Hepatocyte growth factor receptor is a coreceptor for adeno-associated virus type 2 infection

N-glycomic profiling of capsid proteins from Adeno-Associated Virus serotypes

Adeno-associated virus (AAV) vector has become the leading platform for gene delivery. Each serotype exhibits a different tissue tropism, immunogenicity, and in vivo transduction performance. Therefore, selecting the most suitable AAV serotype is critical for efficient gene delivery to target cells or tissues. Genome divergence among different serotypes is due mainly to the hypervariable regions of the AAV capsid proteins. However, the heterogeneity of capsid glycosylation is largely unexplored. In the present study, the N-glycosylation profiles of capsid proteins of AAV serotypes 1 to 9 have been systemically characterized and compared using a previously developed high-throughput and high-sensitivity N-glycan profiling platform. The results showed that all 9 investigated AAV serotypes were glycosylated, with comparable profiles. The most conspicuous feature was the high abundance mannosylated N-glycans, including FM3, M5, M6, M7, M8, and M9, that dominated the chromatograms within a range of 74 to 83%. Another feature was the relatively lower abundance of fucosylated and sialylated N-glycan structures, in the range of 23%-40% and 10%-17%, respectively. However, the exact N-glycan composition differed. These differences may be utilized to identify potential structural relationships between the 9 AAV serotypes. The current research lays the foundation for gaining better understanding of the importance of N-glycans on the AAV capsid surface that may play a significant role in tissue tropism, interaction with cell surface receptors, cellular uptake, and intracellular processing.

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.

AAV2 vector optimization for retinal ganglion cell-targeted delivery of therapeutic genes

Recombinant adeno-associated virus (AAV)-2 has significant potential as a delivery vehicle of therapeutic genes to retinal ganglion cells (RGCs), which are key interventional targets in optic neuropathies. Here we show that when injected intravitreally, AAV2 engineered with a reporter gene driven by cytomegalovirus (CMV) enhancer and chicken β-actin (CBA) promoters, displays ubiquitous and high RGC expression, similar to its synthetic derivative AAV8BP2. A novel AAV2 vector combining the promoter of the human RGC-selective γ-synuclein (hSNCG) gene and woodchuck hepatitis post-transcriptional regulatory element (WPRE) inserted upstream and downstream of a reporter gene, respectively, induces widespread transduction and strong transgene expression in RGCs. High transduction efficiency and selectivity to RGCs is further achieved by incorporating in the vector backbone a leading CMV enhancer and an SV40 intron at the 5' and 3' ends, respectively, of the reporter gene. As a delivery vehicle of hSIRT1, a 2.2-kb therapeutic gene with anti-apoptotic, anti-inflammatory and anti-oxidative stress properties, this recombinant vector displayed improved transduction efficiency, a strong, widespread and selective RGC expression of hSIRT1, and increased RGC survival following optic nerve crush. Thus, AAV2 vector carrying hSNCG promoter with additional regulatory sequences may offer strong potential for enhanced effects of candidate gene therapies targeting RGCs.

Structural basis for nuclear import of bat adeno-associated virus capsid protein

Adeno-associated viruses (AAV) are one of the world's most promising gene therapy vectors and as a result, are one of the most intensively studied viral vectors. Despite a wealth of research into these vectors, the precise characterisation of AAVs to translocate into the host cell nucleus remains unclear. Recently we identified the nuclear localization signals of an AAV porcine strain and determined its mechanism of binding to host importin proteins. To expand our understanding of diverse AAV import mechanisms we sought to determine the mechanism in which the Cap protein from a bat-infecting AAV can interact with transport receptor importins for translocation into the nucleus. Using a high-resolution crystal structure and quantitative assays, we were able to not only determine the exact region and residues of the N-terminal domain of the Cap protein which constitute the functional NLS for binding with the importin alpha two protein, but also reveal the differences in binding affinity across the importin-alpha isoforms. Collectively our results allow for a detailed molecular view of the way AAV Cap proteins interact with host proteins for localization into the cell nucleus.

Various AAV Serotypes and Their Applications in Gene Therapy: An Overview

Despite scientific discoveries in the field of gene and cell therapy, some diseases still have no effective treatment. Advances in genetic engineering methods have enabled the development of effective gene therapy methods for various diseases based on adeno-associated viruses (AAVs). Today, many AAV-based gene therapy medications are being investigated in preclinical and clinical trials, and new ones are appearing on the market. In this article, we present a review of AAV discovery, properties, different serotypes, and tropism, and a following detailed explanation of their uses in gene therapy for disease of different organs and systems.

Adeno-associated virus receptor complexes and implications for adeno-associated virus immune neutralization

Adeno-associated viruses (AAV) are among the foremost vectors for in vivo gene therapy. A number of monoclonal antibodies against several serotypes of AAV have previously been prepared. Many are neutralizing, and the predominant mechanisms have been reported as the inhibition of binding to extracellular glycan receptors or interference with some post-entry step. The identification of a protein receptor and recent structural characterization of its interactions with AAV compel reconsideration of this tenet. AAVs can be divided into two families based on which domain of the receptor is strongly bound. Neighboring domains, unseen in the high-resolution electron microscopy structures have now been located by electron tomography, pointing away from the virus. The epitopes of neutralizing antibodies, previously characterized, are now compared to the distinct protein receptor footprints of the two families of AAV. Comparative structural analysis suggests that antibody interference with protein receptor binding might be the more prevalent mechanism than interference with glycan attachment. Limited competitive binding assays give some support to the hypothesis that inhibition of binding to the protein receptor has been an overlooked mechanism of neutralization. More extensive testing is warranted.

Structural Characterization of Porcine Adeno-Associated Virus Capsid Protein with Nuclear Trafficking Protein Importin Alpha Reveals a Bipartite Nuclear Localization Signal

Adeno-associated viruses (AAV) are important vectors for gene therapy, and accordingly, many aspects of their cell transduction pathway have been well characterized. However, the specific mechanisms that AAV virions use to enter the host nucleus remain largely unresolved. We therefore aimed to reveal the interactions between the AAV Cap protein and the nuclear transport protein importin alpha (IMPα) at an atomic resolution. Herein we expanded upon our earlier research into the Cap nuclear localization signal (NLS) of a porcine AAV isolate, by examining the influence of upstream basic regions (BRs) towards IMPα binding. Using a high-resolution crystal structure, we identified that the IMPα binding determinants of the porcine AAV Cap comprise a bipartite NLS with an N-terminal BR binding at the minor site of IMPα, and the previously identified NLS motif binding at the major site. Quantitative assays showed a vast difference in binding affinity between the previously determined monopartite NLS, and bipartite NLS described in this study. Our results provide a detailed molecular view of the interaction between AAV capsids and the nuclear import receptor, and support the findings that AAV capsids enter the nucleus by binding the nuclear import adapter IMPα using the classical nuclear localization pathway.

