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

Gene therapy for the CNS using AAVs: The impact of systemic delivery by AAV9

Several attempts have been made to discover the ideal vector for gene therapy in central nervous system (CNS). Adeno-associated viruses (AAVs) are currently the preferred vehicle since they exhibit stable transgene expression in post-mitotic cells, neuronal tropism, low risk of insertional mutagenesis and diminished immune responses. Additionally, the discovery that a particular serotype, AAV9, bypasses the blood-brain barrier has raised the possibility of intravascular administration as a non-invasive delivery route to achieve widespread CNS gene expression. AAV9 intravenous delivery has already shown promising results for several diseases in animal models, including lysosomal storage disorders and motor neuron diseases, opening the way to the first clinical trial in the field. This review presents an overview of clinical trials for CNS disorders using AAVs and will focus on preclinical studies based on the systemic gene delivery using AAV9.

Glymphatic fluid transport controls paravascular clearance of AAV vectors from the brain

Adeno-associated viruses (AAV) are currently being evaluated in clinical trials for gene therapy of CNS disorders. However, host factors that influence the spread, clearance, and transduction efficiency of AAV vectors in the brain are not well understood. Recent studies have demonstrated that fluid flow mediated by aquaporin-4 (AQP4) channels located on astroglial end feet is essential for exchange of solutes between interstitial and cerebrospinal fluid. This phenomenon, which is essential for interstitial clearance of solutes from the CNS, has been termed glial-associated lymphatic transport or glymphatic transport. In the current study, we demonstrate that glymphatic transport profoundly affects various aspects of AAV gene transfer in the CNS. Altered localization of AQP4 in aged mouse brains correlated with significantly increased retention of AAV vectors in the parenchyma and reduced systemic leakage following ventricular administration. We observed a similar increase in AAV retention and transgene expression upon i.c.v. administration in AQP4^-/- mice. Consistent with this observation, fluorophore-labeled AAV vectors showed markedly reduced flux from the ventricles of AQP4^-/- mice compared with WT mice. These results were further corroborated by reduced AAV clearance from the AQP4-null brain, as demonstrated by reduced transgene expression and vector genome accumulation in systemic organs. We postulate that deregulation of glymphatic transport in aged and diseased brains could markedly affect the parenchymal spread, clearance, and gene transfer efficiency of AAV vectors. Assessment of biomarkers that report the kinetics of CSF flux in prospective gene therapy patients might inform variable treatment outcomes and guide future clinical trial design.

Astrocytic Acid-Sensing Ion Channel 1a Contributes to the Development of Chronic Epileptogenesis

Unraveling mechanisms underlying epileptogenesis after brain injury is an unmet medical challenge. Although histopathological studies have revealed that reactive astrogliosis and tissue acidosis are prominent features in epileptogenic foci, their roles in epileptogenesis remain unclear. Here, we explored whether astrocytic acid-sensing ion channel-1a (ASIC1a) contributes to the development of chronic epilepsy. High levels of ASIC1a were measured in reactive astrocytes in the hippocampi of patients with temporal lobe epilepsy (TLE) and epileptic mice. Extracellular acidosis caused a significant Ca²⁺ influx in cultured astrocytes, and this influx was sensitive to inhibition by the ASIC1a-specific blocker psalmotoxin 1 (PcTX1). In addition, recombinant adeno-associated virus (rAAV) vectors carrying a GFAP promoter in conjunction with ASIC1a shRNA or cDNA were generated to suppress or restore, respectively, ASIC1a expression in astrocytes. Injection of rAAV-ASIC1a-shRNA into the dentate gyrus of the wide type TLE mouse model resulted in the inhibition of astrocytic ASIC1a expression and a reduction in spontaneous seizures. By contrast, rAAV-ASIC1a-cDNA restored astrocytic ASIC1a expression in an ASIC1a knock-out TLE mouse model and increased the frequency of spontaneous seizures. Taken together, our results reveal that astrocytic ASIC1a may be an attractive new target for the treatment of epilepsy.

