recognize and cleave target genes under the guidance of small guide RNA and realizes gene editing through homologous directional repair and non-homologous recombination repair. In recent years, an increasing number of animal experiments and clinical studies have revealed the great potential of AAV as a vector to deliver the CRISPR/cas9 system for treating genetic diseases and viral infections. However, the immunogenicity, toxicity, low transmission efficiency in brain and ear tissues, packaging size limitations of AAV, and immunogenicity and off-target effects of Cas9 protein pose several clinical challenges. This research reviews the role, challenges, and countermeasures of the AAV-CRISPR/cas9 system in gene therapy.

Emerging trends in virus and virus-like particle gene therapy delivery to the brain

Recent advances in gene therapy and gene-editing techniques offer the very real potential for successful treatment of neurological diseases. However, drug delivery constraints continue to impede viable therapeutic interventions targeting the brain due to its anatomical complexity and highly restrictive microvasculature that is impervious to many molecules. Realizing the therapeutic potential of gene-based therapies requires robust encapsulation and safe and efficient delivery to the target cells. Although viral vectors have been widely used for targeted delivery of gene-based therapies, drawbacks such as host genome integration, prolonged expression, undesired off-target mutations, and immunogenicity have led to the development of alternative strategies. Engineered virus-like particles (eVLPs) are an emerging, promising platform that can be engineered to achieve neurotropism through pseudotyping. This review outlines strategies to improve eVLP neurotropism for therapeutic brain delivery of gene-editing agents.

Current developments of gene therapy in human diseases

Gene therapy has witnessed substantial advancements in recent years, becoming a constructive tactic for treating various human diseases. This review presents a comprehensive overview of these developments, with a focus on their diverse applications in different disease contexts. It explores the evolution of gene delivery systems, encompassing viral (like adeno-associated virus; AAV) and nonviral approaches, and evaluates their inherent strengths and limitations. Moreover, the review delves into the progress made in targeting specific tissues and cell types, spanning the eye, liver, muscles, and central nervous system, among others, using these gene technologies. This targeted approach is crucial in addressing a broad spectrum of genetic disorders, such as inherited lysosomal storage diseases, neurodegenerative disorders, and cardiovascular diseases. Recent clinical trials and successful outcomes in gene therapy, particularly those involving AAV and the clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated proteins, are highlighted, illuminating the transformative potentials of this approach in disease treatment. The review summarizes the current status of gene therapy, its prospects, and its capacity to significantly ameliorate patient outcomes and quality of life. By offering comprehensive analysis, this review provides invaluable insights for researchers, clinicians, and stakeholders, enriching the ongoing discourse on the trajectory of disease treatment.

T-cell specific in vivo gene delivery with DART-AAVs targeted to CD8

One of the biggest challenges for in vivo gene therapy are vectors mediating highly selective gene transfer into a defined population of therapy-relevant cells. Here we present DARPin-targeted AAVs (DART-AAVs) displaying DARPins specific for human and murine CD8. Insertion of DARPins into the GH2/GH3 loop of the capsid protein 1 (VP1) of AAV2 and AAV6 resulted in high selectivity for CD8-positive T cells with unimpaired gene delivery activity. Remarkably, the capsid core structure was unaltered with protruding DARPins detectable. In complex primary cell mixtures, including donor blood or systemic injections into mice, the CD8-targeted AAVs were by far superior to unmodified AAV2 and AAV6 in terms of selectivity, target cell viability, and gene transfer rates. In vivo, up to 80% of activated CD8+ T cells were hit upon a single vector injection into conditioned humanized or immunocompetent mice. While gene transfer rates decreased significantly under non-activated conditions, genomic modification selectively in CD8+ T cells was still detectable upon Cre delivery into indicator mice. In both mouse models, selectivity for CD8+ T cells was close to absolute with exceptional detargeting from liver. The CD8-AAVs described here expand strategies for immunological research and in vivo gene therapy options.

Delivery of mRNA Using Biomimetic Vectors: Progress and Challenges

Messenger RNA (mRNA) is an emerging class of therapeutic agents for treating a wide range of diseases. However, due to the instability and low cell transfection rate of naked mRNA, the expression of delivered mRNA in target cells or tissues in vivo requires delivery strategies. Biomimetic vectors hold advantages such as high biocompatibility, tissue specific targeting ability and efficient delivery mechanisms, potentially overcoming challenges faced by other delivery vectors. In this review, biomimetic vector-based mRNA delivery systems are summarized and discuss the possible challenges and prospects of such delivery systems, which may contribute to the progress and application of mRNA-based therapy in the biomedical field.

Drug product Formulation and Fill/Finish Manufacturing Process Considerations for AAV-Based Genomic Medicines

Adeno-associated viruses (AAVs) have become the delivery medium of choice for a variety of genomic medicine applications i.e., gene therapy, gene editing/regulation, and ex-vivo cell therapy. AAVs are protein-DNA complexes which have unique stability characteristics that are susceptible to various stress exposure conditions commonly seen in the drug product (DP) life cycle. This review takes a comprehensive look at AAV DP formulation and process development considerations that could impact critical quality attributes (CQAs) during manufacturing, packaging, shipping, and clinical use. Additional aspects related to AAV development reviewed herein are: (1) Different AAV serotypes with unique protein sequences and charge characteristics potentially leading to discrete stability profiles; (2) Manufacturing process challenges and optimization efforts to improve yield, recovery and purity especially during early development activities; and (3) Defining and identifying CQAs with analytical methods which are constantly evolving and present unique characterization challenges for AAV-based products.

Probe capsid structure stability and dynamics of adeno-associated virus as an important viral vector for gene therapy by hydrogen-deuterium exchange-mass spectrometry

Adeno-associated virus (AAV), a widely used gene therapy vector, is a small, nonenveloped virus that contains a single-stranded DNA genome with a maximum length of 4.7 kb. Despite extensive biophysical and structural characterization, many aspects of AAV functions remain elusive. This knowledge gap is primarily due to a lack of structurally resolved dynamic information and the absence of structural coverage of functionally critical segments on the AAV capsid. Here, we developed a protocol to study AAV structural dynamics by hydrogen-deuterium exchange mass spectrometry (HDX-MS), a powerful method for monitoring protein structure stability and dynamics in solution. We performed HDX-MS measurements on AAVs without or with different DNA payloads of different sizes, and obtained detailed dynamic information on the entire AAV sequence including the two functionally important segments not previously structurally characterized. The unique N terminus of the capsid protein VP1 (VP1u) was found to adopt a highly dynamic and unstable conformation with low HDX protection across the entire region, whereas the presence of a DNA payload increased its protection. The VP1 and VP2 shared region (VP1/2) showed no measurable protection, with or without DNA. Differential HDX between empty and full capsid samples allowed us to identify potential new DNA-capsid interaction sites located primarily around the five-fold channel, which differ from the three-fold pocket binding site previously identified. Our HDX-MS method for characterizing AAV structural dynamics opens a new way for future efforts to understand AAV structure-function relationships and engineer next-generation AAV vectors with improved gene delivery properties.