Improving adeno-associated viral (AAV) vector-mediated transgene expression in retinal ganglion cells: comparison of five promoters

Recombinant adeno-associated viral vectors (AAVs) are an effective system for gene transfer. AAV serotype 2 (AAV2) is commonly used to deliver transgenes to retinal ganglion cells (RGCs) via intravitreal injection. The AAV serotype however is not the only factor contributing to the effectiveness of gene therapies. Promoters influence the strength and cell-selectivity of transgene expression. This study compares five promoters designed to maximise AAV2 cargo space for gene delivery: chicken β-actin (CBA), cytomegalovirus (CMV), short CMV early enhancer/chicken β-actin/short β-globulin intron (sCAG), mouse phosphoglycerate kinase (PGK), and human synapsin (SYN). The promoters driving enhanced green fluorescent protein (eGFP) were examined in adult C57BL/6J mice eyes and tissues of the visual system. eGFP expression was strongest in the retina, optic nerves and brain when driven by the sCAG and SYN promoters. CBA, CMV, and PGK had moderate expression by comparison. The SYN promoter had almost exclusive transgene expression in RGCs. The PGK promoter had predominant expression in both RGCs and AII amacrine cells. The ubiquitous CBA, CMV, and sCAG promoters expressed eGFP in a variety of cell types across multiple retinal layers including Müller glia and astrocytes. We also found that these promoters could transduce human retina ex vivo, although expression was predominantly in glial cells due to low RGC viability. Taken together, this promoter comparison study contributes to optimising AAV-mediated transduction in the retina, and could be valuable for research in ocular disorders, particularly those with large or complex genetic cargos.

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.

Organoids and microphysiological systems: Promising models for accelerating AAV gene therapy studies

The FDA has predicted that at least 10-20 gene therapy products will be approved by 2025. The surge in the development of such therapies can be attributed to the advent of safe and effective gene delivery vectors such as adeno-associated virus (AAV). The enormous potential of AAV has been demonstrated by its use in over 100 clinical trials and the FDA’s approval of two AAV-based gene therapy products. Despite its demonstrated success in some clinical settings, AAV-based gene therapy is still plagued by issues related to host immunity, and recent studies have suggested that AAV vectors may actually integrate into the host cell genome, raising concerns over the potential for genotoxicity. To better understand these issues and develop means to overcome them, preclinical model systems that accurately recapitulate human physiology are needed. The objective of this review is to provide a brief overview of AAV gene therapy and its current hurdles, to discuss how 3D organoids, microphysiological systems, and body-on-a-chip platforms could serve as powerful models that could be adopted in the preclinical stage, and to provide some examples of the successful application of these models to answer critical questions regarding AAV biology and toxicity that could not have been answered using current animal models. Finally, technical considerations while adopting these models to study AAV gene therapy are also discussed.

Cryo-electron Microscopy of Adeno-associated Virus

Adeno-associated virus (AAV) has a single-stranded DNA genome encapsidated in a small icosahedrally symmetric protein shell with 60 subunits. AAV is the leading delivery vector in emerging gene therapy treatments for inherited disorders, so its structure and molecular interactions with human hosts are of intense interest. A wide array of electron microscopic approaches have been used to visualize the virus and its complexes, depending on the scientific question, technology available, and amenability of the sample. Approaches range from subvolume tomographic analyses of complexes with large and flexible host proteins to detailed analysis of atomic interactions within the virus and with small ligands at resolutions as high as 1.6 Å. Analyses have led to the reclassification of glycan receptors as attachment factors, to structures with a new-found receptor protein, to identification of the epitopes of antibodies, and a new understanding of possible neutralization mechanisms. AAV is now well-enough characterized that it has also become a model system for EM methods development. Heralding a new era, cryo-EM is now also being deployed as an analytic tool in the process development and production quality control of high value pharmaceutical biologics, namely AAV vectors.

Adeno-Associated Virus Receptor-Binding: Flexible Domains and Alternative Conformations through Cryo-Electron Tomography of Adeno-Associated Virus 2 (AAV2) and AAV5 Complexes

Recombinant forms of adeno-associated virus (rAAV) are vectors of choice in the development of treatments for a number of genetic dispositions. Greater understanding of AAV’s molecular virology is needed to underpin needed improvements in efficiency and specificity. Recent advances have included identification of a near-universal entry receptor, AAVR, and structures detected by cryo-electron microscopy (EM) single particle analysis (SPA) that revealed, at high resolution, only the domains of AAVR most tightly bound to AAV. Here, cryogenic electron tomography (cryo-ET) is applied to reveal the neighboring domains of the flexible receptor. For AAV5, where the PKD1 domain is bound strongly, PKD2 is seen in three configurations extending away from the virus. AAV2 binds tightly to the PKD2 domain at a distinct site, and cryo-ET now reveals four configurations of PKD1, all different from that seen in AAV5. The AAV2 receptor complex also shows unmodeled features on the inner surface that appear to be an equilibrium alternate configuration. Other AAV structures start near the 5-fold axis, but now β-strand A is the minor conformer and, for the major conformer, partially ordered N termini near the 2-fold axis join the canonical capsid jellyroll fold at the βA-βB turn. The addition of cryo-ET is revealing unappreciated complexity that is likely relevant to viral entry and to the development of improved gene therapy vectors.

IMPORTANCE With 150 clinical trials for 30 diseases under way, AAV is a leading gene therapy vector. Immunotoxicity at high doses used to overcome inefficient transduction has occasionally proven fatal and highlighted gaps in fundamental virology. AAV enters cells, interacting through distinct sites with different domains of the AAVR receptor, according to AAV clade. Single domains are resolved in structures by cryogenic electron microscopy. Here, the adjoining domains are revealed by cryo-electron tomography of AAV2 and AAV5 complexes. They are in flexible configurations interacting minimally with AAV, despite measurable dependence of AAV2 transduction on both domains.

Adeno-associated virus (AAV) cell entry: structural insights

Adeno-associated virus (AAV) is the leading vector in emerging treatments of inherited diseases. Higher transduction efficiencies and cellular specificity are required for broader clinical application, motivating investigations of virus-host molecular interactions during cell entry. High-throughput methods are identifying host proteins more comprehensively, with subsequent molecular studies revealing unanticipated complexity and serotype specificity. Cryogenic electron microscopy (cryo-EM) provides a path towards structural details of these sometimes heterogeneous virus-host complexes, and is poised to illuminate more fully the steps in entry. Here presented, is progress in understanding the distinct steps of glycan attachment, and receptor-mediated entry/trafficking. Comparison with structures of antibody complexes provides new insights on immune neutralization with implications for the design of improved gene therapy vectors.