AAV Vectors for FRET-Based Analysis of Protein-Protein Interactions in Photoreceptor Outer Segments

Fluorescence resonance energy transfer (FRET) is a powerful method for the detection and quantification of stationary and dynamic protein-protein interactions. Technical limitations have hampered systematic in vivo FRET experiments to study protein-protein interactions in their native environment. Here, we describe a rapid and robust protocol that combines adeno-associated virus (AAV) vector-mediated in vivo delivery of genetically encoded FRET partners with ex vivo FRET measurements. The method was established on acutely isolated outer segments of murine rod and cone photoreceptors and relies on the high co-transduction efficiency of retinal photoreceptors by co-delivered AAV vectors. The procedure can be used for the systematic analysis of protein-protein interactions of wild type or mutant outer segment proteins in their native environment. Conclusively, our protocol can help to characterize the physiological and pathophysiological relevance of photoreceptor specific proteins and, in principle, should also be transferable to other cell types.

CNS-restricted Transduction and CRISPR/Cas9-mediated Gene Deletion with an Engineered AAV Vector

Gene therapy using recombinant adeno-associated viral (AAV) vectors is emerging as a promising approach to treat central nervous system disorders such as Spinal muscular atrophy, Batten, Parkinson and Alzheimer disease amongst others. A critical remaining challenge for central nervous system-targeted gene therapy, silencing or gene editing is to limit potential vector dose-related toxicity in off-target cells and organs. Here, we characterize a lab-derived AAV chimeric (AAV2g9), which displays favorable central nervous system attributes derived from both parental counterparts, AAV2 and AAV9. This synthetic AAV strain displays preferential, robust, and widespread neuronal transduction within the brain and decreased glial tropism. Importantly, we observed minimal systemic leakage, decreased sequestration and gene transfer in off-target organs with AAV2g9, when administered into the cerebrospinal fluid. A single intracranial injection of AAV2g9 vectors encoding guide RNAs targeting the schizophrenia risk gene MIR137 (encoding MIR137) in CRISPR/Cas9 knockin mice resulted in brain-specific gene deletion with no detectable events in the liver. This engineered AAV vector is a promising platform for treating neurological disorders through gene therapy, silencing or editing modalities.

MicroRNA-regulated viral vectors for gene therapy

Safe and effective gene therapy approaches require targeted tissue-specific transfer of a therapeutic transgene. Besides traditional approaches, such as transcriptional and transductional targeting, microRNA-dependent post-transcriptional suppression of transgene expression has been emerging as powerful new technology to increase the specificity of vector-mediated transgene expression. MicroRNAs are small non-coding RNAs and often expressed in a tissue-, lineage-, activation- or differentiation-specific pattern. They typically regulate gene expression by binding to imperfectly complementary sequences in the 3' untranslated region (UTR) of the mRNA. To control exogenous transgene expression, tandem repeats of artificial microRNA target sites are usually incorporated into the 3' UTR of the transgene expression cassette, leading to subsequent degradation of transgene mRNA in cells expressing the corresponding microRNA. This targeting strategy, first shown for lentiviral vectors in antigen presenting cells, has now been used for tissue-specific expression of vector-encoded therapeutic transgenes, to reduce immune response against the transgene, to control virus tropism for oncolytic virotherapy, to increase safety of live attenuated virus vaccines and to identify and select cell subsets for pluripotent stem cell therapies, respectively. This review provides an introduction into the technical mechanism underlying microRNA-regulation, highlights new developments in this field and gives an overview of applications of microRNA-regulated viral vectors for cardiac, suicide gene cancer and hematopoietic stem cell therapy, as well as for treatment of neurological and eye diseases.

rAAV-compatible MiniPromoters for restricted expression in the brain and eye

BACKGROUND

Small promoters that recapitulate endogenous gene expression patterns are important for basic, preclinical, and now clinical research. Recently, there has been a promising revival of gene therapy for diseases with unmet therapeutic needs. To date, most gene therapies have used viral-based ubiquitous promoters-however, promoters that restrict expression to target cells will minimize off-target side effects, broaden the palette of deliverable therapeutics, and thereby improve safety and efficacy. Here, we take steps towards filling the need for such promoters by developing a high-throughput pipeline that goes from genome-based bioinformatic design to rapid testing in vivo.

