Terminal N-linked galactose is the primary receptor for adeno-associated virus 9

Functional analysis of the putative integrin recognition motif on adeno-associated virus 9

Adeno-associated viruses (AAVs) display a highly conserved NGR motif on the capsid surface. Earlier studies have established this tripeptide motif as being essential for integrin-mediated uptake of recombinant AAV serotype 2 (AAV2) in cultured cells. However, functional attributes of this putative integrin recognition motif in other recombinant AAV serotypes displaying systemic transduction in vivo remain unknown. In this study, we dissect the biology of an integrin domain capsid mutant derived from the human isolate AAV9 in mice. The AAV9/NGA mutant shows decreased systemic transduction in mice. This defective phenotype was accompanied by rapid clearance of mutant virions from the blood circulation and nonspecific sequestration by the spleen. Transient vascular hyperpermeability, induced by histamine coinjection, exacerbated AAV9/NGA uptake by the spleen but not the liver. However, such treatment did not affect AAV9 virions, suggesting a potential entry/post-entry defect for the mutant in different tissues. Further characterization revealed modestly decreased cell surface binding but a more pronounced defect in the cellular entry of mutant virions. These findings were corroborated by the observation that blocking multiple integrins adversely affected recombinant AAV9 transduction in different cell types, albeit with variable efficiencies. From a structural perspective, we observed that the integrin recognition motif is located in close proximity to the galactose binding footprint on AAV9 capsids and postulate that this feature could influence cell surface attachment, cellular uptake at the tissue level, and systemic clearance by the reticuloendothelial system.

Intravenous AAV9 efficiently transduces myenteric neurons in neonate and juvenile mice

Gene therapies for neurological diseases with autonomic or gastrointestinal involvement may require global gene expression. Gastrointestinal complications are often associated with Parkinson's disease and autism. Lewy bodies, a pathological hallmark of Parkinson's brains, are routinely identified in the neurons of the enteric nervous system (ENS) following colon biopsies from patients. The ENS is the intrinsic nervous system of the gut, and is responsible for coordinating the secretory and motor functions of the gastrointestinal tract. ENS dysfunction can cause severe patient discomfort, malnourishment, or even death as in intestinal pseudo-obstruction (Ogilvie syndrome). Importantly, ENS transduction following systemic vector administration has not been thoroughly evaluated. Here we show that systemic injection of AAV9 into neonate or juvenile mice results in transduction of 25-57% of ENS myenteric neurons. Transgene expression was prominent in choline acetyltransferase positive cells, but not within vasoactive intestinal peptide or neuronal nitric oxide synthase cells, suggesting a bias for cells involved in excitatory signaling. AAV9 transduction in enteric glia is very low compared to CNS astrocytes. Enteric glial transduction was enhanced by using a glial specific promoter. Furthermore, we show that AAV8 results in comparable transduction in neonatal mice to AAV9 though AAV1, 5, and 6 are less efficient. These data demonstrate that systemic AAV9 has high affinity for peripheral neural tissue and is useful for future therapeutic development and basic studies of the ENS.

Adeno-associated virus: fit to serve

Adeno-associated virus (AAV) is a helper-dependent parvovirus which has not been linked with human disease. This aspect, in combination with its broad cell and tissue tropism, and limited viral host response has made it an attractive vector system for gene therapy. The viral protein capsid, the primary interface with the host, is the main determinant for these phenotypes, is highly variable, and is most subject to pressures during replication. Here, we explore the evolutionary path of AAV and other parvoviruses in respect to these phenotypes, as well as directed evolution and engineering strategies that have exploited the lessons learned from natural selection in order to address remaining limitations of AAV as a therapeutic gene transfer platform.

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

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

The expression pattern of systemically injected AAV9 in the developing mouse retina is determined by age

Systemic delivery of AAV9 offers the potential for widespread and efficient gene delivery to the retina, and may thus be a useful approach for treatment of disease where intraocular injections are not possible, for syndromes affecting multiple organs, or where early intervention is required. The expression resulting from intravenous injection of AAV9 is more efficient in neonates than adults, and here we characterize the effect of age on retinal transduction of AAV9 in the mouse retina. We find that the pattern of expression in neonatal mice is correlated to the development of the retinal vasculature, and that the area of the retinal transduction as well as the cell types infected vary depending on the age at injection. Furthermore, we demonstrate that sequential injections of AAV9 vectors carrying two different transgenes infect adjacent areas of the retina, providing a larger area of coverage. Lastly, we show that the retina's endogenous spatiotemporal expression pattern of Mfsd2a, a protein associated with the maturation of a functional blood-brain barrier, coincides with suppression of retinal transduction by intravenously-delivered AAV9, suggesting that AAV9 crosses the blood-retina barrier through transcytosis.

Adeno-associated viral vectors show serotype specific transduction of equine joint tissue explants and cultured monolayers

Adeno-associated virus (AAV) receptors range from heparan sulfate proteoglycan to sialic acid moieties present on cell surfaces. Abundance of the glycan profiles is greatly influenced by animal species, cell type, and culture conditions. The objective of this study was to determine whether AAV serotypes' transduction efficiencies specifically in the equine monolayer culture model are an accurate representation of transduction efficiencies in tissue explants, a model more closely related to in vivo transduction. It was found that AAV 2 and 2.5 transduced cells more efficiently in explants than in monolayers. Through experiments involving assessing enzyme degradation of cell surface proteoglycans, this change could not be attributed to differences in the extra cellular matrix (ECM), but a similar change in AAV 5 transduction efficiency could be readily explained by differences in cell surface sialylated glycan. Unexpectedly it was found that in a small but diverse sample of horses evidence for serum neutralizing antibodies was only found to AAV 5. This suggests a unique relationship between this capsid and the equine host or an unresolved relationship between similar bovine AAV and the AAV 5 capsid immune response.

Parvovirus glycan interactions

Members of the Parvoviridae utilize glycan receptors for cellular attachment and subsequent interactions determine transduction efficiency or pathogenic outcome. This review focuses on the identity of the glycan receptors utilized, their capsid binding footprints, and a discussion of the overlap of these sites with tropism, transduction, and pathogenicity determinants. Despite high sequence diversity between the different genera, most parvoviruses bind to negatively charged glycans, such as sialic acid and heparan sulfate, abundant on cell surface membranes. The capsid structure of these viruses exhibit high structural homology enabling common regions to be utilized for glycan binding. At the same time the sequence diversity at the common footprints allows for binding of different glycans or differential binding of the same glycan.

Adeno-associated virus vectors as therapeutic and investigational tools in the cardiovascular system

The use of vectors based on the small parvovirus adeno-associated virus has gained significant momentum during the past decade. Their high efficiency of transduction of postmitotic tissues in vivo, such as heart, brain, and retina, renders these vectors extremely attractive for several gene therapy applications affecting these organs. Besides functional correction of different monogenic diseases, the possibility to drive efficient and persistent transgene expression in the heart offers the possibility to develop innovative therapies for prevalent conditions, such as ischemic cardiomyopathy and heart failure. Therapeutic genes are not only restricted to protein-coding complementary DNAs but also include short hairpin RNAs and microRNA genes, thus broadening the spectrum of possible applications. In addition, several spontaneous or engineered variants in the virus capsid have recently improved vector efficiency and expanded their tropism. Apart from their therapeutic potential, adeno-associated virus vectors also represent outstanding investigational tools to explore the function of individual genes or gene combinations in vivo, thus providing information that is conceptually similar to that obtained from genetically modified animals. Finally, their single-stranded DNA genome can drive homology-directed gene repair at high efficiency. Here, we review the main molecular characteristics of adeno-associated virus vectors, with a particular view to their applications in the cardiovascular field.