Adeno-Associated Virus (AAV)-Mediated Gene Therapy for Duchenne Muscular Dystrophy: The Issue of Transgene Persistence

Duchenne muscular dystrophy (DMD) is an X-linked recessive, infancy-onset neuromuscular disorder characterized by progressive muscle weakness and atrophy, leading to delay of motor milestones, loss of autonomous ambulation, respiratory failure, cardiomyopathy, and premature death. DMD originates from mutations in the DMD gene that result in a complete absence of dystrophin. Dystrophin is a cytoskeletal protein which belongs to the dystrophin-associated protein complex, involved in cellular signaling and myofiber membrane stabilization. To date, the few available therapeutic options are aimed at lessening disease progression, but persistent loss of muscle tissue and function and premature death are unavoidable. In this scenario, one of the most promising therapeutic strategies for DMD is represented by adeno-associated virus (AAV)-mediated gene therapy. DMD gene therapy relies on the administration of exogenous micro-dystrophin, a miniature version of the dystrophin gene lacking unnecessary domains and encoding a truncated, but functional, dystrophin protein. Limited transgene persistence represents one of the most significant issues that jeopardize the translatability of DMD gene replacement strategies from the bench to the bedside. Here, we critically review preclinical and clinical studies of AAV-mediated gene therapy in DMD, focusing on long-term transgene persistence in transduced tissues, which can deeply affect effectiveness and sustainability of gene replacement in DMD. We also discuss the role played by the overactivation of the immune host system in limiting long-term expression of genetic material. In this perspective, further studies aimed at better elucidating the need for immune suppression in AAV-treated subjects are warranted in order to allow for life-long therapy in DMD patients.

Hepatocellular Carcinoma Is a Natural Target for Adeno-Associated Virus (AAV) 2 Vectors

Simple Summary

Gene therapy is a novel approach to treat diseases by introducing corrective genetic information into target cells. Adeno-associated virus vectors are the most frequently applied gene delivery tools for in vivo gene therapy and are also studied as part of innovative anticancer strategies. Here, we report on the natural preference of AAV2 vectors for hepatocellular carcinoma (HCC) compared to nonmalignant liver cells in mice and human tissue. This preference in transduction is due to the improved intracellular processing of AAV2 vectors in HCC, resulting in significantly more vector genomes serving as templates for transcription in the cell nucleus. Based on this natural tropism for HCC, novel therapeutic strategies can be designed or existing therapeutic approaches can be strengthened as they currently result in only a minor improvement of the poor prognosis for most liver cancer patients.

Abstract

Although therapeutic options are gradually improving, the overall prognosis for patients with hepatocellular carcinoma (HCC) is still poor. Gene therapy-based strategies are developed to complement the therapeutic armamentarium, both in early and late-stage disease. For efficient delivery of transgenes with antitumor activity, vectors demonstrating preferred tumor tropism are required. Here, we report on the natural tropism of adeno-associated virus (AAV) serotype 2 vectors for HCC. When applied intravenously in transgenic HCC mouse models, similar amounts of vectors were detected in the liver and liver tumor tissue. In contrast, transduction efficiency, as indicated by the level of transgene product, was moderate in the liver but was elevated up to 19-fold in mouse tumor tissue. Preferred transduction of HCC compared to hepatocytes was confirmed in precision-cut liver slices from human patient samples. Our mechanistic studies revealed that this preference is due to the improved intracellular processing of AAV2 vectors in HCC, resulting, for example, in nearly 4-fold more AAV vector episomes that serve as templates for gene transcription. Given this background, AAV2 vectors ought to be considered to strengthen current—or develop novel—strategies for treating HCC.

Intracellular trafficking of adeno-associated virus (AAV) vectors: challenges and future directions

In the last two decades, recombinant adeno-associated virus has emerged as the most popular gene therapy vector. Recently AAV gene therapy has been approved by the FDA for the treatment of two rare genetic disorders, namely the early childhood blindness disease Leber congenital amaurosis and spinal muscular atrophy (SMA). As is the case for the treatment of SMA, if the AAV vector must be administered systemically, very high vector doses are often required for therapeutic efficacy. But higher vector doses inevitably increase the risk of adverse events. The tragic death of three children in a clinical trial to treat X-linked myotubular myopathy with an AAV vector has thrown this limitation into sharp relief. Regardless of the precise cause(s) that led to the death of the two children, it is critical that we develop better AAV vectors to achieve therapeutic levels of expression with lower vector doses. To transduce successfully a target cell, AAV has to overcome both systemic as well as cellular roadblocks. In this review, we discuss some of the most prominent cellular roadblocks that AAV must get past to deliver successfully its therapeutic payload. We also highlight recent advancements in our knowledge of AAV biology that can potentially be harnessed to improve AAV vector performance and thereby make AAV gene therapy safer.

ATF6-mediated unfolded protein response facilitates AAV2 transduction by releasing the suppression of AAV receptor on ER stress

Adeno-associated virus (AAV) is extensively used as a viral vector to deliver therapeutic genes during human gene therapy. A high affinity cellular receptor (AAVR) for most serotypes was recently identified, however, its biological function as a gene product remains unclear. In this study, we used AAVR knockdown cell models to show that AAVR depletion significantly attenuated cells to activate unfolded protein response (UPR) pathways, when exposed to the endoplasmic reticulum (ER) stress inducer, tunicamycin. By analyzing three major UPR pathways, we found that ATF6 signaling was most affected in an AAVR-dependent fashion, distinct to CHOP and XBP1 branches. AAVR capacity in UPR regulation required the full native AAVR protein, and AAV2 capsid binding to the receptor altered ATF6 dynamics. Conversely, the transduction efficiency of AAV2 was associated with changes in ATF6 signaling in host cells following treatment with different small molecules. Thus, AAVR served as an inhibitory molecule to repress UPR responses via a specificity for ATF6 signaling, and the AAV2 infection route involved the release from AAVR-mediated ATF6 repression, thereby facilitating viral intracellular trafficking and transduction. Importance The native function of the AAVR as an ER-Golgi localized protein is largely unknown. We showed that AAVR acted as a functional molecule to regulate UPR signaling under induced ER stress. AAVR inhibited the activation of the transcription factor, ATF6, whereas receptor binding to AAV2 released the suppression effects. This finding has expanded our understanding of AAV infection biology in terms of the physiological properties of AAVR in host cells. Importantly, our research provides a possible strategy which may improve the efficiency of AAV mediated gene delivery during gene therapy.