Studying the ssDNA loaded adeno-associated virus aggregation using coarse-grained molecular dynamics simulations

The aggregation of adeno-associated viral (AAV) capsids in an aqueous environment was investigated via coarse-grained molecular dynamics (CG-MD) simulations. The primary driving force and mechanism of the aggregation were investigated with or without single-strand DNA (ssDNA) loaded at various process temperatures. Capsid aggregation appeared to involve multiple residue interactions (i.e., hydrophobic, polar and charged residues) leading to complex protein aggregation. In addition, two aggregation mechanisms (i.e., the fivefold face-to-face contact and the edge-to-edge contact) were identified from this study. The ssDNA with its asymmetric structure could be the reason for destabilizing protein subunits and enhancing the interaction between the charged residues, and further result in the non-reversible face-to-face contact. At higher temperature, the capsid structure was found to be unstable with the significant size expansion of the loaded ssDNA which could be attributed to reduced number of intramolecular hydrogen bonds, the increased conformational deviations of protein subunits and the higher residue fluctuations. The CG-MD model was further validated with previous experimental and simulation data, including the full capsid size measurement and the capsid internal pressure. Thus, a good understanding of AAV capsid aggregation, instability and the role of ssDNA were revealed by applying the developed computational model.

Gene therapy targeting the blood-brain barrier

Endothelial cells are the building blocks of vessels in the central nervous system (CNS) and form the blood-brain barrier (BBB). An intact BBB limits permeation of large hydrophilic molecules into the CNS. Thus, the healthy BBB is a major obstacle for the treatment of CNS disorders with antibodies, recombinant proteins or viral vectors. Several strategies have been devised to overcome the barrier. A key principle often consists in attaching the therapeutic compound to a ligand of receptors expressed on the BBB, for example, the transferrin receptor (TfR). The fusion molecule will bind to TfR on the luminal side of brain endothelial cells, pass the endothelial layer by transcytosis and be delivered to the brain parenchyma. However, attempts to endow therapeutic compounds with the ability to cross the BBB can be difficult to implement. An alternative and possibly more straight-forward approach is to produce therapeutic proteins in the endothelial cells that form the barrier. These cells are accessible from blood circulation and have a large interface with the brain parenchyma. They may be an ideal production site for therapeutic protein and afford direct supply to the CNS.

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.

Therapeutic potential of AAV-FL-Klotho in obesity: Impact on weight loss and lipid metabolism in mice

Klotho, an anti-aging protein, has gained attention for its protective effects against various diseases, including metabolic disorders, through recombinant Klotho administration. However, the potential of Klotho as a target for gene therapy requires further exploration, as it remains relatively understudied in the context of metabolic disorders. In this study, we demonstrate that AAV-full length(FL)-Klotho administration induces weight loss in mice and provides protection against high-fat diet (HFD)-induced obesity and hepatic steatosis, concurrently reducing the weights of white adipose tissue and liver. AAV-FL-Klotho administration also enhanced thermogenic gene expression in brown adipose tissue (BAT) and improved the morphology of interscapular BAT. The weight loss effect of AAV-FL-Klotho was found to be, at least in part, mediated by UCP1-dependent thermogenesis in brown adipocytes, potentially influenced by hepatokines secreted from AAV-FL-Klotho-transduced hepatocytes. These findings suggest that AAV-FL-Klotho is an attractive candidate for gene therapy to combat obesity. Nevertheless, unbiased experiments have also revealed disturbances in lipid metabolism due to AAV-FL-Klotho, as evidenced by the emergence of lipomas and increased expression of hepatic lipogenic proteins.

A New Generation of Gene Therapies as the Future of Wet AMD Treatment

Age-related macular degeneration (AMD) is an eye disease and the most common cause of vision loss in the Western World. In its advanced stage, AMD occurs in two clinically distinguished forms, dry and wet, but only wet AMD is treatable. However, the treatment based on repeated injections with vascular endothelial growth factor A (VEGFA) antagonists may at best stop the disease progression and prevent or delay vision loss but without an improvement of visual dysfunction. Moreover, it is a serious mental and financial burden for patients and may be linked with some complications. The recent first success of intravitreal gene therapy with ADVM-022, which transformed retinal cells to continuous production of aflibercept, a VEGF antagonist, after a single injection, has opened a revolutionary perspective in wet AMD treatment. Promising results obtained so far in other ongoing clinical trials support this perspective. In this narrative/hypothesis review, we present basic information on wet AMD pathogenesis and treatment, the concept of gene therapy in retinal diseases, update evidence on completed and ongoing clinical trials with gene therapy for wet AMD, and perspectives on the progress to the clinic of "one and done" therapy for wet AMD to replace a lifetime of injections. Gene editing targeting the VEGFA gene is also presented as another gene therapy strategy to improve wet AMD management.

Intravitreal injection of a rationally designed AAV capsid library in non-human primate identifies variants with enhanced retinal transduction and neutralizing antibody evasion

Adeno-associated virus (AAV) continues to be the gold standard vector for therapeutic gene delivery and has proven especially useful for treating ocular disease. Intravitreal injection (IVtI) is a promising delivery route because it increases accessibility of gene therapies to larger patient populations. However, data from clinical and non-human primate (NHP) studies utilizing currently available capsids indicate that anatomical barriers to AAV and pre-existing neutralizing antibodies can restrict gene expression to levels that are "sub-therapeutic" in a substantial proportion of patients. Here, we performed a combination of directed evolution in NHPs of an AAV2-based capsid library with simultaneous mutations across six surface-exposed variable regions and rational design to identify novel capsid variants with improved retinal transduction following IVtI. Following two rounds of screening in NHP, enriched variants were characterized in intravitreally injected mice and NHPs and shown to have increased transduction relative to AAV2. Lead capsid variant, P2-V1, demonstrated an increased ability to evade neutralizing antibodies in human vitreous samples relative to AAV2 and AAV2.7m8. Taken together, this study further contributed to our understanding of the selective pressures associated with retinal transduction via the vitreous and identified promising novel AAV capsid variants for clinical consideration.