Glycan Engagement by Viruses: Receptor Switches and Specificity

A large number of viruses, including many human pathogens, bind cell-surface glycans during the initial steps of infection. Viral glycan receptors such as glycosaminoglycans and sialic acid-containing carbohydrates are often negatively charged, but neutral glycans such as histo-blood group antigens can also function as receptors. The engagement of glycans facilitates attachment and entry and, consequently, is often a key determinant of the host range, tissue tropism, pathogenicity, and transmissibility of viruses. Here, we review current knowledge about virus-glycan interactions using representative crystal structures of viral attachment proteins in complex with glycans. We illuminate the determinants of specificity utilized by different glycan-binding viruses and explore the potential of these interactions for switching receptor specificities within or even between glycan classes. A detailed understanding of these parameters is important for the prediction of binding sites where structural information is not available, and is invaluable for the development of antiviral therapeutics.

Gene transfer properties and structural modeling of human stem cell-derived AAV

Adeno-associated virus (AAV) vectors are proving to be remarkably successful for in vivo gene delivery. Based upon reports of abundant AAV in the human marrow, we tested CD34(+) hematopoietic stem cells for the presence of natural AAV. Here, we report for the first time, the presence of novel AAV variants in healthy CD34(+) human peripheral blood stem cells. The majority of healthy peripheral blood stem cell donors were found to harbor AAV in their CD34(+) cells. Every AAV isolated from CD34(+) cells mapped to AAV Clade F. Gene transfer vectors derived from these novel AAVs efficiently underwent entry and postentry processing in human cord blood stem cells and supported stable gene transfer into long-term, in vivo engrafting human HSCs significantly better than other serotypes. AAVHSC-transduced human CD34(+) cells engrafted in vivo and gave rise to differentiated transgene-expressing progeny. Importantly, gene-marked CD34(+) stem cells persisted long term in xenograft recipients, indicating transduction of primitive progenitors. Notably, correlation of structure with function permitted identification of potential capsid components important for HSC transduction. Thus, AAVHSCs represent a new class of genetic vectors for the manipulation of HSC genomes.

Biodistribution of adeno-associated virus serotype 9 (AAV9) vector after intrathecal and intravenous delivery in mouse

Adeno-associated virus serotype 9 (AAV9)-mediated gene transfer has been reported in central nervous system (CNS) and peripheral tissues. The current study compared the pattern of expression of Green Fluorescent Protein (GFP) across the mouse CNS and selected peripheral tissues after intrathecal (i.t.) or intravenous (i.v.) delivery of equivalent doses of single-stranded AAV9 vector. After i.t. delivery, GFP immunoreactivity (-ir) was observed in spinal neurons, primary afferent fibers and corresponding primary sensory neurons at all spinal levels. Robust transduction was seen in small and large dorsal root ganglion (DRG) neurons as well as trigeminal and vagal primary afferent neurons. Transduction efficiency in sensory ganglia was substantially lower in i.v. treated mice. In brain, i.v. delivery yielded GFP-immunoreactivity (-ir) primarily in spinal trigeminal tract, pituitary, and scattered isolated neurons and astrocytes. In contrast, after i.t. delivery, GFP-ir was widespread throughout CNS, with greater intensity and more abundant neuropil-like staining at 6 weeks compared to 3 weeks. Brain regions with prominent GFP-ir included cranial nerve nuclei, ventral pons, cerebellar cortex, hippocampus, pituitary, choroid plexus, and selected nuclei of midbrain, thalamus and hypothalamus. In cortex, GFP-ir was associated with blood vessels, and was seen in both neurons and astrocytes. In the periphery, GFP-ir in colon and ileum was present in the enteric nervous system in both i.v. and i.t. treated mice. Liver and adrenal cortex, but not adrenal medulla, also showed abundant GFP-ir after both routes of delivery. In summary, i.t. delivery yielded higher transduction efficiency in sensory neurons and the CNS. The observation of comparable gene transfer to peripheral tissues using the two routes indicates that a component of i.t. delivered vector is redistributed from the subarachnoid space to the systemic circulation.

Systemic gene delivery to the central nervous system using Adeno-associated virus

Adeno-associated virus (AAV)-mediated gene delivery has emerged as an effective and safe tool for both preclinical and clinical studies of neurological disorders. The recent discovery that several serotypes are able to cross the blood–brain barrier when administered systemically has been a real breakthrough in the field of neurodegenerative diseases. Widespread transgene expression after systemic injection could spark interest as a therapeutic approach. Such strategy will avoid invasive brain surgery and allow non-focal gene therapy promising for CNS diseases affecting large portion of the brain. Here, we will review the recent results achieved through different systemic routes of injection generated in the last decade using systemic AAV-mediated delivery and propose a brief assessment of their values. In particular, we emphasize how the methods used for virus engineering could improve brain transduction after peripheral delivery.

rAAV9 combined with renal vein injection is optimal for kidney-targeted gene delivery: conclusion of a comparative study

Effective gene therapy strategies for the treatment of kidney disorders remain elusive. We report an optimized kidney-targeted gene delivery strategy using recombinant adeno-associated virus (rAAV) administered via retrograde renal vein injection in mice. Renal vein injection of rAAV consistently resulted in superior kidney transduction compared with tail vein injection using as little as half the tail vein dose. We compared rAAV5, 6, 8 and 9, containing either green fluorescent protein (GFP) or luciferase reporter genes driven by the Cytomegalovirus promoter. We demonstrated that although rAAV6 and 8 injected via renal vein transduced the kidney, transgene expression was mainly restricted to the medulla. Transgene expression was systematically low after rAAV5 injection, attributed to T-cell immune response, which could be overcome by transient immunosuppression. However, rAAV9 was the only serotype that permitted high-transduction efficiency of both the cortex and medulla. Moreover, both the glomeruli and tubules were targeted, with a higher efficiency within the glomeruli. To improve the specificity of kidney-targeted gene delivery with rAAV9, we used the parathyroid hormone receptor 'kidney-specific' promoter. We obtained a more efficient transgene expression within the kidney, and a significant reduction in other tissues. Our work represents the first comprehensive and clinically relevant study for kidney gene delivery.

Adeno-associated virus serotypes 1, 8, and 9 share conserved mechanisms for anterograde and retrograde axonal transport

Adeno-associated virus (AAV) vectors often undergo long-distance axonal transport after brain injection. This leads to transduction of brain regions distal to the injection site, although the extent of axonal transport and distal transduction varies widely among AAV serotypes. The mechanisms driving this variability are poorly understood. This is a critical problem for applications that require focal gene expression within a specific brain region, and also impedes the utilization of vector transport for applications requiring widespread delivery of transgene to the brain. Here, we compared AAV serotypes 1 and 9, which frequently demonstrate distal transduction, with serotype 8, which rarely spreads beyond the injection site. To examine directional AAV transport in vitro, we used a microfluidic chamber to apply dye-labeled AAV to the axon termini or to the cell bodies of primary rat embryonic cortical neurons. All three serotypes were actively transported along axons, with transport characterized by high velocities and prolonged runs in both the anterograde and retrograde directions. Coinfection with pairs of serotypes indicated that AAV1, 8, and 9 share the same intracellular compartments for axonal transport. In vivo, both AAV8 and 9 demonstrated anterograde and retrograde transport within a nonreciprocal circuit after injection into adult mouse brain, with highly similar distributions of distal transduction. However, in mass-cultured neurons, we found that AAV1 was more frequently transported than AAV8 or 9, and that the frequency of AAV9 transport could be enhanced by increasing receptor availability. Thus, while these serotypes share conserved mechanisms for axonal transport both in vitro and in vivo, the frequency of transport can vary among serotypes, and axonal transport can be markedly increased by enhancing vector uptake. This suggests that variability in distal transduction in vivo likely results from differential uptake at the plasma membrane, rather than fundamental differences in transport mechanisms among AAV serotypes.