METHODS

For much of this work, therapeutically interesting Pleiades MiniPromoters (MiniPs; ~4 kb human DNA regulatory elements), previously tested in knock-in mice, were "cut down" to ~2.5 kb and tested in recombinant adeno-associated virus (rAAV), the virus of choice for gene therapy of the central nervous system. To evaluate our methods, we generated 29 experimental rAAV2/9 viruses carrying 19 different MiniPs, which were injected intravenously into neonatal mice to allow broad unbiased distribution, and characterized in neural tissues by X-gal immunohistochemistry for icre, or immunofluorescent detection of GFP.

RESULTS

The data showed that 16 of the 19 (84 %) MiniPs recapitulated the expression pattern of their design source. This included expression of: Ple67 in brain raphe nuclei; Ple155 in Purkinje cells of the cerebellum, and retinal bipolar ON cells; Ple261 in endothelial cells of brain blood vessels; and Ple264 in retinal Müller glia.

CONCLUSIONS

Overall, the methodology and MiniPs presented here represent important advances for basic and preclinical research, and may enable a paradigm shift in gene therapy.

In Vivo Delivery Systems for Therapeutic Genome Editing

Therapeutic genome editing technology has been widely used as a powerful tool for directly correcting genetic mutations in target pathological tissues and cells to cure of diseases. The modification of specific genomic sequences can be achieved by utilizing programmable nucleases, such as Meganucleases, zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the clustered regularly-interspaced short palindromic repeat-associated nuclease Cas9 (CRISPR/Cas9). However, given the properties, such as large size, negative charge, low membrane penetrating ability, as well as weak tolerance for serum, and low endosomal escape, of these nucleases genome editing cannot be successfully applied unless in vivo delivery of related programmable nucleases into target organisms or cells is achieved. Here, we look back at delivery strategies having been used in the in vivo delivery of three main genome editing nucleases, followed by methodologies currently undergoing testing in clinical trials, and potential delivery strategies provided by analyzing characteristics of nucleases and commonly used vectors.

Gene suppression strategies for dominantly inherited neurodegenerative diseases: lessons from Huntington's disease and spinocerebellar ataxia

RNA-targeting approaches are emerging as viable therapeutics that offer an alternative method to modulate traditionally 'undrugable' targets. In the case of dominantly inherited neurodegenerative diseases, gene suppression strategies can target the underlying cause of these intractable disorders. Polyglutamine diseases are caused by CAG expansions in discrete genes, making them ideal candidates for gene suppression therapies. Here, we discuss the current state of gene suppression approaches for Huntington's disease and the spinocerebellar ataxias, including the use of antisense oligonucleotides, short-interfering RNAs, as well as viral vector-mediated delivery of short hairpin RNAs and artificial microRNAs. We focus on lessons learned from preclinical studies investigating gene suppression therapies for these disorders, particularly in rodent models of disease and in non-human primates. In animal models, recent advances in gene suppression technologies have not only prevented disease progression in a number of cases, but have also reversed existing disease, providing evidence that reducing the expression of disease-causing genes may be of benefit in symptomatic patients. Both allele- and non-allele-specific approaches to gene suppression have made great strides over the past decade, showing efficacy and safety in both small and large animal models. Advances in delivery techniques allow for broad and durable suppression of target genes, have been validated in non-human primates and in some cases, are currently being evaluated in human patients. Finally, we discuss the challenges of developing and delivering gene suppression constructs into the CNS and recent advances of potential therapeutics into the clinic.