Interspecies Scaling of Transgene Products for Viral Vector Gene Therapies: Method Assessment Using Data from Eleven Viral Vectors

The prediction of transgene product expression in human is important to guide first-in-human (FIH) dose selection for viral vector-based gene replacement therapies. Recently, allometric scaling from preclinical data and interspecies normalization of dose-response (D-R) relationship have been used to predict human transgene product expression of adeno-associated virus (AAV) vectors. In this study, we assessed two interspecies allometric scaling methods and two dose-response methods in predicting human transgene product expression of nine intravenously administered AAV vectors, one intramuscularly administered AAV vector, and one intravesical administered adenoviral vector. Among the four methods, normalized D-R method generated the highest prediction accuracy, with geometric mean fold error (GMFE) of 2.9 folds and 75% predictions within fivefold deviations of observed human transgene product levels. The vg/kg-based D-R method worked well for locally delivered vectors but substantially overpredicted human transgene product levels of some hemophilia A and B vectors. For both intravenously and locally administered vectors, the prediction accuracy of allometric scaling using body weight^-0.25 (AS by W^-0.25) was superior to allometric scaling using log(body weight) (AS by logW). This study successfully extended the use of allometric scaling and interspecies D-R normalization methods for human transgene product prediction from intravenous viral vectors to locally delivered viral vectors.

Losing, preserving, and restoring vision from neurodegeneration in the eye

The retina is a part of the brain that sits at the back of the eye, looking out onto the world. The first neurons of the retina are the rod and cone photoreceptors, which convert changes in photon flux into electrical signals that are the basis of vision. Rods and cones are frequent targets of heritable neurodegenerative diseases that cause visual impairment, including blindness, in millions of people worldwide. This review summarizes the diverse genetic causes of inherited retinal degenerations (IRDs) and their convergence onto common pathogenic mechanisms of vision loss. Currently, there are few effective treatments for IRDs, but recent advances in disparate areas of biology and technology (e.g., genome editing, viral engineering, 3D organoids, optogenetics, semiconductor arrays) discussed here enable promising efforts to preserve and restore vision in IRD patients with implications for neurodegeneration in less approachable brain areas.

Combining Surface-Induced Dissociation and Charge Detection Mass Spectrometry to Reveal the Native Topology of Heterogeneous Protein Complexes

Charge detection mass spectrometry (CDMS) enables the direct mass measurement of heterogeneous samples on the megadalton scale, as the charge state for a single ion is determined simultaneously with the mass-to-charge ratio (m/z). Surface-induced dissociation (SID) is an effective activation method to dissociate non-intertwined, non-covalent protein complexes without extensive gas-phase restructuring, producing various subcomplexes reflective of the native protein topology. Here, we demonstrate that using CDMS after SID on an Orbitrap platform offers subunit connectivity, topology, proteoform information, and relative interfacial strengths of the intact macromolecular assemblies. SID dissects the capsids (∼3.7 MDa) of adeno-associated viruses (AAVs) into trimer-containing fragments (3mer, 6mer, 9mer, 15mer, etc.) that can be detected by the individual ion mass spectrometry (I2MS) approach on Orbitrap instruments. SID coupled to CDMS provides unique structural insights into heterogeneous assemblies that are not readily obtained by traditional MS measurements.

Peptide ligands for the affinity purification of adeno-associated viruses from HEK 293 cell lysates

Adeno-associated viruses (AAVs) are the vector of choice for delivering gene therapies that can cure inherited and acquired diseases. Clinical research on various AAV serotypes significantly increased in recent years alongside regulatory approvals of AAV-based therapies. The current AAV purification platform hinges on the capture step, for which several affinity resins are commercially available. These adsorbents rely on protein ligands-typically camelid antibodies-that provide high binding capacity and selectivity, but suffer from low biochemical stability and high cost, and impose harsh elution conditions (pH < 3) that can harm the transduction activity of recovered AAVs. Addressing these challenges, this study introduces peptide ligands that selectively capture AAVs and release them under mild conditions (pH = 6.0). The peptide sequences were identified by screening a focused library and modeled in silico against AAV serotypes 2 and 9 (AAV2 and AAV9) to select candidate ligands that target homologous sites at the interface of the VP1-VP2 and VP2-VP3 virion proteins with mild binding strength (KD  ~ 10-5 -10- 6  M). Selected peptides were conjugated to Toyopearl resin and evaluated via binding studies against AAV2 and AAV9, demonstrating the ability to target both serotypes with values of dynamic binding capacity (DBC10%  > 1013 vp/mL of resin) and product yields (~50%-80%) on par with commercial adsorbents. The peptide-based adsorbents were finally utilized to purify AAV2 from a HEK 293 cell lysate, affording high recovery (50%-80%), 80- to 400-fold reduction of host cell proteins (HCPs), and high transduction activity (up to 80%) of the purified viruses.

Treatment to cure: advancing AAV gene therapy manufacture

Advanced therapy medicinal products are a reality. With the opportunity to treat patients at the genetic level, the pharmaceutical industry has extended the treatment paradigm to innovative and potentially curative approaches. Gene therapy modifies or manipulates the expression of a gene, through gene repair, replacement, or modification, to alter living cells for therapeutic use, requiring delivery mechanisms through viral vectors. Market analysis not only demonstrates that the gene therapy sector has strong growth potential, but also indicates infancy with the number of currently approved products. Within gene therapy, adeno-associated viruses (AAVs) have high prominence, allowing for the targeted delivery of a transgene for therapeutic effect. To be able to realise the full potential of AAV-based gene therapy, focus has shifted to the ability to manufacture and deliver high titre, high quality, and efficacious product. However, manufacturing is not simple, with multiple complex challenges ranging from starting material generation, ensuring cellular production of high titres of viral vectors, to purification, where not all AAV particles contain the intended genetic payload. As an industry, we must learn from established manufacturing processes, such as for monoclonal antibodies (mAbs), to deliver rapidly scalable, robust, and cost-effective platform solutions that can be truly multiproduct, while working hand-in-hand with regulatory agencies. Additionally future innovation remains important and there are several opportunities for disruptive and further advanced manufacturing approaches. With a true end in mind approach, can we turn the tide from treatment to cure? Teaser: The gene therapy market shows considerable opportunity for growth, with the potential to change the treatment paradigm toward curative approaches. However, manufacture remains a challenge. Focussing on AAV, we look what is required for these products to be delivered to patients.