Structural Study of Aavrh.10 Receptor and Antibody Interactions

Recombinant adeno-associated virus (rAAV) vectors are one of the leading tools for the delivery of therapeutic genes in human gene therapy applications. For a successful transfer of their payload, the AAV vectors have to circumvent potential preexisting neutralizing host antibodies and bind to the receptors of the target cells. Both of these aspects have not been structurally analyzed for AAVrh.10. Here, cryo-electron microscopy and three-dimensional image reconstruction were used to map the binding site of sulfated N-acetyllactosamine (LacNAc; previously shown to bind AAVrh.10) and a series of four monoclonal antibodies (MAbs). LacNAc was found to bind to a pocket located on the side of the 3-fold capsid protrusion that is mostly conserved to AAV9 and equivalent to its galactose-binding site. As a result, AAVrh.10 was also shown to be able to bind to cell surface glycans with terminal galactose. For the antigenic characterization, it was observed that several anti-AAV8 MAbs cross-react with AAVrh.10. The binding sites of these antibodies were mapped to the 3-fold capsid protrusions. Based on these observations, the AAVrh.10 capsid surface was engineered to create variant capsids that escape these antibodies while maintaining infectivity. IMPORTANCE Gene therapy vectors based on adeno-associated virus rhesus isolate 10 (AAVrh.10) have been used in several clinical trials to treat monogenetic diseases. However, compared to other AAV serotypes little is known about receptor binding and antigenicity of the AAVrh.10 capsid. Particularly, preexisting neutralizing antibodies against capsids are an important challenge that can hamper treatment efficiency. This study addresses both topics and identifies critical regions of the AAVrh.10 capsid for receptor and antibody binding. The insights gained were utilized to generate AAVrh.10 variants capable of evading known neutralizing antibodies. The findings of this study could further aid the utilization of AAVrh.10 vectors in clinical trials and help the approval of the subsequent biologics.

Structural characterization of the porcine adeno-associated virus Po1 capsid protein binding to the nuclear trafficking protein importin alpha

Adeno-associated viruses (AAVs) are key vectors for gene therapy; thus, many aspects of their cell transduction pathway have been revealed in detail. However, the specific mechanisms AAV virions use to enter the host nucleus remain largely unresolved. We therefore aimed to reveal the structural interactions between the AAV capsid (Cap) protein and the nuclear transport protein importin alpha (IMPα). A putative nuclear localization sequence (NLS) in the virion protein 1 capsid protein of the porcine AAV Po1 was identified. This region was complexed with IMPα and a structure solved at 2.26 Å. This is the first time that an NLS of AAV Cap complexed with IMPα has been determined structurally. Our results support the findings that AAV capsids enter the nucleus through binding the nuclear import adapter IMPα.

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.

Development of a Bispecific Antibody-Based Platform for Retargeting of Capsid Modified AAV Vectors

A major limiting factor for systemically delivered gene therapies is the lack of novel tissue specific AAV (Adeno-associated virus) derived vectors. Bispecific antibodies can be used to redirect AAVs to specific target receptors. Here, we demonstrate that the insertion of a short linear epitope “2E3” derived from human proprotein-convertase subtilisin/kexin type 9 (PCSK9) into different surface loops of the VP capsid proteins can be used for AAV de-targeting from its natural receptor(s), combined with a bispecific antibody-mediated retargeting. We chose to target a set of distinct disease relevant membrane proteins—fibroblast activation protein (FAP), which is upregulated on activated fibroblasts within the tumor stroma and in fibrotic tissues, as well as programmed death-ligand 1 (PD-L1), which is strongly upregulated in many cancers. Upon incubation with a bispecific antibody recognizing the 2E3 epitope and FAP or PD-L1, the bispecific antibody/rAAV complex was able to selectively transduce receptor positive cells. In summary, we developed a novel, rationally designed vector retargeting platform that can target AAVs to a new set of cellular receptors in a modular fashion. This versatile platform may serve as a valuable tool to investigate the role of disease relevant cell types and basis for novel gene therapy approaches.

Applications of adeno-associated virus vector-mediated gene delivery for neurodegenerative diseases and psychiatric diseases: Progress, advances, and challenges

Neurodegeneration is the most common disease in the elderly population due to its slowly progressive nature of neuronal deterioration, eventually leading to executive dysfunction. The pathological markers of neurological disorders are relatively well-established, however, detailed molecular mechanisms of progression and therapeutic targets are needed to develop novel treatments in human patients. Treating known therapeutic targets of neurological diseases has been aided by recent advancements in adeno-associated virus (AAV) technology. AAVs are known for their low-immunogenicity, blood-brain barrier (BBB) penetrating ability, selective neuronal tropism, stable transgene expression, and pleiotropy. In addition, the usage of AAVs has enormous potential to be optimized. Therefore, AAV can be a powerful tool used to uncover the underlying pathophysiology of neurological disorders and to increase the success in human gene therapy. This review summarizes different optimization approaches of AAV vectors with their current applications in disease modeling, neural tracing and gene therapy, hence exploring progressive mechanisms of neurodegenerative diseases as well as effective therapy. Lastly, this review discusses the limitations and future perspectives of the AAV-mediated transgene delivery system.

Adeno-Associated Virus (AAV) Gene Delivery: Dissecting Molecular Interactions upon Cell Entry

Human gene therapy has advanced from twentieth-century conception to twenty-first-century reality. The recombinant Adeno-Associated Virus (rAAV) is a major gene therapy vector. Research continues to improve rAAV safety and efficacy using a variety of AAV capsid modification strategies. Significant factors influencing rAAV transduction efficiency include neutralizing antibodies, attachment factor interactions and receptor binding. Advances in understanding the molecular interactions during rAAV cell entry combined with improved capsid modulation strategies will help guide the design and engineering of safer and more efficient rAAV gene therapy vectors.

Receptor switching in newly evolved adeno-associated viruses

Adeno-associated viruses utilize different glycans and the AAV receptor (AAVR) for cellular attachment and entry. Directed evolution has yielded new AAV variants; however, structure-function correlates underlying their improved transduction are generally overlooked. Here, we report that infectious cycling of structurally diverse AAV surface loop libraries yields functionally distinct variants. Newly evolved variants show enhanced cellular binding, uptake, and transduction, but through distinct mechanisms. Using glycan-based and genome-wide CRISPR knockout screens, we discover that one AAV variant acquires the ability to recognize sulfated glycosaminoglycans, while another displays receptor switching from AAVR to integrin β1 (ITGB1). A previously evolved variant, AAVhum.8, preferentially utilizes the ITGB1 receptor over AAVR. Visualization of the AAVhum.8 capsid by cryoelectron microscopy at 2.49-Å resolution localizes the newly acquired integrin recognition motif adjacent to the AAVR footprint. These observations underscore the new finding that distinct AAV surface epitopes can be evolved to exploit different cellular receptors for enhanced transduction.