Intracisternal delivery of AAV9 results in oligodendrocyte and motor neuron transduction in the whole central nervous system of cats

Systemic and intracerebrospinal fluid delivery of adeno-associated virus serotype 9 (AAV9) has been shown to achieve widespread gene delivery to the central nervous system (CNS). However, after systemic injection, the neurotropism of the vector has been reported to vary according to age at injection, with greater neuronal transduction in newborns and preferential glial cell tropism in adults. This difference has not yet been reported after cerebrospinal fluid (CSF) delivery. The present study analyzed both neuronal and glial cell transduction in the CNS of cats according to age of AAV9 CSF injection. In both newborns and young cats, administration of AAV9-GFP in the cisterna magna resulted in high levels of motor neurons (MNs) transduction from the cervical (84±5%) to the lumbar (99±1%) spinal cord, demonstrating that the remarkable tropism of AAV9 for MNs is not affected by age at CSF delivery. Surprisingly, numerous oligodendrocytes were also transduced in the brain and in the spinal cord white matter of young cats, but not of neonates, indicating that (i) age of CSF delivery influences the tropism of AAV9 for glial cells and (ii) AAV9 intracisternal delivery could be relevant for both the treatment of MN and demyelinating disorders.

Adeno-associated virus vectors and neurological gene therapy

Gene therapy has strong potential for treating a variety of genetic disorders, as demonstrated in recent clinical trials. There is unfortunately no scarcity of disease targets, and the grand challenge in this field has instead been the development of safe and efficient gene delivery platforms. To date, approximately two thirds of the 1800 gene therapy clinical trials completed worldwide have used viral vectors. Among these, adeno-associated virus (AAV) has emerged as particularly promising because of its impressive safety profile and efficiency in transducing a wide range of cell types. Gene delivery to the CNS involves both considerable promise and unique challenges, and better AAV vectors are thus needed to translate CNS gene therapy approaches to the clinic. This review discusses strategies for vector design, potential routes of administration, immune responses, and clinical applications of AAV in the CNS.

Profiling of glycan receptors for minute virus of mice in permissive cell lines towards understanding the mechanism of cell recognition

The recognition of sialic acids by two strains of minute virus of mice (MVM), MVMp (prototype) and MVMi (immunosuppressive), is an essential requirement for successful infection. To understand the potential for recognition of different modifications of sialic acid by MVM, three types of capsids, virus-like particles, wild type empty (no DNA) capsids, and DNA packaged virions, were screened on a sialylated glycan microarray (SGM). Both viruses demonstrated a preference for binding to 9-O-methylated sialic acid derivatives, while MVMp showed additional binding to 9-O-acetylated and 9-O-lactoylated sialic acid derivatives, indicating recognition differences. The glycans recognized contained a type-2 Galβ1-4GlcNAc motif (Neu5Acα2-3Galβ1-4GlcNAc or 3′SIA-LN) and were biantennary complex-type N-glycans with the exception of one. To correlate the recognition of the 3′SIA-LN glycan motif as well as the biantennary structures to their natural expression in cell lines permissive for MVMp, MVMi, or both strains, the N- and O-glycans, and polar glycolipids present in three cell lines used for in vitro studies, A9 fibroblasts, EL4 T lymphocytes, and the SV40 transformed NB324K cells, were analyzed by MALDI-TOF/TOF mass spectrometry. The cells showed an abundance of the sialylated glycan motifs recognized by the viruses in the SGM and previous glycan microarrays supporting their role in cellular recognition by MVM. Significantly, the NB324K showed fucosylation at the non-reducing end of their biantennary glycans, suggesting that recognition of these cells is possibly mediated by the Lewis X motif as in 3′SIA-Le^X identified in a previous glycan microarray screen.

Methods for gene transfer to the central nervous system

Gene transfer is an increasingly utilized approach for research and clinical applications involving the central nervous system (CNS). Vectors for gene transfer can be as simple as an unmodified plasmid, but more commonly involve complex modifications to viruses to make them suitable gene delivery vehicles. This chapter will explain how tools for CNS gene transfer have been derived from naturally occurring viruses. The current capabilities of plasmid, retroviral, adeno-associated virus, adenovirus, and herpes simplex virus vectors for CNS gene delivery will be described. These include both focal and global CNS gene transfer strategies, with short- or long-term gene expression. As is described in this chapter, an important aspect of any vector is the cis-acting regulatory elements incorporated into the vector genome that control when, where, and how the transgene is expressed.

Differential adeno-associated virus serotype-specific interaction patterns with synthetic heparins and other glycans

All currently identified primary receptors of adeno-associated virus (AAV) are glycans. Depending on the AAV serotype, these carbohydrates range from heparan sulfate proteoglycans (HSPG), through glycans with terminal α2-3 or α2-6 sialic acids, to terminal galactose moieties. Receptor identification has largely relied on binding to natural compounds, defined glycan-presenting cell lines, or enzyme-mediated glycan modifications. Here, we describe a comparative binding analysis of highly purified, fluorescent-dye-labeled AAV vectors of various serotypes on arrays displaying over 600 different glycans and on a specialized array with natural and synthetic heparins. Few glycans bind AAV specifically in a serotype-dependent manner. Differential glycan binding was detected for the described sialic acid-binding AAV serotypes 1, 6, 5, and 4. The natural heparin binding serotypes AAV2, -3, -6, and -13 displayed differential binding to selected synthetic heparins. AAV7, -8, -rh.10, and -12 did not bind to any of the glycans present on the arrays. For discrimination of AAV serotypes 1 to 6 and 13, minimal binding moieties are identified. This is the first study to differentiate the natural mixed heparin binding AAV serotypes 2, 3, 6, and 13 by differential binding to specific synthetic heparins. Also, sialic acid binding AAVs display differential glycan binding specificities. The findings are relevant for further dissection of AAV host cell interaction. Moreover, the definition of single AAV-discriminating glycan binders opens the possibility for glycan microarray-based discrimination of AAV serotypes in gene therapy.

An emerging adeno-associated viral vector pipeline for cardiac gene therapy

The naturally occurring adeno-associated virus (AAV) isolates display diverse tissue tropisms in different hosts. Robust cardiac transduction in particular has been reported for certain AAV strains. Successful applications of these AAV strains in preclinical and clinical settings with a focus on treating cardiovascular disease continue to be reported. At the same time, these studies have highlighted challenges such as cross-species variability in AAV tropism, transduction efficiency, and immunity. Continued progress in our understanding of AAV capsid structure and biology has provided the rationale for designing improved vectors that can possibly address these concerns. The current report provides an overview of cardiotropic AAV, existing gaps in our knowledge, and newly engineered AAV strains that are viable candidates for the cardiac gene therapy clinic.