Molecular engineering of cyclic azobenzene-peptide hybrid ligands for the purification of human blood Factor VIII via photo-affinity chromatography

The use of benign stimuli to control the binding and release of labile biologics for their isolation from complex feedstocks is a key goal of modern biopharmaceutical technology. This study introduces cyclic azobenzene-peptide (CAP) hybrid ligands for the rapid and discrete photo-responsive capture and release of blood coagulation Factor VIII (FVIII). A predictive method - based on amino acid sequence and molecular architecture of CAPs - was developed to correlate the conformation of cis/trans CAP photo-isomers to FVIII binding and release. The combined in silico and in vitro analysis of FVIII:peptide interactions guided the design of a rational approach to optimize isomerization kinetics and biorecognition of CAPs. A photoaffinity adsorbent, prepared by conjugating selected CAP G-cycloAZOB[Lys-YYKHLYN-Lys]-G on translucent chromatographic beads, featured high binding capacity (> 6 mg of FVIII per mL of resin) and rapid photo-isomerization kinetics (τ < 30s) when exposed to 420-450 nm light at the intensity of 0.1 W·cm-2. The adsorbent purified FVIII from a recombinant harvest using a single mobile phase, affording high product yield (>90%), purity (>95%), and blood clotting activity. The CAPs introduced in this report demonstrate a novel route integrating gentle operational conditions in a rapid and efficient bioprocess for the purification of life-saving biotherapeutics.

AAV genome modification for efficient AAV production

The adeno-associated virus (AAV) is one of the most potent vectors in gene therapy. The experimental profile of this vector shows its efficiency and accepted safety, which explains its increased usage by scientists for the research and treatment of a wide range of diseases. These studies require using functional, pure, and high titers of vector particles. In fact, the current knowledge of AAV structure and genome helps improve the scalable production of AAV vectors. In this review, we summarize the latest studies on the optimization of scalable AAV production through modifying the AAV genome or biological processes inside the cell.

AAV Serotypes and Their Suitability for Retinal Gene Therapy

Throughout the last 25 years, exceptional progress in retinal gene therapy was achieved. The major breakthrough was realized in 2017 when the FDA approved the adeno-associated virus (AAV)-based gene therapy for treatment of the monogenetic disorder Leber congenital amaurosis type 2 (LCA2). Since then, many therapies for inherited retinal diseases (IRD) reached phase I/II clinical trials, targeting diseases like achromatopsia, choroideremia, retinitis pigmentosa, Stargardt disease, and many more (reviewed in (Trapani and Auricchio, Trends Mol Med 24:669-681, 2018)). Advanced vector and capsid design technologies as well as improved gene transfer and gene editing methods may lead to refined therapies for various eye diseases. Many research departments worldwide focus on optimizing transgene expression by designing novel AAV serotypes. Besides serotype tropism, the method of injection (intravitreal, subretinal, or suprachoroidal) (Han et al., Hum Gene Ther 31:1288-1299, 2020) defines the efficiency outcome along with the use of tissue-specific promotors which play a critical role for cell targeting.

Neurodegenerative Diseases: From Dysproteostasis, Altered Calcium Signalosome to Selective Neuronal Vulnerability to AAV-Mediated Gene Therapy

Despite intense research into the multifaceted etiology of neurodegenerative diseases (ND), they remain incurable. Here we provide a brief overview of several major ND and explore novel therapeutic approaches. Although the cause (s) of ND are not fully understood, the accumulation of misfolded/aggregated proteins in the brain is a common pathological feature. This aggregation may initiate disruption of Ca⁺⁺ signaling, which is an early pathological event leading to altered dendritic structure, neuronal dysfunction, and cell death. Presently, ND gene therapies remain unidimensional, elusive, and limited to modifying one pathological feature while ignoring others. Considering the complexity of signaling cascades in ND, we discuss emerging therapeutic concepts and suggest that deciphering the molecular mechanisms involved in dendritic pathology may broaden the phenotypic spectrum of ND treatment. An innovative multiplexed gene transfer strategy that employs silencing and/or over-expressing multiple effectors could preserve vulnerable neurons before they are lost. Such therapeutic approaches may extend brain health span and ameliorate burdensome chronic disease states.

Rosa26-LSL-dCas9-VPR: a versatile mouse model for tissue specific and simultaneous activation of multiple genes for drug discovery

Transgenic animals with increased or abrogated target gene expression are powerful tools for drug discovery research. Here, we developed a CRISPR-based Rosa26-LSL-dCas9-VPR mouse model for targeted induction of endogenous gene expression using different Adeno-associated virus (AAV) capsid variants for tissue-specific gRNAs delivery. To show applicability of the model, we targeted low-density lipoprotein receptor (LDLR) and proprotein convertase subtilisin/kexin type 9 (PCSK9), either individually or together. We induced up to ninefold higher expression of hepatocellular proteins. In consequence of LDLR upregulation, plasma LDL levels almost abolished, whereas upregulation of PCSK9 led to increased plasma LDL and cholesterol levels. Strikingly, simultaneous upregulation of both LDLR and PCSK9 resulted in almost unaltered LDL levels. Additionally, we used our model to achieve expression of all α₁-Antitrypsin (AAT) gene paralogues simultaneously. These results show the potential of our model as a versatile tool for optimized targeted gene expression, alone or in combination.

Circumventing the packaging limit of AAV-mediated gene replacement therapy for neurological disorders

INTRODUCTION

Gene therapy provides the exciting opportunity of a curative single treatment for devastating diseases, eradicating the need for chronic medication. Adeno-associated viruses (AAVs) are among the most attractive vector carriers for gene replacement in vivo. Yet, despite the success of recent AAV-based clinical trials, the clinical use of these vectors has been limited. For instance, the AAV packaging capacity is restricted to ~4.7 kb, making it a substantial challenge to deliver large gene products.

AREAS COVERED

In this review, we explore established and emerging strategies that circumvent the packaging limit of AAVs to make them effective vehicles for gene replacement therapy of monogenic disorders, with a particular focus on diseases affecting the nervous system. We report historical references, design remarks, as well as strengths and weaknesses of these approaches. We additionally discuss examples of neurological disorders for which such strategies have been attempted.

EXPERT OPINION

The field of AAV-gene therapy has experienced enormous advancements in the last decade. However, there is still ample space for improvement aimed at overcoming existing challenges that are slowing down the progressive trajectory of this field.