IMPORTANCE Understanding how viruses interact with host cells through cell surface receptors is central to discovery and development of antiviral therapeutics, vaccines, and gene transfer vectors. Here, we demonstrate that distinct epitopes on the surface of adeno-associated viruses can be evolved by infectious cycling to recognize different cell surface carbohydrates and glycoprotein receptors and solve the three-dimensional structure of one such newly evolved AAV capsid, which provides a roadmap for designing viruses with improved attributes for gene therapy applications.

Synthetic Biology: Emerging Concepts to Design and Advance Adeno-Associated Viral Vectors for Gene Therapy

Three recent approvals and over 100 ongoing clinical trials make adeno-associated virus (AAV)-based vectors the leading gene delivery vehicles in gene therapy. Pharmaceutical companies are investing in this small and nonpathogenic gene shuttle to increase the therapeutic portfolios within the coming years. This prospect of marking a new era in gene therapy has fostered both investigations of the fundamental AAV biology as well as engineering studies to enhance delivery vehicles. Driven by the high clinical potential, a new generation of synthetic-biologically engineered AAV vectors is on the rise. Concepts from synthetic biology enable the control and fine-tuning of vector function at different stages of cellular transduction and gene expression. It is anticipated that the emerging field of synthetic-biologically engineered AAV vectors can shape future gene therapeutic approaches and thus the design of tomorrow's gene delivery vectors. This review describes and discusses the recent trends in capsid and vector genome engineering, with particular emphasis on synthetic-biological approaches.

Current progress and limitations of AAV mediated delivery of protein therapeutic genes and the importance of developing quantitative pharmacokinetic/pharmacodynamic (PK/PD) models

While protein therapeutics are one of the most successful class of drug molecules, they are expensive and not suited for treating chronic disorders that require long-term dosing. Adeno-associated virus (AAV) mediated in vivo gene therapy represents a viable alternative, which can deliver the genes of protein therapeutics to produce long-term expression of proteins in target tissues. Ongoing clinical trials and recent regulatory approvals demonstrate great interest in these therapeutics, however, there is a lack of understanding regarding their cellular disposition, whole-body disposition, dose-exposure relationship, exposure-response relationship, and how product quality and immunogenicity affects these important properties. In addition, there is a lack of quantitative studies to support the development of pharmacokinetic-pharmacodynamic models, which can support the discovery, development, and clinical translation of this delivery system. In this review, we have provided a state-of-the-art overview of current progress and limitations related to AAV mediated delivery of protein therapeutic genes, along with our perspective on the steps that need to be taken to improve clinical translation of this therapeutic modality.

Completion of the AAV Structural Atlas: Serotype Capsid Structures Reveals Clade-Specific Features

The capsid structures of most Adeno-associated virus (AAV) serotypes, already assigned to an antigenic clade, have been previously determined. This study reports the remaining capsid structures of AAV7, AAV11, AAV12, and AAV13 determined by cryo-electron microscopy and three-dimensional image reconstruction to 2.96, 2.86, 2.54, and 2.76 Å resolution, respectively. These structures complete the structural atlas of the AAV serotype capsids. AAV7 represents the first clade D capsid structure; AAV11 and AAV12 are of a currently unassigned clade that would include AAV4; and AAV13 represents the first AAV2-AAV3 hybrid clade C capsid structure. These newly determined capsid structures all exhibit the AAV capsid features including 5-fold channels, 3-fold protrusions, 2-fold depressions, and a nucleotide binding pocket with an ordered nucleotide in genome-containing capsids. However, these structures have viral proteins that display clade-specific loop conformations. This structural characterization completes our three-dimensional library of the current AAV serotypes to provide an atlas of surface loop configurations compatible with capsid assembly and amenable for future vector engineering efforts. Derived vectors could improve gene delivery success with respect to specific tissue targeting, transduction efficiency, antigenicity or receptor retargeting.

Of rAAV and Men: From Genetic Neuromuscular Disorder Efficacy and Toxicity Preclinical Studies to Clinical Trials and Back

Neuromuscular disorders are a large group of rare pathologies characterised by skeletal muscle atrophy and weakness, with the common involvement of respiratory and/or cardiac muscles. These diseases lead to life-long motor deficiencies and specific organ failures, and are, in their worst-case scenarios, life threatening. Amongst other causes, they can be genetically inherited through mutations in more than 500 different genes. In the last 20 years, specific pharmacological treatments have been approved for human usage. However, these "à-la-carte" therapies cover only a very small portion of the clinical needs and are often partially efficient in alleviating the symptoms of the disease, even less so in curing it. Recombinant adeno-associated virus vector-mediated gene transfer is a more general strategy that could be adapted for a large majority of these diseases and has proved very efficient in rescuing the symptoms in many neuropathological animal models. On this solid ground, several clinical trials are currently being conducted with the whole-body delivery of the therapeutic vectors. This review recapitulates the state-of-the-art tools for neuron and muscle-targeted gene therapy, and summarises the main findings of the spinal muscular atrophy (SMA), Duchenne muscular dystrophy (DMD) and X-linked myotubular myopathy (XLMTM) trials. Despite promising efficacy results, serious adverse events of various severities were observed in these trials. Possible leads for second-generation products are also discussed.

Immune responses to retinal gene therapy using adeno-associated viral vectors - Implications for treatment success and safety

Recombinant adeno-associated virus (AAV) is the leading vector for gene therapy in the retina. As non-pathogenic, non-integrating, replication deficient vector, the recombinant virus efficiently transduces all key retinal cell populations. Successful testing of AAV vectors in clinical trials of inherited retinal diseases led to the recent approval of voretigene neparvovec (Luxturna) for the treatment of RPE65 mutation-associated retinal dystrophies. However, studies applying AAV-mediated retinal gene therapy independently reported intraocular inflammation and/or loss of efficacy after initial functional improvements. Both observations might be explained by targeted removal of transduced cells via anti-viral defence mechanisms. AAV has been shown to activate innate pattern recognition receptors (PRRs) such as toll-like receptor (TLR)-2 and TLR-9 resulting in the release of inflammatory cytokines and type I interferons. The vector can also induce capsid-specific and transgene-specific T cell responses and neutralizing anti-AAV antibodies which both limit the therapeutic effect. However, the target organ of retinal gene therapy, the eye, is known as an immune-privileged site. It is characterized by suppression of inflammation and promotion of immune tolerance which might prevent AAV-induced immune responses. This review evaluates AAV-related immune responses, toxicity and inflammation in studies of retinal gene therapy, identifies influencing variables of these responses and discusses potential strategies to modulate immune reactions to AAV vectors to increase the safety and efficacy of ocular gene therapy.