Structural insights into adeno-associated virus serotype 5

The adeno-associated viruses (AAVs) display differential cell binding, transduction, and antigenic characteristics specified by their capsid viral protein (VP) composition. Toward structure-function annotation, the crystal structure of AAV5, one of the most sequence diverse AAV serotypes, was determined to 3.45-Å resolution. The AAV5 VP and capsid conserve topological features previously described for other AAVs but uniquely differ in the surface-exposed HI loop between βH and βI of the core β-barrel motif and have pronounced conformational differences in two of the AAV surface variable regions (VRs), VR-IV and VR-VII. The HI loop is structurally conserved in other AAVs despite amino acid differences but is smaller in AAV5 due to an amino acid deletion. This HI loop is adjacent to VR-VII, which is largest in AAV5. The VR-IV, which forms the larger outermost finger-like loop contributing to the protrusions surrounding the icosahedral 3-fold axes of the AAVs, is shorter in AAV5, creating a smoother capsid surface topology. The HI loop plays a role in AAV capsid assembly and genome packaging, and VR-IV and VR-VII are associated with transduction and antigenic differences, respectively, between the AAVs. A comparison of interior capsid surface charge and volume of AAV5 to AAV2 and AAV4 showed a higher propensity of acidic residues but similar volumes, consistent with comparable DNA packaging capacities. This structure provided a three-dimensional (3D) template for functional annotation of the AAV5 capsid with respect to regions that confer assembly efficiency, dictate cellular transduction phenotypes, and control antigenicity.

Analysis of transduction efficiency, tropism and axonal transport of AAV serotypes 1, 2, 5, 6, 8 and 9 in the mouse brain

Recombinant Adeno-associated virus vectors (rAAV) are widely used for gene delivery and multiple naturally occurring serotypes have been harnessed to target cells in different tissues and organs including the brain. Here, we provide a detailed and quantitative analysis of the transduction profiles of rAAV vectors based on six of the most commonly used serotypes (AAV1, AAV2, AAV5, AAV6, AAV8, AAV9) that allows systematic comparison and selection of the optimal vector for a specific application. In our studies we observed marked differences among serotypes in the efficiency to transduce three different brain regions namely the striatum, hippocampus and neocortex of the mouse. Despite the fact that the analyzed serotypes have the general ability to transduce all major cell types in the brain (neurons, microglia, astrocytes and oligodendrocytes), the expression level of a reporter gene driven from a ubiquitous promoter varies significantly for specific cell type / serotype combinations. For example, rAAV8 is particularly efficient to drive transgene expression in astrocytes while rAAV9 appears well suited for the transduction of cortical neurons. Interestingly, we demonstrate selective retrograde transport of rAAV5 along axons projecting from the ventral part of the entorhinal cortex to the dentate gyrus. Furthermore, we show that self-complementing rAAV can be used to significantly decrease the time required for the onset of transgene expression in the mouse brain.

Comparative analysis of adeno-associated virus capsid stability and dynamics

Icosahedral viral capsids are obligated to perform a thermodynamic balancing act. Capsids must be stable enough to protect the genome until a suitable host cell is encountered yet be poised to bind receptor, initiate cell entry, navigate the cellular milieu, and release their genome in the appropriate replication compartment. In this study, serotypes of adeno-associated virus (AAV), AAV1, AAV2, AAV5, and AAV8, were compared with respect to the physical properties of their capsids that influence thermodynamic stability. Thermal stability measurements using differential scanning fluorimetry, differential scanning calorimetry, and electron microscopy showed that capsid melting temperatures differed by more than 20°C between the least and most stable serotypes, AAV2 and AAV5, respectively. Limited proteolysis and peptide mass mapping of intact particles were used to investigate capsid protein dynamics. Active hot spots mapped to the region surrounding the 3-fold axis of symmetry for all serotypes. Cleavages also mapped to the unique region of VP1 which contains a phospholipase domain, indicating transient exposure on the surface of the capsid. Data on the biophysical properties of the different AAV serotypes are important for understanding cellular trafficking and is critical to their production, storage, and use for gene therapy. The distinct differences reported here provide direction for future studies on entry and vector production.

Multiple roles for sialylated glycans in determining the cardiopulmonary tropism of adeno-associated virus 4

Adeno-associated virus 4 (AAV4) is one of the most divergent serotypes among known AAV isolates. Mucins or O-linked sialoglycans have been identified as the primary attachment receptors for AAV4 in vitro. However, little is known about the role(s) played by sialic acid interactions in determining AAV4 tissue tropism in vivo. In the current study, we first characterized two loss-of-function mutants obtained by screening a randomly mutated AAV4 capsid library. Both mutants harbored several amino acid residue changes localized to the 3-fold icosahedral symmetry axes on the AAV4 capsid and displayed low transduction efficiency in vitro. This defect was attributed to decreased cell surface binding as well as uptake of mutant virions. These results were further corroborated by low transgene expression and recovery of mutant viral genomes in cardiac and lung tissue following intravenous administration in mice. Pharmacokinetic analysis revealed rapid clearance of AAV4 mutants from the blood circulation in conjunction with low hemagglutination potential ex vivo. These results were recapitulated with mice pretreated intravenously with sialidase, directly confirming the role of sialic acids in determining AAV4 tissue tropism. Taken together, our results support the notion that blood-borne AAV4 particles interact sequentially with O-linked sialoglycans expressed abundantly on erythrocytes followed by cardiopulmonary tissues and subsequently for viral cell entry.

Widespread spinal cord transduction by intrathecal injection of rAAV delivers efficacious RNAi therapy for amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) causes motor neuron degeneration and paralysis. No treatment can significantly slow or arrest the disease progression. Mutations in the SOD1 gene cause a subset of familial ALS by a gain of toxicity. In principle, these cases could be treated with RNAi that destroys the mutant mRNA, thereby abolishing the toxic protein. However, no system is available to efficiently deliver the RNAi therapy. Recombinant adenoassociated virus (rAAV) is a promising vehicle due to its long-lasting gene expression and low toxicity. However, ALS afflicts broad areas of the central nervous system (CNS). A lack of practical means to spread rAAV broadly has hindered its application in treatment of ALS. To overcome this barrier, we injected several rAAV serotypes into the cerebrospinal fluid. We found that some rAAV serotypes such as rAAVrh10 and rAAV9 transduced cells throughout the length of the spinal cord following a single intrathecal injection and in the broad forebrain following a single injection into the third ventricle. Furthermore, a single intrathecal injection of rAAVrh10 robustly transduced motor neurons throughout the spinal cord in a non-human primate. These results suggested a therapeutic potential of this vector for ALS. To test this, we injected a rAAVrh10 vector that expressed an artificial miRNA targeting SOD1 into the SOD1G93A mice. This treatment knocked down the mutant SOD1 expression and slowed the disease progression. Our results demonstrate the potential of rAAVs for delivering gene therapy to treat ALS and other diseases that afflict broad areas of the CNS.

Electron microscopy analysis of a disaccharide analog complex reveals receptor interactions of adeno-associated virus

Mechanistic studies of macromolecular complexes often feature X-ray structures of complexes with bound ligands. The attachment of adeno-associated virus (AAV) to cell surface glycosaminoglycans (GAGs) is an example that has not proven amenable to crystallography, because the binding of GAG analogs disrupts lattice contacts. The interactions of AAV with GAGs are of interest in mediating the cell specificity of AAV-based gene therapy vectors. Previous electron microscopy led to differing conclusions on the exact binding site and the existence of large ligand-induced conformational changes in the virus. Conformational changes are expected during cell entry, but it has remained unclear whether the electron microscopy provided evidence of their induction by GAG-binding. Taking advantage of automated data collection, careful processing and new methods of structure refinement, the structure of AAV-DJ complexed with sucrose octasulfate is determined by electron microscopy difference map analysis to 4.8Å resolution. At this higher resolution, individual sulfate groups are discernible, providing a stereochemical validation of map interpretation, and highlighting interactions with two surface arginines that have been implicated in genetic studies. Conformational changes induced by the SOS are modest and limited to the loop most directly interacting with the ligand. While the resolution attainable will depend on sample order and other factors, there are an increasing number of macromolecular complexes that can be studied by cryo-electron microscopy at resolutions beyond 5Å, for which the approaches used here could be used to characterize the binding of inhibitors and other small molecule effectors when crystallography is not tractable.