Therapeutic in vivo delivery of gene editing agents

In vivo gene editing therapies offer the potential to treat the root causes of many genetic diseases. Realizing the promise of therapeutic in vivo gene editing requires the ability to safely and efficiently deliver gene editing agents to relevant organs and tissues in vivo. Here, we review current delivery technologies that have been used to enable therapeutic in vivo gene editing, including viral vectors, lipid nanoparticles, and virus-like particles. Since no single delivery modality is likely to be appropriate for every possible application, we compare the benefits and drawbacks of each method and highlight opportunities for future improvements.

A precise gene delivery approach for human induced pluripotent stem cells using Cas9 RNP complex and recombinant AAV6 donor vectors

Genome editing in human induced pluripotent stem cells (hiPSCs) offers a potential tool for studying gene functions in disease models and correcting genetic mutations for cell-based therapy. Precise transgene insertion in hiPSCs represents a significant challenge. In the past decade, viral transduction has been widely used due to its high transduction efficiency; however, it can result in random transgene integration and variable transgene copy numbers. Non-viral-based strategies are generally safer but limited by their low transfection efficiency in hiPSCs. Recently, genome engineering using adeno-associated virus (AAV) vectors has emerged as a promising gene delivery approach due to AAVs’ low immunogenicity, toxicity, and ability to infect a broad range of cells. The following protocol describes the workflow for genome editing in hiPSCs using the CRISPR/Cas9 ribonucleoprotein (RNP) complex combined with the recombinant AAV serotype 6 (AAV6) donor vectors to introduce a gene of interest (GOI) fused with mCherry fluorescent reporter gene into the AAVS1 safe harbor site. This approach leads to efficient transgene insertion and is applicable to precise genome editing of hiPSCs or other types of stem cells for research purposes.

Functional analysis of mutations endowing rAAV2-retro with retrograde tracing capacity

Recombinant adeno-associated viruses (rAAVs) are widespread vectors in neuroscience research. However, the nearly absent retrograde access to projection neurons hampers their application in functional dissection of neural circuits and in therapeutic intervention. Recently, engineering of the AAV2 capsid has generated an AAV variant, called rAAV2-retro, with exceptional retrograde functionality. This variant comprises a 10-mer peptide insertion at residue 587 and two point mutations (LADQDYTKTA + V708I + N382D). Here, we evaluated the contribution of each mutation to retrograde transport in prefrontal cortex -striatum and amygdala-striatum pathways, respectively. Results showed that disruption of the inserted decapeptide almost completely abolishes the retrograde access to neurons projecting to striatum. Eliminating N382D has little effect on the retrograde functionality. Restoring another mutation V708I, however, even improves its performance in amygdala-striatum pathway. Parallel comparison within same animal further confirms this conflicting effect of V708I. These results demonstrate a pivotal role of decapeptide insertion in gaining the capacity of retrograde transport and highlight a neural circuit-dependent contribution of V708I. It suggests constant and custom engineering of rAAV2-retro might be required to tackle the challenge of tremendous neuronal heterogeneity.

Transduction of Brain Neurons in Juvenile Chum Salmon (Oncorhynchus keta) with Recombinant Adeno-Associated Hippocampal Virus Injected into the Cerebellum during Long-Term Monitoring

Corpus cerebelli in juvenile chum salmon is a multiprojective region of the brain connected via afferent and efferent projections with the higher regions of the brainstem and synencephalon, as well as with multiprojection regions of the medulla oblongata and spinal cord. During the postembryonic development of the cerebellum in chum salmon, Oncorhynchus keta, the lateral part of the juvenile cerebellum gives rise to the caudomedial part of the definitive cerebellum, which is consistent with the data reported for zebrafish and mouse cerebellum. Thus, the topographic organization of the cerebellum and its efferents are similar between fish (chum salmon and zebrafish) and mammals, including mice and humans. The distributions of recombinant adeno-associated viral vectors (rAAVs) after an injection of the base vector into the cerebellum have shown highly specific patterns of transgene expression in bipolar neurons in the latero-caudal lobe of the juvenile chum tectum opticum. The distribution of rAAVs in the dorsal thalamus, epithalamus, nucleus rotundus, and pretectal complex indicates the targeted distribution of the transgene via the thalamo-cerebellar projections. The detection of GFP expression in the cells of the epiphysis and posterior tubercle of juvenile chum salmon is associated with the transgene’s distribution and with the cerebrospinal fluid flow, the brain ventricles and its outer surface. The direct delivery of the rAAV into the central nervous system by intracerebroventricular administration allows it to spread widely in the brain. Thus, the presence of special projection areas in the juvenile chum salmon cerebellum, as well as outside it, and the identification of the transgene’s expression in them confirm the potential ability of rAAVs to distribute in both intracerebellar and afferent and efferent extracerebellar projections of the cerebellum.

Novel human liver-tropic AAV variants define transferable domains that markedly enhance the human tropism of AAV7 and AAV8

Recent clinical successes have intensified interest in using adeno-associated virus (AAV) vectors for therapeutic gene delivery. The liver is a key clinical target, given its critical physiological functions and involvement in a wide range of genetic diseases. Here, we report the bioengineering of a set of next-generation AAV vectors, named AAV-SYDs (where "SYD" stands for Sydney, Australia), with increased human hepato-tropism in a liver xenograft mouse model repopulated with primary human hepatocytes. We followed a two-step process that staggered directed evolution and domain-swapping approaches. Using DNA-family shuffling, we first mapped key AAV capsid regions responsible for efficient human hepatocyte transduction in vivo. Focusing on these regions, we next applied domain-swapping strategies to identify and study key capsid residues that enhance primary human hepatocyte uptake and transgene expression. Our findings underscore the potential of AAV-SYDs as liver gene therapy vectors and provide insights into the mechanism responsible for their enhanced transduction profile.

Genome DNA leakage of Adeno-Associated virus under freeze-thaw stress

Adeno-associated virus (AAV) has become an emerging tool for human gene therapies. Currently, AAV gene therapies are subjected to multiple freeze-thaw cycles during manufacturing, storage, transportation, and administration. While studies have shown that multiple freeze-thaw cycles led to a decrease in transduction efficiency, the AAV degradation mechanism during freeze-thaw is not well understood. Here, we have characterized the impact of freeze-thaw on AAV8 by employing a variety of assays, which revealed significant increases in the amount of free single-stranded DNA (ssDNA) in AAV8 formulations after multiple freeze-thaw cycles. Subsequent analysis using Next Generation Sequencing (NGS) revealed that the ssDNA primarily consisted of genome DNA, indicating that the increased ssDNA leaked out from AAV8. Experiments performed using different serotypes of AAV confirmed the pervasiveness of such behavior amongst AAVs. In addition, formulation screening studies were performed to understand the impact on genome DNA leakage from AAV. The formulation screening results showed that the addition of 10% sucrose and 0.1% poloxamer 188 to Dulbecco's phosphate-buffered saline (DPBS) reduced the leakage of ssDNA in AAV samples after freeze-thaw cycles compared to the base formulation of DPBS alone. These findings shed new light on the degradation mechanism of AAVs and stabilization of the AAV-based gene therapies.