Microglia-specific targeting by novel capsid-modified AAV6 vectors

Recombinant adeno-associated viruses (rAAV) have been widely used in gene therapy applications for central nervous system diseases. Though rAAV can efficiently target neurons and astrocytes in mouse brains, microglia, the immune cells of the brain, are refractile to rAAV. To identify AAV capsids with microglia-specific transduction properties, we initially screened the most commonly used serotypes, AAV1–9 and rh10, on primary mouse microglia cultures. While these capsids were not permissive, we then tested the microglial targeting properties of a newly characterized set of modified rAAV6 capsid variants with high tropism for monocytes. Indeed, these newly characterized rAAV6 capsid variants, specially a triply mutated Y731F/Y705F/T492V form, carrying a self-complementary genome and microglia-specific promoters (F4/80 or CD68) could efficiently and selectively transduce microglia in vitro. Delivery of these constructs in mice brains resulted in microglia-specific expression of green fluorescent protein, albeit at modest levels. We further show that CD68 promoter–driven expression of the inflammatory cytokine, interleukin-6, using this capsid variant leads to increased astrogliosis in the brains of wild-type mice. Our study describes the first instance of AAV-targeted microglial gene expression leading to functional modulation of the innate immune system in mice brains. This provides the rationale for utilizing these unique capsid/promoter combinations for microglia-specific gene targeting for modeling or functional studies.

Viral vector-based tools advance knowledge of basal ganglia anatomy and physiology

Viral vectors were originally developed to deliver genes into host cells for therapeutic potential. However, viral vector use in neuroscience research has increased because they enhance interpretation of the anatomy and physiology of brain circuits compared with conventional tract tracing or electrical stimulation techniques. Viral vectors enable neuronal or glial subpopulations to be labeled or stimulated, which can be spatially restricted to a single target nucleus or pathway. Here we review the use of viral vectors to examine the structure and function of motor and limbic basal ganglia (BG) networks in normal and pathological states. We outline the use of viral vectors, particularly lentivirus and adeno-associated virus, in circuit tracing, optogenetic stimulation, and designer drug stimulation experiments. Key studies that have used viral vectors to trace and image pathways and connectivity at gross or ultrastructural levels are reviewed. We explain how optogenetic stimulation and designer drugs used to modulate a distinct pathway and neuronal subpopulation have enhanced our mechanistic understanding of BG function in health and pathophysiology in disease. Finally, we outline how viral vector technology may be applied to neurological and psychiatric conditions to offer new treatments with enhanced outcomes for patients.

Proof-of-concept: neonatal intravenous injection of adeno-associated virus vectors results in successful transduction of myenteric and submucosal neurons in the mouse small and large intestine

BACKGROUND

Despite the success of viral vector technology in the transduction of the central nervous system in both preclinical research and gene therapy, its potential in neurogastroenterological research remains largely unexploited. This study asked whether and to what extent myenteric and submucosal neurons in the ileum and distal colon of the mouse were transduced after neonatal systemic delivery of recombinant adeno-associated viral vectors (AAVs).

METHODS

Mice were intravenously injected at postnatal day one with AAV pseudotypes AAV8 or AAV9 carrying a cassette encoding enhanced green fluorescent protein (eGFP) as a reporter under the control of a cytomegalovirus promoter. At postnatal day 35, transduction of the myenteric and submucosal plexuses of the ileum and distal colon was evaluated in whole-mount preparations, using immunohistochemistry to neurochemically identify transduced enteric neurons.

KEY RESULTS

The pseudotypes AAV8 and AAV9 showed equal potential in transducing the enteric nervous system (ENS), with 25-30% of the neurons expressing eGFP. However, the percentage of eGFP-expressing colonic submucosal neurons was significantly lower. Neurochemical analysis showed that all enteric neuron subtypes, but not glia, expressed the reporter protein. Intrinsic sensory neurons were most efficiently transduced as nearly 80% of calcitonin gene-related peptide-positive neurons expressed the transgene.

CONCLUSIONS & INFERENCES

The pseudotypes AAV8 and AAV9 can be employed for gene delivery to both the myenteric and the submucosal plexus, although the transduction efficiency in the latter is region-dependent. These findings open perspectives for novel preclinical applications aimed at manipulating and imaging the ENS in the short term, and in gene therapy in the longer term.