Engraftment of a galactose receptor footprint onto adeno-associated viral capsids improves transduction efficiency

New viral strains can be evolved to recognize different host glycans through mutagenesis and experimental adaptation. However, such mutants generally harbor amino acid changes that affect viral binding to a single class of carbohydrate receptors. We describe the rational design and synthesis of novel, chimeric adeno-associated virus (AAV) strains that exploit an orthogonal glycan receptor for transduction. A dual glycan-binding AAV strain was first engineered as proof of concept by grafting a galactose (Gal)-binding footprint from AAV serotype 9 onto the heparan sulfate-binding AAV serotype 2. The resulting chimera, AAV2G9, continues to bind heparin affinity columns but interchangeably exploits Gal and heparan sulfate receptors for infection, as evidenced by competitive inhibition assays with lectins, glycans, and parental AAV strains. Although remaining hepatotropic like AAV2, the AAV2G9 chimera mediates rapid onset and higher transgene expression in mice. Similarly, engraftment of the Gal footprint onto the laboratory-derived strain AAV2i8 yielded an enhanced AAV2i8G9 chimera. This new strain remains liver-detargeted like AAV2i8 while selectively transducing muscle tissues at high efficiency, comparable with AAV9. The AAV2i8G9 chimera is a promising vector candidate for targeted gene therapy of cardiac and musculoskeletal diseases. In addition to demonstrating the modularity of glycan receptor footprints on viral capsids, our approach provides design strategies to expand the AAV vector toolkit.

C-reactive protein (CRP) is essential for efficient systemic transduction of recombinant adeno-associated virus vector 1 (rAAV-1) and rAAV-6 in mice

The clinical relevance of gene therapy using the recombinant adeno-associated virus (rAAV) vectors often requires widespread distribution of the vector, and in this case, systemic delivery is the optimal route of administration. Humoral blood factors, such as antibodies or complement, are the first barriers met by the vectors administered systemically. We have found that other blood proteins, galectin 3 binding protein (G3BP) and C-reactive protein (CRP), can interact with different AAV serotypes in a species-specific manner. While interactions of rAAV vectors with G3BP, antibodies, or complement lead to a decrease in vector efficacy, systemic transduction of the CRP-deficient mouse and its respective control clearly established that binding to mouse CRP (mCRP) boosts rAAV vector 1 (rAAV-1) and rAAV-6 transduction efficiency in skeletal muscles over 10 times. Notably, the high efficacy of rAAV-6 in CRP-deficient mice can be restored by reconstitution of the CRP-deficient mouse with mCRP. Human CRP (hCRP) does not interact with either rAAV-1 or rAAV-6, and, consequently, the high efficiency of mCRP-mediated muscle transduction by these serotypes in mice cannot be translated to humans. No interaction of mCRP or hCRP was observed with rAAV-8 and rAAV-9. We show, for the first time, that serum components can significantly enhance rAAV-mediated tissue transduction in a serotype- and species-specific manner. Bioprocessing in body fluids should be considered when transfer of a preclinical proof of concept for AAV-based gene therapy to humans is planned.

Gene transfer to the CNS using recombinant adeno-associated virus

Recombinant adeno-associated virus (rAAV) vectors are great tools for gene transfer due to their ability to mediate long-term gene expression. rAAVs have been used successfully as gene transfer vehicles in multiple animal models of CNS disorders, and several clinical trials are currently underway. rAAV vectors have been used at various stages of development with no apparent toxicity. There are multiple ways of delivering AAV vectors to the mouse CNS, depending on the stage of development. In neonates, intravascular injections into the facial vein are often used. In adults, direct injections into target regions of the brain are achieved with great spatiotemporal control through stereotaxic surgeries. Recently, discoveries of new AAV vectors with the ability to cross the blood brain barrier have made it possible to target the adult CNS by intravascular injections.

Efficient gene delivery and selective transduction of astrocytes in the mammalian brain using viral vectors

Astrocytes are now considered as key players in brain information processing because of their newly discovered roles in synapse formation and plasticity, energy metabolism and blood flow regulation. However, our understanding of astrocyte function is still fragmented compared to other brain cell types. A better appreciation of the biology of astrocytes requires the development of tools to generate animal models in which astrocyte-specific proteins and pathways can be manipulated. In addition, it is becoming increasingly evident that astrocytes are also important players in many neurological disorders. Targeted modulation of protein expression in astrocytes would be critical for the development of new therapeutic strategies. Gene transfer is valuable to target a subpopulation of cells and explore their function in experimental models. In particular, viral-mediated gene transfer provides a rapid, highly flexible and cost-effective, in vivo paradigm to study the impact of genes of interest during central nervous system development or in adult animals. We will review the different strategies that led to the recent development of efficient viral vectors that can be successfully used to selectively transduce astrocytes in the mammalian brain.

Predictors of hepatitis B cure using gene therapy to deliver DNA cleavage enzymes: a mathematical modeling approach

Most chronic viral infections are managed with small molecule therapies that inhibit replication but are not curative because non-replicating viral forms can persist despite decades of suppressive treatment. There are therefore numerous strategies in development to eradicate all non-replicating viruses from the body. We are currently engineering DNA cleavage enzymes that specifically target hepatitis B virus covalently closed circular DNA (HBV cccDNA), the episomal form of the virus that persists despite potent antiviral therapies. DNA cleavage enzymes, including homing endonucleases or meganucleases, zinc-finger nucleases (ZFNs), TAL effector nucleases (TALENs), and CRISPR-associated system 9 (Cas9) proteins, can disrupt specific regions of viral DNA. Because DNA repair is error prone, the virus can be neutralized after repeated cleavage events when a target sequence becomes mutated. DNA cleavage enzymes will be delivered as genes within viral vectors that enter hepatocytes. Here we develop mathematical models that describe the delivery and intracellular activity of DNA cleavage enzymes. Model simulations predict that high vector to target cell ratio, limited removal of delivery vectors by humoral immunity, and avid binding between enzyme and its DNA target will promote the highest level of cccDNA disruption. Development of de novo resistance to cleavage enzymes may occur if DNA cleavage and error prone repair does not render the viral episome replication incompetent: our model predicts that concurrent delivery of multiple enzymes which target different vital cccDNA regions, or sequential delivery of different enzymes, are both potentially useful strategies for avoiding multi-enzyme resistance. The underlying dynamics of cccDNA persistence are unlikely to impact the probability of cure provided that antiviral therapy is given concurrently during eradication trials. We conclude by describing experiments that can be used to validate the model, which will in turn provide vital information for dose selection for potential curative trials in animals and ultimately humans.

Innovative new approaches are being developed to eradicate viral infections that until recently were considered incurable. We are interested in engineering DNA cleavage enzymes that can cut and incapacitate persistent viruses. One hurdle is that these enzymes must be delivered to infected cells as genes within viral vectors that are not harmful to humans. In this paper, we developed a series of equations that describe the delivery of these enzymes to their intended targets, as well the activity of DNA cutting within the cell. While our mathematical model is catered towards hepatitis B virus infection, it is widely applicable to other infections such as HIV, as well as oncologic and metabolic diseases characterized by aberrant gene expression. Certain enzymes may bind DNA more avidly than others, while different enzymes may also bind cooperatively if targeted to different regions of viral DNA. We predict that such enzymes, if delivered efficiently to a high proportion of infected cells, will be critical to increase the probability of cure. We also demonstrate that our equations will serve as a useful tool for identifying the most important features of a curative regimen, and ultimately for guiding clinical trial dosing schedules to ensure hepatitis B eradication with the smallest number of possible doses.