Supramolecular arrangement of protein in nanoparticle structures predicts nanoparticle tropism for neutrophils in acute lung inflammation

This study shows that the supramolecular arrangement of proteins in nanoparticle structures predicts nanoparticle accumulation in neutrophils in acute lung inflammation (ALI). We observed homing to inflamed lungs for a variety of nanoparticles with agglutinated protein (NAPs), defined by arrangement of protein in or on the nanoparticles via hydrophobic interactions, crosslinking and electrostatic interactions. Nanoparticles with symmetric protein arrangement (for example, viral capsids) had no selectivity for inflamed lungs. Flow cytometry and immunohistochemistry showed NAPs have tropism for pulmonary neutrophils. Protein-conjugated liposomes were engineered to recapitulate NAP tropism for pulmonary neutrophils. NAP uptake in neutrophils was shown to depend on complement opsonization. We demonstrate diagnostic imaging of ALI with NAPs; show NAP tropism for inflamed human donor lungs; and show that NAPs can remediate pulmonary oedema in ALI. This work demonstrates that structure-dependent tropism for neutrophils drives NAPs to inflamed lungs and shows NAPs can detect and treat ALI.

Capsid-Engineering for Central Nervous System-Directed Gene Therapy with Adeno-Associated Virus Vectors

Closing the gap in knowledge on the cause of neurodegenerative disorders is paving the way toward innovative treatment strategies, among which gene therapy has emerged as a top candidate. Both conventional gene therapy and genome editing approaches are being developed, and a great number of human clinical trials are ongoing. Already 2 years ago, the first gene therapy for a neurodegenerative disease, spinal muscular atrophy type 1 (SMA1), obtained market approval. To realize such innovative strategies, gene therapy delivery tools are key assets. Here, we focus on recombinant adeno-associated virus (AAV) vectors and report on strategies to improve first-generation vectors. Current efforts focus on the viral capsid to modify the host-vector interaction aiming at increasing the efficacy of target cell transduction, at simplifying vector administration, and at reducing the risk of vector dose-related side effects.

Therapeutics Development for Alagille Syndrome

Advancements in treatment for the rare genetic disorder known as Alagille Syndrome (ALGS) have been regrettably slow. The large variety of mutations to the JAG1 and NOTCH2 genes which lead to ALGS pose a unique challenge for developing targeted treatments. Due to the central role of the Notch signaling pathway in several cancers, traditional treatment modalities which compensate for the loss in activity caused by mutation are rightly excluded. Unfortunately, current treatment plans for ALGS focus on relieving symptoms of the disorder and do not address the underlying causes of disease. Here we review several of the current and potential key technologies and strategies which may yield a significant leap in developing targeted therapies for this disorder.

Development of a stable lyophilized adeno-associated virus gene therapy formulation

Adeno-associated viruses (AAV) are among the most actively investigated vectors for gene therapy. Supply of early clinical studies with frozen drug product (DP) can accelerate timelines and minimize degradation risks. In the long-term, logistical challenges of frozen DP may limit patient access. In this work, we developed a lyophilized (freeze-dried) formulation of AAV. The mass concentration of AAV is typically low, and AAV also requires a minimum ionic strength to inhibit aggregation. These factors result in a low collapse temperature, which is limiting to lyophilization. Mannitol crystallization was found to cause extensive degradation and potency loss of AAV during the freezing step. With further development, we determined that AAV could be lyophilized in a sucrose and citrate formulation with a more desirable high glass transition temperature of the dried cake. An optimal residual moisture range (1-3%) was found to be critical to maintaining AAV8 stability. Glycerol was found to protect AAV8 from over-drying by preventing capsid damage and genome DNA release. A lyophilized formulation was identified that maintained potency for 24 months at 2-8 °C, indicating the feasibility of a dried formulation for AAV gene therapy.

Tissue and cell-type-specific transduction using rAAV vectors in lung diseases

Gene therapy of genetically determined diseases, including some pathologies of the respiratory system, requires an efficient method for transgene delivery. Recombinant adeno-associated viral (rAAV) vectors are well studied and employed in gene therapy, as they are relatively simple and low immunogenic and able to efficiently transduce eukaryotic cells. To date, many natural and artificial (with modified capsids) AAV serotypes have been isolated, demonstrating preferential tropism toward different tissues and cells in accordance with the prevalent receptors on the cell surface. However, rAAV-mediated delivery is not strictly specific due to wide tropism of some viral serotypes. Thus, the development of the methods allowing modulating specificity of these vectors could be beneficial in some cases. This review describes various approaches for retargeting rAAV to respiratory cells, for example, using different types of capsid modifications and regulation of a transgene expression by tissue-specific promoters. Part of the review is devoted to the issues of transduction of stem and progenitor lung cells using AAV, which is a complicated task today.

Single amino acid insertion allows functional transduction of murine hepatocytes with human liver tropic AAV capsids

Recent successes in clinical gene therapy applications have intensified the interest in using adeno-associated viruses (AAVs) as vectors for gene delivery into human liver. An inherent intriguing characteristic of AAVs is that vector variants vary substantially in their ability to transduce hepatocytes from different species. This has historically limited the value of preclinical studies using rodent models for predicting the efficiency of AAV vectors in liver-targeted gene therapy clinical studies. In this work, we aimed to investigate the key determinants of the observed differential interspecies transduction abilities among AAV variants. We took advantage of domain swapping strategies between AAV-KP1, a newly identified variant with enhanced murine liver tropism, and AAV3b, which functions poorly in mice. The systematic in vivo comparison of AAV3b/AAV-KP1 chimeric variants allowed us to identify a threonine insertion at position 265 within variable region I (VR-I) as the key residue that confers murine hepatic transduction to human-derived clade B (AAV2-like) and clade C (AAV3b-like) variants. We propose to use this insertion to generate phylogenetically related AAV surrogates in support of toxicology and dosing studies in the murine liver model.

Stem Cell Based Exosomes: Are They Effective in Disease or Health?