Journey to the Center of the Cell: Tracing the Path of AAV Transduction

As adeno-associated virus (AAV)-based gene therapies are being increasingly approved for use in humans, it is important that we understand vector-host interactions in detail. With the advances in genome-wide genetic screening tools, a clear picture of AAV-host interactions is beginning to emerge. Understanding these interactions can provide insights into the viral life cycle. Accordingly, novel strategies to circumvent the current limitations of AAV-based vectors may be explored. Here, we summarize our current understanding of the various stages in the journey of the vector from the cell surface to the nucleus and contextualize the roles of recently identified host factors.

The role of growth factor receptors in viral infections: An opportunity for drug repurposing against emerging viral diseases such as COVID-19?

Growth factor receptors are known to be involved in the process of viral infection. Many viruses not only use growth factor receptors to physically attach to the cell surface and internalize, but also divert receptor tyrosine kinase signaling in order to replicate. Thus, repurposing drugs that have initially been developed to target growth factor receptors and their signaling in cancer may prove to be a fast track to effective therapies against emerging new viral infections, including the coronavirus disease 19 (COVID-19).

Parvovirus Capsid-Antibody Complex Structures Reveal Conservation of Antigenic Epitopes Across the Family

The parvoviruses are small nonenveloped single stranded DNA viruses that constitute members that range from apathogenic to pathogenic in humans and animals. The infection with a parvovirus results in the generation of antibodies against the viral capsid by the host immune system to eliminate the virus and to prevent re-infection. For members currently either being developed as delivery vectors for gene therapy applications or as oncolytic biologics for tumor therapy, efforts are aimed at combating the detrimental effects of pre-existing or post-treatment antibodies that can eliminate therapeutic benefits. Therefore, understanding antigenic epitopes of parvoviruses can provide crucial information for the development of vaccination applications and engineering novel capsids able to escape antibody recognition. This review aims to capture the information for the binding regions of ∼30 capsid-antibody complex structures of different parvovirus capsids determined to date by cryo-electron microscopy and three-dimensional image reconstruction. The comparison of all complex structures revealed the conservation of antigenic regions among parvoviruses from different genera despite low sequence identity and indicates that the available data can be used across the family for vaccine development and capsid engineering.

Battling Neurodegenerative Diseases with Adeno-Associated Virus-Based Approaches

Neurodegenerative diseases (NDDs) are most commonly found in adults and remain essentially incurable. Gene therapy using AAV vectors is a rapidly-growing field of experimental medicine that holds promise for the treatment of NDDs. To date, the delivery of a therapeutic gene into target cells via AAV represents a major obstacle in the field. Ideally, transgenes should be delivered into the target cells specifically and efficiently, while promiscuous or off-target gene delivery should be minimized to avoid toxicity. In the pursuit of an ideal vehicle for NDD gene therapy, a broad variety of vector systems have been explored. Here we specifically outline the advantages of adeno-associated virus (AAV)-based vector systems for NDD therapy application. In contrast to many reviews on NDDs that can be found in the literature, this review is rather focused on AAV vector selection and their preclinical testing in experimental and preclinical NDD models. Preclinical and in vitro data reveal the strong potential of AAV for NDD-related diagnostics and therapeutic strategies.

Tracking Adeno-Associated Virus Capsid Evolution by High-Throughput Sequencing

Despite early successes using recombinant adeno-associated virus (rAAV) vectors in clinical gene therapy trials, limitations remain making additional advancements a necessity. Some of the challenges include variable levels of pre-existing neutralizing antibodies and poor transduction in specific target tissues and/or diseases. In addition, readministration of an rAAV vector is in general not possible due to the immune response against the capsid. Recombinant adeno-associated virus (AAV) vectors with novel capsids can be isolated in nature or developed through different directed evolution strategies. However, in most cases, the process of AAV selection is not well understood and new strategies are required to define the best parameters to develop more efficient and functional rAAV capsids. Therefore, the use of barcoding for AAV capsid libraries, which can be screened by high-throughput sequencing, provides a powerful tool to track AAV capsid evolution and potentially improve AAV capsid library screens. In this study, we examined how different parameters affect the screen of two different AAV libraries in two human cell types. We uncovered new and unexpected insights in how to maximize the likelihood of obtaining AAV variants with the desired properties. The major findings of the study are the following. (1) Inclusion of helper-virus for AAV replication can selectively propagate variants that can replicate to higher titers, but are not necessarily better at transduction. (2) Competition between AAVs with specific capsids can take place in cells that have been infected with different AAVs. (3) The use of low multiplicity of infections for infection results in more variation between screens and is not optimal at selecting the most desired capsids. (4) Using multiple rounds of selection can be counterproductive. We conclude that each of these parameters should be taken into consideration when screening AAV libraries for enhanced properties of interest.

Comparative Analysis of the Capsid Structures of AAVrh.10, AAVrh.39, and AAV8

Adeno-associated viruses (AAVs) from clade E are often used as vectors in gene delivery applications. This clade includes rhesus isolate 10 (AAVrh.10) and 39 (AAVrh.39) which, unlike representative AAV8, are capable of crossing the blood-brain barrier (BBB), thereby enabling the delivery of therapeutic genes to the central nervous system. Here, the capsid structures of AAV8, AAVrh.10 and AAVrh.39 have been determined by cryo-electron microscopy and three-dimensional image reconstruction to 3.08-, 2.75-, and 3.39-Å resolution, respectively, to enable a direct structural comparison. AAVrh.10 and AAVrh.39 are 98% identical in amino acid sequence but only ∼93.5% identical to AAV8. However, the capsid structures of all three viruses are similar, with only minor differences observed in the previously described surface variable regions, suggesting that specific residues S269 and N472, absent in AAV8, may confer the ability to cross the BBB in AAVrh.10 and AAVrh.39. Head-to-head comparison of empty and genome-containing particles showed DNA ordered in the previously described nucleotide-binding pocket, supporting the suggested role of this pocket in DNA packaging for the Dependoparvovirus The structural characterization of these viruses provides a platform for future vector engineering efforts toward improved gene delivery success with respect to specific tissue targeting, transduction efficiency, antigenicity, or receptor retargeting.IMPORTANCE Recombinant adeno-associated virus vectors (rAAVs), based on AAV8 and AAVrh.10, have been utilized in multiple clinical trials to treat different monogenetic diseases. The closely related AAVrh.39 has also shown promise in vivo As recently attained for other AAV biologics, e.g., Luxturna and Zolgensma, based on AAV2 and AAV9, respectively, the vectors in this study will likely gain U.S. Food and Drug Administration approval for commercialization in the near future. This study characterized the capsid structures of these clinical vectors at atomic resolution using cryo-electron microscopy and image reconstruction for comparative analysis. The analysis suggested two key residues, S269 and N472, as determinants of BBB crossing for AAVrh.10 and AAVrh.39, a feature utilized for central nervous system delivery of therapeutic genes. The structure information thus provides a platform for engineering to improve receptor retargeting or tissue specificity. These are important challenges in the field that need attention. Capsid structure information also provides knowledge potentially applicable for regulatory product approval.

rAAV-Mediated Cochlear Gene Therapy: Prospects and Challenges for Clinical Application

Over the last decade, pioneering molecular gene therapy for inner-ear disorders have achieved experimental hearing improvements after a single local or systemic injection of adeno-associated, virus-derived vectors (rAAV for recombinant AAV) encoding an extra copy of a normal gene, or ribozymes used to modify a genome. These results hold promise for treating congenital or later-onset hearing loss resulting from monogenic disorders with gene therapy approaches in patients. In this review, we summarize the current state of rAAV-mediated inner-ear gene therapies including the choice of vectors and delivery routes, and discuss the prospects and obstacles for the future development of efficient clinical rAAV-mediated cochlear gene medicine therapy.