Capsid antibodies to different adeno-associated virus serotypes bind common regions

Interactions between viruses and the host antibody immune response are critical in the development and control of disease, and antibodies are also known to interfere with the efficacy of viral vector-based gene delivery. The adeno-associated viruses (AAVs) being developed as vectors for corrective human gene delivery have shown promise in clinical trials, but preexisting antibodies are detrimental to successful outcomes. However, the antigenic epitopes on AAV capsids remain poorly characterized. Cryo-electron microscopy and three-dimensional image reconstruction were used to define the locations of epitopes to which monoclonal fragment antibodies (Fabs) against AAV1, AAV2, AAV5, and AAV6 bind. Pseudoatomic modeling showed that, in each serotype, Fabs bound to a limited number of sites near the protrusions surrounding the 3-fold axes of the T=1 icosahedral capsids. For the closely related AAV1 and AAV6, a common Fab exhibited substoichiometric binding, with one Fab bound, on average, between two of the three protrusions as a consequence of steric crowding. The other AAV Fabs saturated the capsid and bound to the walls of all 60 protrusions, with the footprint for the AAV5 antibody extending toward the 5-fold axis. The angle of incidence for each bound Fab on the AAVs varied and resulted in significant differences in how much of each viral capsid surface was occluded beyond the Fab footprints. The AAV-antibody interactions showed a common set of footprints that overlapped some known receptor-binding sites and transduction determinants, thus suggesting potential mechanisms for virus neutralization by the antibodies.

Adeno-associated virus serotype 9 efficiently targets ischemic skeletal muscle following systemic delivery

Targeting therapeutic gene expression to the skeletal muscle following intravenous (IV) administration is an attractive strategy for treating peripheral arterial disease (PAD), except that vector access to the ischemic limb could be a limiting factor. As adeno-associated virus serotype 9 (AAV-9) transduces skeletal muscle at high efficiency following systemic delivery, we employed AAV-9 vectors bearing luciferase or enhanced green fluorescent protein (eGFP) reporter genes to test the hypothesis that increased desialylation of cell-surface glycans secondary to hindlimb ischemia (HLI) might help offset the reduction in tissue perfusion that occurs in mouse models of PAD. The utility of the creatine kinase-based (CK6) promoter for restricting gene expression to the skeletal muscle was also examined by comparing it with the cytomegalovirus (CMV) promoter after systemic administration following surgically induced HLI. Despite reduced blood flow to the ischemic limbs, CK6 promoter-driven luciferase activities in the ischemic gastrocnemius (GA) muscles were ∼34-, ∼28- and ∼150-fold higher than in the fully perfused contralateral GA, heart and liver, respectively, 10 days after IV administration. Furthermore, luciferase activity from the CK6 promoter in the ischemic GA muscles was ∼twofold higher than with CMV, while in the liver CK6-driven activity was ∼42-fold lower than with CMV, demonstrating that the specificity of ischemic skeletal muscle transduction can be further improved with the muscle-specific promoters. Studies with Evans blue dye and fluorescently labeled lectins revealed that vascular permeability and desialylation of the cell-surface glycans were increased in the ischemic hindlimbs. Furthermore, AAV9/CK6/Luc vector genome copy numbers were ∼sixfold higher in the ischemic muscle compared with the non-ischemic muscle in the HLI model, whereas this trend was reversed when the same genome was packaged in the AAV-1 capsid (which binds sialylated, as opposed to desialylated glycans), further underscoring the importance of desialylation in the ischemic enhancement of transduction displayed by AAV-9. Taken together, these findings suggest two complementary mechanisms contributing to the preferential transduction of ischemic muscle by AAV-9: increased vascular permeability and desialylation. In conclusion, ischemic muscle is preferentially targeted following systemic administration of AAV-9 in a mouse model of HLI. Unmasking of the primary AAV-9 receptor as a result of ischemia may contribute importantly to this effect.

Structure and dynamics of adeno-associated virus serotype 1 VP1-unique N-terminal domain and its role in capsid trafficking

The importance of the phospholipase A2 domain located within the unique N terminus of the capsid viral protein VP1 (VP1u) in parvovirus infection has been reported. This study used computational methods to characterize the VP1 sequence for adeno-associated virus (AAV) serotypes 1 to 12 and circular dichroism and electron microscopy to monitor conformational changes in the AAV1 capsid induced by temperature and the pHs encountered during trafficking through the endocytic pathway. Circular dichroism was also used to monitor conformational changes in AAV6 capsids assembled from VP2 and VP3 or VP1, VP2, and VP3 at pH 7.5. VP1u was predicted (computationally) and confirmed (in solution) to be structurally ordered. This VP domain was observed to undergo a reversible pH-induced unfolding/refolding process, a loss/gain of α-helical structure, which did not disrupt the capsid integrity and is likely facilitated by its difference in isoelectric point compared to the other VP sequences assembling the capsid. This study is the first to physically document conformational changes in the VP1u region that likely facilitate its externalization from the capsid interior during infection and establishes the order of events in the escape of the AAV capsid from the endosome en route to the nucleus.

AAV9 targets cone photoreceptors in the nonhuman primate retina

Transduction of retinal pigment epithelial cells with an adeno-associated viral vector (AAV) based on serotype 2 has partially corrected retinal blindness in Leber congenital amaurosis type 2. However, many applications of gene therapy for retinal blindness rely on the efficient transduction of rod and cone photoreceptor which is difficult to achieve with first generation vector technology. To address this translational need, we evaluated rod and cone photoreceptor targeting of 4 novel AAV capsids (AAV7, AAV9, rh.64R1 and rh.8R) versus AAV2 and AAV8 in a foveated retina. Eyes of 20 nonhuman primates were injected subretinally in the proximity of the fovea. While numerous vectors efficiently transduced rods, only AAV9 targeted cones both centrally and peripherally efficiently at low doses, likely due to the abundance of galactosylated glycans, the primary receptor for AAV9, on cone photoreceptors. We conclude AAV9 is an ideal candidate for strategies that require restoration of cone photoreceptor function.

Viral vectors for gene delivery to the central nervous system

The potential benefits of gene therapy for neurological diseases such as Parkinson's, Amyotrophic Lateral Sclerosis (ALS), Epilepsy, and Alzheimer's are enormous. Even a delay in the onset of severe symptoms would be invaluable to patients suffering from these and other diseases. Significant effort has been placed in developing vectors capable of delivering therapeutic genes to the CNS in order to treat neurological disorders. At the forefront of potential vectors, viral systems have evolved to efficiently deliver their genetic material to a cell. The biology of different viruses offers unique solutions to the challenges of gene therapy, such as cell targeting, transgene expression and vector production. It is important to consider the natural biology of a vector when deciding whether it will be the most effective for a specific therapeutic function. In this review, we outline desired features of the ideal vector for gene delivery to the CNS and discuss how well available viral vectors compare to this model. Adeno-associated virus, retrovirus, adenovirus and herpesvirus vectors are covered. Focus is placed on features of the natural biology that have made these viruses effective tools for gene delivery with emphasis on their application in the CNS. Our goal is to provide insight into features of the optimal vector and which viral vectors can provide these features.

Adeno-associated virus serotype 9 administered systemically after reperfusion preferentially targets cardiomyocytes in the infarct border zone with pharmacodynamics suitable for the attenuation of left ventricular remodeling

BACKGROUND

Adeno-associated virus serotype 9 (AAV9) vectors provide efficient and uniform gene expression to normal myocardium following systemic administration, with kinetics that approach steady-state within 2-3 weeks. However, as a result of the delayed onset of gene expression, AAV vectors have not previously been administered intravenously after reperfusion for post-infarct gene therapy applications. The present study evaluated the therapeutic potential of post-myocardial infarction gene delivery using intravenous AAV9.