Exosomes are nano-sized vesicles involved in intercellular communication via delivery of molecules including lipids, nucleic acids, proteins, or other cellular components to distant or neighboring sites. Their ability to pass biological barriers, stability in physiological fluids without degradation, and distinctive affinity to target cells make exosomes very remarkable therapeutic vehicles. Virus-based approaches are some of the most widely used gene therapy methods; however, there are many issues need to be clarified such as high immunogenicity. Using of the exosomes procures the functional transfer of their cargo with minimal intervention from the immune system and it has been reported to be secure and well-tolerated. When the regenerative medicine is taken into consideration, stem cell-based approaches have been aimed to utilize but the general efficacy and safety profile of stem cell therapy has still not been enlightened. At this point, stem cell-derived exosomes exhibit a way to procure cell-free regenerative medicine with their unique characteristics. Exosomes are considered as appropriate and highly stable biological nano-vectors taking part in a wide variety of healthy and pathological processes for advanced targeted therapies. However, there are still crucial obstacles to achieve efficient isolation of large amount of specific and pure exosomes. Thus, large-scale exosome production under good manufacturing practice is required. The purpose of this review is to focus on stem cell-based exosomes for gene delivery and to introduce synthetic exosome-mimics as a potential alternative in the field of targeted gene therapies. Further, we aim to highlight the biobanking and large-scale manufacturing methods of exosomes.

Development of New Strategies Using Extracellular Vesicles Loaded with Exogenous Nucleic Acid

Gene therapy is a therapeutic strategy of delivering foreign genetic material (encoding for an important protein) into a patient's target cell to replace a defective gene. Nucleic acids are embedded within the adeno-associated virus (AAVs) vectors; however, preexisting immunity to AAVs remains a significant concern that impairs their clinical application. Extracellular vesicles (EVs) hold great potential for therapeutic applications as vectors of nucleic acids due to their endogenous intercellular communication functions through their cargo delivery, including lipids and proteins. So far, small RNAs (siRNA and micro (mi)RNA) have been mainly loaded into EVs to treat several diseases, but the potential use of EVs to load and deliver exogenous plasmid DNA has not been thoroughly described. This review provides a comprehensive overview of the principal methodologies currently employed to load foreign genetic material into EVs, highlighting the need to find the most effective strategies for their successful clinical translations.

Emerging approaches for restoration of hearing and vision

Impairments of vision and hearing are highly prevalent conditions limiting the quality of life and presenting a major socioeconomic burden. For long, retinal and cochlear disorders have remained intractable for causal therapies, with sensory rehabilitation limited to glasses, hearing aids, and electrical cochlear or retinal implants. Recently, the application of gene therapy and optogenetics to eye and ear has generated hope for a fundamental improvement of vision and hearing restoration. To date, one gene therapy for the restoration of vision has been approved and undergoing clinical trials will broaden its application including gene replacement, genome editing, and regenerative approaches. Moreover, optogenetics, i.e. controlling the activity of cells by light, offers a more general alternative strategy. Over little more than a decade, optogenetic approaches have been developed and applied to better understand the function of biological systems, while protein engineers have identified and designed new opsin variants with desired physiological features. Considering potential clinical applications of optogenetics, the spotlight is on the sensory systems. Multiple efforts have been undertaken to restore lost or hampered function in eye and ear. Optogenetic stimulation promises to overcome fundamental shortcomings of electrical stimulation, namely poor spatial resolution and cellular specificity, and accordingly to deliver more detailed sensory information. This review aims at providing a comprehensive reference on current gene therapeutic and optogenetic research relevant to the restoration of hearing and vision. We will introduce gene-therapeutic approaches and discuss the biotechnological and optoelectronic aspects of optogenetic hearing and vision restoration.

AA, Lukashev AN. Emerging Concepts and Challenges in Rheumatoid Arthritis Gene Therapy

Rheumatoid arthritis (RA) is a systemic inflammatory joint disease affecting about 1% of the population worldwide. Current treatment approaches do not ensure a cure for every patient. Moreover, classical regimens are based on nontargeted systemic immune suppression and have significant side effects. Biological treatment has advanced considerably but efficacy and specificity issues remain. Gene therapy is one of the potential future directions for RA therapy, which is rapidly developing. Several gene therapy trials done so far have been of moderate success, but experimental and genetics studies have yielded novel targets. As a result, the arsenal of gene therapy tools keeps growing. Currently, both viral and nonviral delivery systems are used for RA therapy. Herein, we review recent approaches for RA gene therapy.

Adeno-associated viral vector-mediated immune responses: Understanding barriers to gene delivery

Adeno-associated viral (AAV) vectors have emerged as the leading gene delivery platform for gene therapy and vaccination. Three AAV-based gene therapy drugs, Glybera, LUXTURNA, and ZOLGENSMA were approved between 2012 and 2019 by the European Medicines Agency and the United States Food and Drug Administration as treatments for genetic diseases hereditary lipoprotein lipase deficiency (LPLD), inherited retinal disease (IRD), and spinal muscular atrophy (SMA), respectively. Despite these therapeutic successes, clinical trials have demonstrated that host anti-viral immune responses can prevent the long-term gene expression of AAV vector-encoded genes. Therefore, it is critical that we understand the complex relationship between AAV vectors and the host immune response. This knowledge could allow for the rational design of optimized gene transfer vectors capable of either subverting host immune responses in the context of gene therapy applications, or stimulating desirable immune responses that generate protective immunity in vaccine applications to AAV vector-encoded antigens. This review provides an overview of our current understanding of the AAV-induced immune response and discusses potential strategies by which these responses can be manipulated to improve AAV vector-mediated gene transfer.

Adeno-Associated Virus Technologies and Methods for Targeted Neuronal Manipulation

Cell-type-specific expression of molecular tools and sensors is critical to construct circuit diagrams and to investigate the activity and function of neurons within the nervous system. Strategies for targeted manipulation include combinations of classical genetic tools such as Cre/loxP and Flp/FRT, use of cis-regulatory elements, targeted knock-in transgenic mice, and gene delivery by AAV and other viral vectors. The combination of these complex technologies with the goal of precise neuronal targeting is a challenge in the lab. This report will discuss the theoretical and practical aspects of combining current technologies and establish best practices for achieving targeted manipulation of specific cell types. Novel applications and tools, as well as areas for development, will be envisioned and discussed.

Adeno-associated virus vector as a platform for gene therapy delivery

Adeno-associated virus (AAV) vectors are the leading platform for gene delivery for the treatment of a variety of human diseases. Recent advances in developing clinically desirable AAV capsids, optimizing genome designs and harnessing revolutionary biotechnologies have contributed substantially to the growth of the gene therapy field. Preclinical and clinical successes in AAV-mediated gene replacement, gene silencing and gene editing have helped AAV gain popularity as the ideal therapeutic vector, with two AAV-based therapeutics gaining regulatory approval in Europe or the United States. Continued study of AAV biology and increased understanding of the associated therapeutic challenges and limitations will build the foundation for future clinical success.