The Use of Perinatal 6-Hydroxydopamine to Produce a Rodent Model of Lesch-Nyhan Disease

Lesch-Nyhan disease is a neurologically, metabolically, and behaviorally devastating condition that has eluded complete characterization and adequate treatment. While it is known that the disease is intimately associated with dysfunction of the hypoxanthine phosphoribosyltransferase 1 (HPRT1) gene that codes for an enzyme of purine metabolism (hypoxanthine-guanine phosphoribosyltransferase) and is associated with neurological, behavioral, as well as metabolic dysfunction, the mechanisms of the neurobehavioral manifestations are as yet unclear. However, discoveries over the past few decades not only have created useful novel animal models (e.g., the HPRT-deficient mouse and the serendipitously discovered perinatal 6-hydroxydopamine (6-OHDA lesion model), but also have expanded into epigenetic, genomic, and proteomic approaches to better understand the mechanisms underlying this disease. The perinatal 6-OHDA model, in addition to modeling self-injury and dopamine depletion in the clinical condition, also underscores the profound importance of development in the differential course of maladaptive progression in the face of a common/single neurotoxic insult at different ages. Recent developments from clinical and basic science efforts attest to the fact that while the disease would seem to have a simple single gene defect at its core, the manifestations of this defect are profound and unexpectedly diverse. Future efforts employing the 6-OHDA model and others in the context of the novel technologies of genome editing, chemo- and opto-genetics, epigenetics, and further studies on the mechanisms of stress-induced maladaptations in brain all hold promise in taking our understanding of this disease to the next level.

Applications of CRISPR-Cas systems in neuroscience

Genome-editing tools, and in particular those based on CRISPR-Cas (clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR-associated protein) systems, are accelerating the pace of biological research and enabling targeted genetic interrogation in almost any organism and cell type. These tools have opened the door to the development of new model systems for studying the complexity of the nervous system, including animal models and stem cell-derived in vitro models. Precise and efficient gene editing using CRISPR-Cas systems has the potential to advance both basic and translational neuroscience research.

A Comprehensive Map of CNS Transduction by Eight Recombinant Adeno-associated Virus Serotypes Upon Cerebrospinal Fluid Administration in Pigs

Cerebrospinal fluid administration of recombinant adeno-associated viral (rAAV) vectors has been demonstrated to be effective in delivering therapeutic genes to the central nervous system (CNS) in different disease animal models. However, a quantitative and qualitative analysis of transduction patterns of the most promising rAAV serotypes for brain targeting in large animal models is missing. Here, we characterize distribution, transduction efficiency, and cellular targeting of rAAV serotypes 1, 2, 5, 7, 9, rh.10, rh.39, and rh.43 delivered into the cisterna magna of wild-type pigs. rAAV9 showed the highest transduction efficiency and the widest distribution capability among the vectors tested. Moreover, rAAV9 robustly transduced both glia and neurons, including the motor neurons of the spinal cord. Relevant cell transduction specificity of the glia was observed after rAAV1 and rAAV7 delivery. rAAV7 also displayed a specific tropism to Purkinje cells. Evaluation of biochemical and hematological markers suggested that all rAAV serotypes tested were well tolerated. This study provides a comprehensive CNS transduction map in a useful preclinical large animal model enabling the selection of potentially clinically transferable rAAV serotypes based on disease specificity. Therefore, our data are instrumental for the clinical evaluation of these rAAV vectors in human neurodegenerative diseases.

Intranasal gene delivery for treating Parkinson's disease: overcoming the blood-brain barrier

INTRODUCTION

Developing a disease-modifying gene therapy for Parkinson's disease (PD) has been a high priority for over a decade. However, due to the inability of large biomolecules to cross the blood-brain barrier (BBB), the only means of delivery to the brain has been intracerebral infusion. Intranasal administration offers a non-surgical means of bypassing the BBB to deliver neurotrophic factors, and the genes encoding them, directly to the brain.

AREAS COVERED

This review summarizes: i) evidence demonstrating intranasal delivery to the brain of a number of biomolecules having therapeutic potential for various CNS disorders; and ii) evidence demonstrating neuroprotective efficacy of a subset of biomolecules specifically for PD. The intersection of these two spheres represents the area of opportunity for development of new intranasal gene therapies for PD. To that end, our laboratory showed that intranasal administration of glial cell line-derived neurotrophic factor (GDNF), or plasmid DNA nanoparticles encoding GDNF, provides neuroprotection in a rat model of PD, and that the cells transfected by the nanoparticle vector are likely to be pericytes.