Preparation and Administration of Adeno-associated Virus Vectors for Corneal Gene Delivery

Gene delivery approaches using adeno-associated virus (AAV) vectors are currently the preferred method for human gene therapy applications and have demonstrated success in clinical trials for a diverse set of diseases including retinal blindness. To date, no clinical trials using AAV gene therapy in the anterior eye have been initiated; however, corneal gene delivery appears to be an attractive approach for treating both corneal and ocular surface diseases. Multiple preclinical studies by our lab and others have demonstrated efficient AAV vector-mediated gene delivery to the cornea for immunomodulation, anti-vascularization, and enzyme supplementation. Interestingly, the route of AAV vector administration and nuances such as administered volume influence vector tropism and transduction efficiency. In this chapter, a detailed protocol for AAV vector production and specific approaches for AAV-mediated gene transfer to the cornea via subconjunctival and intrastromal injections are described.

Structural and cellular biology of adeno-associated virus attachment and entry

Adeno-associated virus (AAV) is a nonenveloped, ssDNA virus in the parvovirus family, which has become one of the leading candidate vectors for human gene therapy. AAV has been studied extensively to identify host cellular factors involved in infection, as well as to identify capsid variants that confer clinically favorable transduction profiles ex vivo and in vivo. Recent advances in technology have allowed for direct genetic approaches to be used to more comprehensively characterize host factors required for AAV infection and allowed for identification of a critical multi-serotype receptor, adeno-associated virus receptor (AAVR). In this chapter, we will discuss the interactions of AAV with its glycan and proteinaceous receptors and describe the host and viral components involved in AAV entry, which requires cellular attachment, endocytosis, trafficking to the trans-Golgi network and nuclear import. AAV serves as a paradigm for entry of nonenveloped viruses. Furthermore, we will discuss the potential of utilizing our increased understanding of virus-host interactions during AAV entry to develop better AAV-based therapeutics, with a focus on host factors and capsid interactions involved in in vivo tropism.

GPR108 Is a Highly Conserved AAV Entry Factor

Adeno-associated virus (AAV) is a highly promising gene transfer vector, yet major cellular requirements for AAV entry are poorly understood. Using a genome-wide CRISPR screen for entry of evolutionarily divergent serotype AAVrh32.33, we identified GPR108, a member of the G protein-coupled receptor superfamily, as an AAV entry factor. Of greater than 20 divergent AAVs across all AAV clades tested in human cell lines, only AAV5 transduction was unaffected in the GPR108 knockout (KO). GPR108 dependency was further shown in murine and primary cells in vitro. These findings are further validated in vivo, as the Gpr108 KO mouse demonstrates 10- to 100-fold reduced expression for AAV8 and rh32.33 but not AAV5. Mechanistically, both GPR108 N- and C-terminal domains are required for transduction, and on the capsid, a VP1 unique domain that is not conserved on AAV5 can be transferred to confer GPR108 independence onto AAV2 chimeras. In vitro binding and fractionation studies indicate reduced nuclear import and cytosolic accumulation in the absence of GPR108. We thus have identified the second of two AAV entry factors that is conserved between mice and humans relevant both in vitro and in vivo, further providing a mechanistic understanding to the tropism of AAV gene therapy vectors.

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.

Targeting microglia with lentivirus and AAV: Recent advances and remaining challenges

Microglia have emerged as a critical component of neurodegenerative diseases. Genetic manipulation of microglia can elucidate their functional impact in disease. In neuroscience, recombinant viruses such as lentiviruses and adeno-associated viruses (AAVs) have been successfully used to target various cell types in the brain, although effective transduction of microglia is rare. In this review, we provide a short background of lentiviruses and AAVs, and strategies for designing recombinant viral vectors. Then, we will summarize recent literature on successful microglial transductions in vitro and in vivo, and discuss the current challenges. Finally, we provide guidelines for reporting the efficiency and specificity of viral targeting in microglia, which will enable the microglial research community to assess and improve methodologies for future studies.

Polarized AAVR expression determines infectivity by AAV gene therapy vectors

Adeno-associated virus (AAV) has been investigated to transfer the cystic fibrosis transmembrane conductance regulator (CFTR) to airways. Inhaled AAV2-CFTR in people with cystic fibrosis (CF) is safe, but inefficient. In vitro, AAV2 transduction of human airway epithelia on the apical (luminal) side is inefficient, but efficient basolaterally. We previously selected AAV2.5T, a novel capsid that apically transduces CF human airway epithelia and efficiently restores CFTR function. We hypothesize the AAV receptor (AAVR) is basolaterally localized, and that AAV2.5T utilizes an alternative apical receptor. We found AAVR in human airway epithelia by western blot and RNA-Seq analyses. Using immunocytochemistry we did not find endogenous AAVR at membranes but overexpression localized AAVR to the basolateral membrane, where it preferentially increased transduction. Anti-AAVR antibodies blocked transduction by AAV2 from the basolateral side but not AAV2.5T from the apical side, suggesting a unique apical receptor. Finally, we found infection by AAV2 but not AAV2.5T was blocked by CRISPR knockout of AAVR in cell lines. Our data suggest the absence of apical AAVR is rate limiting for AAV2, and efficient transduction by AAV2.5T is accomplished using an AAVR independent pathway. Our findings inform the development of gene therapy for CF, and AAV vectors in general.

Reverse transduction can improve efficiency of AAV vectors in transduction-resistant cells

Reverse transduction, also known as substrate-mediated gene delivery, is a strategy in which viral vectors are first coated onto a surface that subsequently comes into contact with mammalian cells. The cells internalize the surface-attached vectors, resulting in transgene expression. We hypothesized that forcing the interaction between cells and adeno-associated virus (AAV) vectors through a reverse transduction format would increase in vitro gene delivery efficiencies of the vectors in transduction-resistant cells. We tested this hypothesis by comparing the gene delivery efficiencies of three AAV serotypes using either standard or reverse transduction approaches. Our study reveals reverse transduction of AAV7 and AAV9 can significantly improve their delivery efficiencies. In contrast, AAV2 does not perform better under the reverse transduction format. Interestingly, increased vector uptake by cells does not provide a complete explanation for the increased transduction efficiency. Our findings offer a simple and practical method for improving transduction outcomes in vitro in cell types less permissive to a particular AAV vector.