METHODS

AAV9 vectors expressing firefly luciferase, enhanced green fluorescent protein (eGFP) or extracellular superoxide dismutase genes from the cardiac troponin-T (cTnT) promoter (AcTnTLuc, AcTnTeGFP, AcTnTEcSOD) were employed. AcTnTLuc was administered intravenously at 10 min and at 1, 2 and 3 days post-ischemia/reperfusion (IR), and the kinetics of luciferase expression were assessed with bioluminescence imaging. AcTnTeGFP was used to evaluate the distribution of eGFP expression. High-resolution echocardiography was used to evaluate the effects of AcTnTEcSOD on left ventricular (LV) remodeling when injected 10 min post-IR.

RESULTS

Compared to sham animals, luciferase expression at 2 days after vector administration was elevated by four-, 24-, 210- and 213-fold in groups injected at 10 min, 1 day, 2 days and 3 days post-IR, respectively. The expression of cTnT-driven eGFP was strongest in cardiomyocytes bordering the infarct zone. In the efficacy study of EcSOD, post-infarct LV end-systolic and end-diastolic volumes at days 14 and 28 were significantly smaller in the EcSOD group compared to the control.

CONCLUSIONS

Systemic administration of AAV9 vectors after IR both elevates and accelerates gene expression that preferentially targets cardiomyocytes in the border zone with pharmacodynamics suitable for the attenuation of LV remodeling.

Entry of influenza A Virus with a α2,6-linked sialic acid binding preference requires host fibronectin

The receptor binding specificity of influenza A virus is one of the major determinants of viral tropism and host specificity. In general, avian viral hemagglutinin prefers to bind to α2,3-linked sialic acid, whereas the human viral hemagglutinin prefers to bind to α2,6-linked sialic acid. Here, we demonstrate that host fibronectin protein plays an important role in the life cycle of some influenza A viruses. Treating cells with anti-fibronectin antibodies or fibronectin-specific small interfering RNA can inhibit the virus replication of human H1N1 influenza A viruses. Strikingly, these inhibitory effects cannot be observed in cells infected with H5N1 viruses. By using reverse genetics techniques, we observed that the receptor binding specificity, but not the origin of the hemagglutinin subtype, is responsible for this differential inhibitory effect. Changing the binding preference of hemagglutinin from α2,6-linked sialic acid to α2,3-linked sialic acid can make the virus resistant to the anti-fibronectin antibody treatment and vice versa. Our further characterizations indicate that anti-fibronectin antibody acts on the early phase of viral replication cycle, but it has no effect on the initial binding of influenza A virus to cell surface. Our subsequent investigations further show that anti-fibronectin antibody can block the postattachment entry of influenza virus. Overall, these results indicate that the sialic acid binding preference of influenza viral hemagglutinin can modulate the preferences of viral entry pathways, suggesting that there are subtle differences between the virus entries of human and avian influenza viruses.

Glycan binding avidity determines the systemic fate of adeno-associated virus type 9

Glycans are key determinants of host range and transmissibility in several pathogens. In the case of adeno-associated viruses (AAV), different carbohydrates serve as cellular receptors in vitro; however, their contributions in vivo are less clear. A particularly interesting example is adeno-associated virus serotype 9 (AAV9), which displays systemic tropism in mice despite low endogenous levels of its primary receptor (galactose) in murine tissues. To understand this further, we studied the effect of modulating glycan binding avidity on the systemic fate of AAV9 in mice. Intravenous administration of recombinant sialidase increased tissue levels of terminally galactosylated glycans in several murine tissues. These conditions altered the systemic tropism of AAV9 into a hepatotropic phenotype, characterized by markedly increased sequestration within the liver sinusoidal endothelium and Kupffer cells. In contrast, an AAV9 mutant with decreased glycan binding avidity displayed a liver-detargeted phenotype. Altering glycan binding avidity also profoundly affected AAV9 persistence in blood circulation. Our results support the notion that high glycan receptor binding avidity appears to impart increased liver tropism, while decreased avidity favors systemic spread of AAV vectors. These findings may not only help predict species-specific differences in tropism for AAV9 on the basis of tissue glycosylation profiles, but also provide a general approach to tailor AAV vectors for systemic or hepatic gene transfer by reengineering capsid-glycan interactions.

Recombinant adeno-associated virus: clinical application and development as a gene-therapy vector

Gene therapy is gaining momentum as a method of treating human disease. Initially conceived as a strategy to complement defective genes in monogenic disorders, the scope of gene therapy has expanded to encompass a variety of applications. Likewise, the molecular tools for gene delivery have evolved and diversified to meet these various therapeutic needs. Recombinant adeno-associated virus (rAAV) has made significant strides toward clinical application with an excellent safety profile and successes in several clinical trials. This review covers the basic biology of rAAV as a gene therapy vector as well as its advantages compared with other methods of gene delivery. The status of clinical trials utilizing rAAV is also discussed in detail. In conclusion, methods of engineering the vector to overcome challenges identified from these trials are covered, with emphasis on modification of the viral capsid to increase the tissue/cell-specific targeting and transduction efficiency.

Intracellular transport of recombinant adeno-associated virus vectors

Recombinant adeno-associated viral vectors (rAAVs) have been widely used for gene delivery in animal models, and are currently evaluated for human gene therapy after successful clinical trials in the treatment of inherited, degenerative or acquired diseases, such as Leber congenital amaurosis, Parkinson disease or heart failure. However, limitations in vector tropism, such as limited tissue specificity and insufficient transduction efficiencies of particular tissues and cell types, still preclude therapeutic applications in certain tissues. Wild-type adeno-associated viruses (AAVs) are defective viruses that require the presence of a helper virus to complete their life cycle. On the one hand, this unique property makes AAV vectors one of the safest available viral vectors for gene delivery. On the other, it also represents a potential obstacle because rAAV vectors have to overcome several biological barriers in the absence of a helper virus to transduce successfully a cell. Consequently, a better understanding of the cellular roadblocks that limit rAAV gene delivery is crucial and, during the last 15 years, numerous studies resulted in an expanding body of knowledge of the intracellular trafficking pathways of rAAV vectors. This review describes our current understanding of the mechanisms involved in rAAV attachment to target cells, endocytosis, intracellular trafficking, capsid processing, nuclear import and genome release with an emphasis on the most recent discoveries in the field and the emerging strategies used to improve the efficiency of AAV-derived vectors.

Structural insight into the unique properties of adeno-associated virus serotype 9

Adeno-associated virus serotype 9 (AAV9) has enhanced capsid-associated tropism for cardiac muscle and the ability to cross the blood-brain barrier compared to other AAV serotypes. To help identify the structural features facilitating these properties, we have used cryo-electron microscopy (cryo-EM) and three-dimensional image reconstruction (cryo-reconstruction) and X-ray crystallography to determine the structure of the AAV9 capsid at 9.7- and 2.8-Å resolutions, respectively. The AAV9 capsid exhibits the surface topology conserved in all AAVs: depressions at each icosahedral two-fold symmetry axis and surrounding each five-fold axis, three separate protrusions surrounding each three-fold axis, and a channel at each five-fold axis. The AAV9 viral protein (VP) has a conserved core structure, consisting of an eight-stranded, β-barrel motif and the αA helix, which are present in all parvovirus structures. The AAV9 VP differs in nine variable surface regions (VR-I to -IX) compared to AAV4, but at only three (VR-I, VR-II, and VR-IV) compared to AAV2 and AAV8. VR-I differences modify the raised region of the capsid surface between the two-fold and five-fold depressions. The VR-IV difference produces smaller three-fold protrusions in AAV9 that are less "pointed" than AAV2 and AAV8. Significantly, residues in the AAV9 VRs have been identified as important determinants of cellular tropism and transduction and dictate its antigenic diversity from AAV2. Hence, the AAV9 VRs likely confer the unique infection phenotypes of this serotype.