From fiction to science: clinical potentials and regulatory considerations of gene editing

Gene editing technologies such as CRISPR/Cas9 have emerged as an attractive tool not only for scientific research but also for the development of medicinal products. Their ability to induce precise double strand breaks into DNA enables targeted modifications of the genome including selective knockout of genes, correction of mutations or precise insertion of new genetic material into specific loci. Gene editing-based therapies hold a great potential for the treatment of numerous diseases and the first products are already being tested in clinical trials. The treatment indications include oncological malignancies, HIV, diseases of the hematopoietic system and metabolic disorders. This article reviews ongoing preclinical and clinical studies and discusses how gene editing technologies are altering the gene therapy landscape. In addition, it focusses on the regulatory challenges associated with such therapies and how they can be tackled during the drug development process.

Clinical Considerations for Capsid Choice in the Development of Liver-Targeted AAV-Based Gene Transfer

As gene transfer with adeno-associated virus (AAV) vectors is starting to enter clinical practice, this review examines the impact of vector capsid choice in liver-directed gene transfer for hemophilia. Given that there are multiple clinical trials completed and ongoing in this field, it is important to review the clinical evidence, particularly as a range of AAV-vector serotypes including AAV2, AAV5, AAV8, and AAV10 have been tested. Although there have been a number of successful trials, the development of two investigational AAV vectors for hemophilia B has been discontinued because they did not meet efficacy and/or safety expectations. Whether this difference between success and failure of gene transfer approaches reflects capsid choice, vector design, manufacturing system, or other variables is a question of great interest. Here, we examine the body of evidence across trials to determine the possible influences of serotype choice on key clinical outcomes such as safety, vector clearance, treatment eligibility, occurrence of transaminase elevations, activation of capsid-directed cytotoxic T cell responses, and clinical efficacy. In summary, gene transfer requires a balance between achieving sufficient transgene expression and minimizing destructive immune responses, which may be affected by AAV-vector serotype choice.

A Student's Guide to Neural Circuit Tracing

The mammalian nervous system is comprised of a seemingly infinitely complex network of specialized synaptic connections that coordinate the flow of information through it. The field of connectomics seeks to map the structure that underlies brain function at resolutions that range from the ultrastructural, which examines the organization of individual synapses that impinge upon a neuron, to the macroscopic, which examines gross connectivity between large brain regions. At the mesoscopic level, distant and local connections between neuronal populations are identified, providing insights into circuit-level architecture. Although neural tract tracing techniques have been available to experimental neuroscientists for many decades, considerable methodological advances have been made in the last 20 years due to synergies between the fields of molecular biology, virology, microscopy, computer science and genetics. As a consequence, investigators now enjoy an unprecedented toolbox of reagents that can be directed against selected subpopulations of neurons to identify their efferent and afferent connectomes. Unfortunately, the intersectional nature of this progress presents newcomers to the field with a daunting array of technologies that have emerged from disciplines they may not be familiar with. This review outlines the current state of mesoscale connectomic approaches, from data collection to analysis, written for the novice to this field. A brief history of neuroanatomy is followed by an assessment of the techniques used by contemporary neuroscientists to resolve mesoscale organization, such as conventional and viral tracers, and methods of selecting for sub-populations of neurons. We consider some weaknesses and bottlenecks of the most widely used approaches for the analysis and dissemination of tracing data and explore the trajectories that rapidly developing neuroanatomy technologies are likely to take.

A novel adeno-associated virus capsid with enhanced neurotropism corrects a lysosomal transmembrane enzyme deficiency

Recombinant adeno-associated viruses (AAVs) are popular in vivo gene transfer vehicles. However, vector doses needed to achieve therapeutic effect are high and some target tissues in the central nervous system remain difficult to transduce. Gene therapy trials using AAV for the treatment of neurological disorders have seldom led to demonstrated clinical efficacy. Important contributing factors are low transduction rates and inefficient distribution of the vector. To overcome these hurdles, a variety of capsid engineering methods have been utilized to generate capsids with improved transduction properties. Here we describe an alternative approach to capsid engineering, which draws on the natural evolution of the virus and aims to yield capsids that are better suited to infect human tissues. We generated an AAV capsid to include amino acids that are conserved among natural AAV2 isolates and tested its biodistribution properties in mice and rats. Intriguingly, this novel variant, AAV-TT, demonstrates strong neurotropism in rodents and displays significantly improved distribution throughout the central nervous system as compared to AAV2. Additionally, sub-retinal injections in mice revealed markedly enhanced transduction of photoreceptor cells when compared to AAV2. Importantly, AAV-TT exceeds the distribution abilities of benchmark neurotropic serotypes AAV9 and AAVrh10 in the central nervous system of mice, and is the only virus, when administered at low dose, that is able to correct the neurological phenotype in a mouse model of mucopolysaccharidosis IIIC, a transmembrane enzyme lysosomal storage disease, which requires delivery to every cell for biochemical correction. These data represent unprecedented correction of a lysosomal transmembrane enzyme deficiency in mice and suggest that AAV-TT-based gene therapies may be suitable for treatment of human neurological diseases such as mucopolysaccharidosis IIIC, which is characterized by global neuropathology.

AAV9 Vector: a Novel modality in gene therapy for spinal muscular atrophy

Spinal muscular atrophy (SMA), the leading genetic cause of infant mortality, is characterized by the deterioration of alpha motor neurons in the brainstem and spinal cord. Currently, there is no cure for SMA, which calls for an urgent need to explore affordable and effective therapies and to maximize patients' independence and quality of life. Adeno-associated virus (AAV) vector, one of the most promising and well-investigated vehicles for delivering transgenes, is a compelling candidate for gene therapy. Some of the hallmarks of AAVs are their nonpathogenicity, inability to incur an immune response, potential to achieve robust transgene expression, and varied tropism for several tissues of the body. Recently, these features were harnessed in a clinical trial conducted by AveXis in SMA patients, where AAV9 was employed as a vehicle for one-time administration of the SMN gene, the causative gene in SMA. The trial demonstrated remarkable improvements in motor milestones and rates of survival in the patients. This review focuses on the advent of SMA gene therapy and summarizes different preclinical studies that were conducted leading up to the AAV9-SMA trial in SMA patients.