EXPERT OPINION

A number of genes encoding neurotrophic factors have therapeutic potential for PD, but few have been tested by the intranasal route and shown to be neuroprotective in a model of PD. Intranasal delivery provides a largely unexplored, promising approach for development of a non-invasive gene therapy for PD.

Adeno-Associated Viral Vector Delivery to the Enteric Nervous System: A Review

Gene therapy to the gastrointestinal tract has remarkable potential for treating gastrointestinal disorders that currently lack effective treatments. Adeno-associated viral vectors (AAVs) have been extensively applied to the central nervous system, and have repeatedly demonstrated safety and efficacy in animal models. The enteric nervous system (ENS) represents a vast collection of neurons and glial cells that may also be subject to treatment by AAV, however little work has been conducted on AAV delivery to the ENS. Challenges for gastrointestinal gene therapy include identifying gene targets, optimizing gene delivery, and target cell selection. Researchers are now beginning to tackle the later of the two challenges with AAV, and the same AAV technology can be used to identify novel gene targets in the future. Continued efforts to understand AAV delivery and improve vector design are essential for therapeutic development. This review summarizes the current knowledge about AAV delivery to the ENS.

Neuroanatomy goes viral!

The nervous system is complex not simply because of the enormous number of neurons it contains but by virtue of the specificity with which they are connected. Unraveling this specificity is the task of neuroanatomy. In this endeavor, neuroanatomists have traditionally exploited an impressive array of tools ranging from the Golgi method to electron microscopy. An ideal method for studying anatomy would label neurons that are interconnected, and, in addition, allow expression of foreign genes in these neurons. Fortuitously, nature has already partially developed such a method in the form of neurotropic viruses, which have evolved to deliver their genetic material between synaptically connected neurons while largely eluding glia and the immune system. While these characteristics make some of these viruses a threat to human health, simple modifications allow them to be used in controlled experimental settings, thus enabling neuroanatomists to trace multi-synaptic connections within and across brain regions. Wild-type neurotropic viruses, such as rabies and alpha-herpes virus, have already contributed greatly to our understanding of brain connectivity, and modern molecular techniques have enabled the construction of recombinant forms of these and other viruses. These newly engineered reagents are particularly useful, as they can target genetically defined populations of neurons, spread only one synapse to either inputs or outputs, and carry instructions by which the targeted neurons can be made to express exogenous proteins, such as calcium sensors or light-sensitive ion channels, that can be used to study neuronal function. In this review, we address these uniquely powerful features of the viruses already in the neuroanatomist's toolbox, as well as the aspects of their biology that currently limit their utility. Based on the latter, we consider strategies for improving viral tracing methods by reducing toxicity, improving control of transsynaptic spread, and extending the range of species that can be studied.

Cre Activated and Inactivated Recombinant Adeno-Associated Viral Vectors for Neuronal Anatomical Tracing or Activity Manipulation

Recombinant adeno-associated viruses (rAAVs) transcriptionally activated by Cre recombinase (Cre-On) are powerful tools for determining the anatomy and function of genetically defined neuronal types in transgenic Cre driver mice. Here we describe how rAAVs transcriptionally inactivated by Cre (Cre-Off) can be used in conjunction with Cre-On rAAVs or genomic Cre-reporter alleles to study brain circuits. Intracranial injection of Cre-On/Cre-Off rAAVs into spatially intermingled Cre(+) and Cre(-) neurons allows these populations to be differentially labeled or manipulated within individual animals. This comparison helps define the unique properties of Cre(+) neurons, highlighting the specialized role they play in their constituent brain circuits. This protocol touches on the conceptual and experimental background of Cre-Off rAAV systems, including caveats and methods of validation.