Sustained expression and safety of human GNE in normal mice after gene transfer based on AAV8 systemic delivery

GNE myopathy is an autosomal recessive adult onset disorder caused by mutations in the GNE gene. GNE encodes the bifunctional enzyme UDP-N-acetylglucosamine 2-epimerase/N-acetyl mannosamine kinase, the key enzyme in the biosynthesis pathway of sialic acid. Additional functions for GNE have been described recently, but the mechanism leading from GNE mutation to this myopathy is unclear. Therefore a gene therapy approach could address all potential defects caused by GNE mutations in muscle. We show that AAV8 viral vectors carrying wild type human GNE cDNA are able to transduce murine muscle cells and human GNE myopathy-derived muscle cells in culture and to express the transgene in these cells. Furthermore, the intravenous administration of this viral vector to healthy mice allows expression of the GNE transgene mRNA and of the coexpressed luciferase protein, for at least 6months in skeletal muscles, with no clinical or pathological signs of focal or general toxicity, neither from the virus particles nor from the wild type human GNE overexpression. Our results support the future use of an AAV8 based vector platform for a safe and efficient therapy of muscle in GNE myopathy.

Sialic acid deposition impairs the utility of AAV9, but not peptide-modified AAVs for brain gene therapy in a mouse model of lysosomal storage disease

Recombinant vector systems have been recently identified that when delivered systemically can transduce neurons, glia, and endothelia in the central nervous system (CNS), providing an opportunity to develop therapies for diseases affecting the brain without performing direct intracranial injections. Vector systems based on adeno-associated virus (AAV) include AAV serotype 9 (AAV9) and AAVs that have been re-engineered at the capsid level for CNS tropism. Here, we performed a head-to-head comparison of AAV9 and a capsid modified AAV for their abilities to rescue CNS and peripheral disease in an animal model of lysosomal storage disease (LSD), the mucopolysacharidoses (MPS) VII mouse. While the peptide-modified AAV reversed cognitive deficits, improved storage burden in the brain, and substantially prolonged survival, we were surprised to find that AAV9 provided no CNS benefit. Additional experiments demonstrated that sialic acid, a known inhibitor of AAV9, is elevated in the CNS of MPS VII mice. These studies highlight how disease manifestations can dramatically impact the known tropism of recombinant vectors, and raise awareness to assuming similar transduction profiles between normal and disease models.

Identification of the galactose binding domain of the adeno-associated virus serotype 9 capsid

Adeno-associated virus serotype 9 (AAV9) vectors show promise for gene therapy of a variety of diseases due to their ability to transduce multiple tissues, including heart, skeletal muscle, and the alveolar epithelium of the lung. In addition, AAV9 is unique compared to other AAV serotypes in that it is capable of surpassing the blood-brain barrier and transducing neurons in the brain and spinal cord. It has recently been shown that AAV9 uses galactose as a receptor to transduce many different cell types in vitro, as well as cells of the mouse airway in vivo. In this study, we sought to identify the specific amino acids of the AAV9 capsid necessary for binding to galactose. By site-directed mutagenesis and cell binding assays, plus computational ligand docking studies, we discovered five amino acids, including N470, D271, N272, Y446, and W503, which are required for galactose binding that form a pocket at the base of the protrusions around the icosahedral 3-fold axes of symmetry. The importance of these amino acids for tissue tropism was also confirmed by in vivo studies in the mouse lung. Identifying the interactions necessary for AAV9 binding to galactose may lead to advances in vector engineering.

The AAV vector toolkit: poised at the clinical crossroads

The discovery of naturally occurring adeno-associated virus (AAV) isolates in different animal species and the generation of engineered AAV strains using molecular genetics tools have yielded a versatile AAV vector toolkit. Promising results in preclinical animal models of human disease spurred the much awaited transition toward clinical application, and early successes in phase I/II clinical trials for a broad spectrum of genetic diseases have recently been reported. As the gene therapy community forges ahead with cautious optimism, both preclinical and clinical studies using first generation AAV vectors have highlighted potential challenges. These include cross-species variation in vector tissue tropism and gene transfer efficiency, pre-existing humoral immunity to AAV capsids and vector dose-dependent toxicity in patients. A battery of second generation AAV vectors, engineered through rational and combinatorial approaches to address the aforementioned concerns, are now available. This review will provide an overview of preclinical studies with the ever-expanding AAV vector portfolio in large animal models and an update on new lead AAV vector candidates poised for clinical translation.

Hybrid- and complex-type N-glycans are not essential for Newcastle disease virus infection and fusion of host cells

N-linked glycans are composed of three major types: high-mannose (Man), hybrid or complex. The functional role of hybrid- and complex-type N-glycans in Newcastle disease virus (NDV) infection and fusion was examined in N-acetylglucosaminyltransferase I (GnT I)-deficient Lec1 cells, a mutant Chinese hamster ovary (CHO) cell incapable of synthesizing hybrid- and complex-type N-glycans. We used recombinant NDV expressing green fluorescence protein or red fluorescence protein to monitor NDV infection, syncytium formation and viral yield. Flow cytometry showed that CHO-K1 and Lec1 cells had essentially the same degree of NDV infection. In contrast, Lec2 cells were found to be resistant to NDV infection. Compared with CHO-K1 cells, Lec1 cells were shown to more sensitive to fusion induced by NDV. Viral attachment was found to be comparable in both lines. We found that there were no significant differences in the yield of progeny virus produced by both CHO-K1 and Lec1 cells. Quantitative analysis revealed that NDV infection and fusion in Lec1 cells were also inhibited by treatment with sialidase. Pretreatment of Lec1 cells with Galanthus nivalis agglutinin specific for terminal α1-3-linked Man prior to inoculation with NDV rendered Lec1 cells less sensitive to cell-to-cell fusion compared with mock-treated Lec1 cells. Treatment of CHO-K1 and Lec1 cells with tunicamycin, an inhibitor of N-glycosylation, significantly blocked fusion and infection. In conclusion, our results suggest that hybrid- and complex-type N-glycans are not required for NDV infection and fusion. We propose that high-Man-type N-glycans could play an important role in the cell-to-cell fusion induced by NDV.

Parvoviruses: structure and infection

Parvoviruses package a ssDNA genome. Both nonpathogenic and pathogenic members exist, including those that cause fetal infections, encompassing the entire spectrum of virus phenotypes. Their small genomes and simple coding strategy has enabled functional annotation of many steps in the infectious life cycle. They assemble a multifunctional capsid responsible for cell recognition and the transport of the packaged genome to the nucleus for replication and progeny virus production. It is also the target of the host immune response. Understanding how the capsid structure relates to the function of parvoviruses provides a platform for recombinant engineering of viral gene delivery vectors for the treatment of clinical diseases, and is fundamental for dissecting the viral determinants of pathogenicity. This review focuses on our current understanding of parvovirus capsid structure and function with respect to the infectious life cycle.

Advances in cell and gene-based therapies for cystic fibrosis lung disease

Cystic fibrosis (CF) is a disease characterized by airway infection, inflammation, remodeling, and obstruction that gradually destroy the lungs. Direct delivery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene to airway epithelia may offer advantages, as the tissue is accessible for topical delivery of vectors. Yet, physical and host immune barriers in the lung present challenges for successful gene transfer to the respiratory tract. Advances in gene transfer approaches, tissue engineering, and novel animal models are generating excitement within the CF research field. This review discusses current challenges and advancements in viral and nonviral vectors, cell-based therapies, and CF animal models.