AAV-8 and AAV-9 Vectors Cooperate with Serum Proteins Differently Than AAV-1 and AAV-6

Under intravenous delivery, recombinant adeno-associated vectors (rAAVs) interact with blood-borne components in ways that can critically alter their therapeutic efficiencies. We have previously shown that interaction with human galectin 3 binding protein dramatically reduces rAAV-6 efficacy, whereas binding of mouse C-reactive protein improves rAAV-1 and rAAV-6 transduction effectiveness. Herein we have assessed, through qualitative and quantitative studies, the proteins from mouse and human sera that bind with rAAV-8 and rAAV-9, two vectors that are being considered for clinical trials for patients with neuromuscular disorders. We show that, in contrast to rAAV-1 and rAAV-6, there was a substantial similarity in protein binding patterns between mouse and human sera for these vector serotypes. To establish an in vivo role for the vector binding of these sera proteins, we chose to study platelet factor 4 (PF4), which interacts with both vectors in both mouse and human sera. Experiments using PF4-knockout mice showed that a complete lack of PF4 did not alter skeletal muscle transduction for these vectors, whereas heart transduction was moderately improved. Our results strongly support our position that the impact of serum proteins on the transduction properties of rAAV-8 and rAAV-9, already observed in mouse models, should be similar in human preclinical trials.

High-Throughput Dissection of AAV-Host Interactions: The Fast and the Curious

Over 50 years after its initial description, adeno-associated virus (AAV) remains the most exciting but also most elusive study object in basic or applied virology. On the one hand, its simple structure not only facilitates investigations into virus biology but, combined with the availability of numerous natural AAV variants with distinct infection efficiency and specificity, also makes AAV a preferred substrate for engineering of gene delivery vectors. On the other hand, it is striking to witness a recent flurry of reports that highlight and partially close persistent gaps in our understanding of AAV virus and vector biology. This is all the more perplexing considering that recombinant AAVs have already been used in >160 clinical trials and recently been commercialized as gene therapeutics. Here, we discuss a reason for these advances in AAV research, namely, the advent and application of powerful high-throughput technology for dissection of AAV-host interactions and optimization of AAV gene therapy vectors. As relevant examples, we focus on the discovery of (i) a "new" cellular AAV receptor, AAVR, (ii) host restriction factors for AAV entry, and (iii) AAV capsid determinants that mediate trafficking through the blood-brain barrier. While items i/ii are prototypes of extra- or intracellular AAV host factors that were identified via high-throughput screenings, item iii exemplifies the power of molecular evolution to investigate the virus itself. In the future, we anticipate that these and other key technologies will continue to accelerate the dissection of AAV biology and will yield a wealth of new designer viruses for clinical use.

Recent progress and considerations for AAV gene therapies targeting the central nervous system

BACKGROUND

Neurodevelopmental disorders, as a class of diseases, have been particularly difficult to treat even when the underlying cause(s), such as genetic alterations, are understood. What treatments do exist are generally not curative and instead seek to improve quality of life for affected individuals. The advent of gene therapy via gene replacement offers the potential for transformative therapies to slow or even stop disease progression for current patients and perhaps minimize or prevent the appearance of symptoms in future patients.

MAIN BODY

This review focuses on adeno-associated virus (AAV) gene therapies for diseases of the central nervous system. An overview of advances in AAV vector design for therapy is provided, along with a description of current strategies to develop AAV vectors with tailored tropism. Next, progress towards treatment of neurodegenerative diseases is presented at both the pre-clinical and clinical stages, focusing on a few select diseases to highlight broad categories of therapeutic parameters. Special considerations for more challenging cases are then discussed in addition to the immunological aspects of gene therapy.

CONCLUSION

With the promising clinical trial results that have been observed for the latest AAV gene therapies and continued pre-clinical successes, the question is no longer whether a therapy can be developed for certain neurodevelopmental disorders, but rather, how quickly.

Adeno-Associated Virus (AAV) as a Vector for Gene Therapy

There has been a resurgence in gene therapy efforts that is partly fueled by the identification and understanding of new gene delivery vectors. Adeno-associated virus (AAV) is a non-enveloped virus that can be engineered to deliver DNA to target cells, and has attracted a significant amount of attention in the field, especially in clinical-stage experimental therapeutic strategies. The ability to generate recombinant AAV particles lacking any viral genes and containing DNA sequences of interest for various therapeutic applications has thus far proven to be one of the safest strategies for gene therapies. This review will provide an overview of some important factors to consider in the use of AAV as a vector for gene therapy.

An Alternate Route for Adeno-associated Virus (AAV) Entry Independent of AAV Receptor

Determinants and mechanisms of cell attachment and entry steer adeno-associated virus (AAV) in its utility as a gene therapy vector. Thus far, a systematic assessment of how diverse AAV serotypes engage their proteinaceous receptor AAVR (KIAA0319L) to establish transduction has been lacking, despite potential implications for cell and tissue tropism. Here, a large set of human and simian AAVs as well as in silico-reconstructed ancestral AAV capsids were interrogated for AAVR usage. We identified a distinct AAV capsid lineage comprised of AAV4 and AAVrh32.33 that can bind and transduce cells in the absence of AAVR, independent of the multiplicity of infection. Virus overlay assays and rescue experiments in nonpermissive cells demonstrate that these AAVs are unable to bind to or use the AAVR protein for entry. Further evidence for a distinct entry pathway was observed in vivo, as AAVR knockout mice were equally as permissive to transduction by AAVrh32.33 as wild-type mice upon systemic injection. We interestingly observe that some AAV capsids undergo a low level of transduction in the absence of AAVR, both in vitro and in vivo, suggesting that some capsids may have a multimodal entry pathway. In aggregate, our results demonstrate that AAVR usage is conserved among all primate AAVs except for those of the AAV4 lineage, and a non-AAVR pathway may be available to other serotypes. This work furthers our understanding of the entry of AAV, a vector system of broad utility in gene therapy.IMPORTANCE Adeno-associated virus (AAV) is a nonpathogenic virus that is used as a vehicle for gene delivery. Here, we have identified several situations in which transduction is retained in both cell lines and a mouse model in the absence of a previously defined entry receptor, AAVR. Defining the molecular determinants of the infectious pathway of this highly relevant viral vector system can help refine future applications and therapies with this vector.