Recombinant adeno-associated virus vectors in the treatment of rare diseases

INTRODUCTION

An estimated 25 million Americans are living with rare diseases. Adeno-associated virus (AAV)-mediated gene therapy is an emerging therapeutic option for the more than 7,000 identified rare diseases. This paper highlights the benefits of AAV therapy compared to conventional small molecules, discusses current pre-clinical and clinical applications of AAV-mediated gene therapy, and offers insights into cutting edge research that will shape the future of AAV for broad therapeutic use.

AREAS COVERED

In this review the biology of AAV and our ability to generate disease-specific variants is summarized. Limitations of current therapy are reviewed, with an emphasis on immune detection of virus, viral tropism and tissue targeting, and limitations of gene expression. Information for this review was found using PubMed and clinicaltrials.gov.

EXPERT OPINION

Currently the scope of clinical trials of AAV gene therapy is concentrated in an array of phase I/II safety trials with less than two dozen rare diseases featured. Pre-clinical, translational studies are expanding in number as developments within the last decade have made generation of improved AAV vectors available to more researchers. Further, one bottleneck that is being overcome is the availability of disease models, which will allow for improved preclinical testing and advancement of AAV to more clinical applications.

Unique glycan signatures regulate adeno-associated virus tropism in the developing brain

Adeno-associated viruses (AAV) are thought to spread through the central nervous system (CNS) by exploiting cerebrospinal fluid (CSF) flux and hijacking axonal transport pathways. The role of host receptors that mediate these processes is not well understood. In the current study, we utilized AAV serotype 4 (AAV4) as a model to evaluate whether ubiquitously expressed 2,3-linked sialic acid and the developmentally regulated marker 2,8-linked polysialic acid (PSA) regulate viral transport and tropism in the neonatal brain. Modulation of the levels of SA and PSA in cell culture studies using specific neuraminidases revealed possibly opposing roles of the two glycans in AAV4 transduction. Interestingly, upon intracranial injection into lateral ventricles of the neonatal mouse brain, a low-affinity AAV4 mutant (AAV4.18) displayed a striking shift in cellular tropism from 2,3-linked SA(+) ependymal lining to 2,8-linked PSA(+) migrating progenitors in the rostral migratory stream and olfactory bulb. In addition, this gain-of-function phenotype correlated with robust CNS spread of AAV4.18 through paravascular transport pathways. Consistent with these observations, altering glycan dynamics within the brain by coadministering SA- and PSA-specific neuraminidases resulted in striking changes to the cellular tropisms and transduction efficiencies of both parental and mutant vectors. We postulate that glycan signatures associated with host development can be exploited to redirect novel AAV vectors to specific cell types in the brain.

IMPORTANCE

Viruses invade the CNS through various mechanisms. In the current study, we utilized AAV as a model to study the dynamics of virus-carbohydrate interactions in the developing brain and their impact on viral tropism. Our findings suggest that carbohydrate content can be exploited to regulate viral transport and tropism in the brain.

A promising gene delivery system developed from PEGylated MoS2 nanosheets for gene therapy

A new class of two-dimensional (2D) nanomaterial, transition metal dichalcogenides (TMDCs) such as MoS2, MoSe2, WS2, and WSe2 which have fantastic physical and chemical properties, has drawn tremendous attention in different fields recently. Herein, we for the first time take advantage of the great potential of MoS2 with well-engineered surface as a novel type of 2D nanocarriers for gene delivery and therapy of cancer. In our system, positively charged MoS2-PEG-PEI is synthesized with lipoic acid-modified polyethylene glycol (LA-PEG) and branched polyethylenimine (PEI). The amino end of positively charged nanomaterials can bind to the negatively charged small interfering RNA (siRNA). After detection of physical and chemical characteristics of the nanomaterial, cell toxicity was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Polo-like kinase 1 (PLK1) was investigated as a well-known oncogene, which was a critical regulator of cell cycle transmission at multiple levels. Through knockdown of PLK1 with siRNA carried by novel nanovector, qPCR and Western blot were used to measure the interfering efficiency; apoptosis assay was used to detect the transfection effect of PLK1. All results showed that the novel nanocarrier revealed good biocompatibility, reduced cytotoxicity, as well as high gene-carrying ability without serum interference, thus would have great potential for gene delivery and therapy.