Tropism and toxicity of adeno-associated viral vector serotypes 1, 2, 5, 6, 7, 8, and 9 in rat neurons and glia in vitro

Excitotoxic glutamate levels cause the secretion of resident endoplasmic reticulum proteins

Dysregulation of synaptic glutamate levels can lead to excitotoxicity such as that observed in stroke, traumatic brain injury, and epilepsy. The role of increased intracellular calcium (Ca2+) in the development of excitotoxicity is well established. However, less is known regarding the impact of glutamate on endoplasmic reticulum (ER)-Ca2+-mediated processes such as proteostasis. To investigate this, we expressed a secreted ER Ca2+ modulated protein (SERCaMP) in primary cortical neurons to monitor exodosis, a phenomenon whereby ER calcium depletion causes the secretion of ER-resident proteins that perform essential functions to the ER and the cell. Activation of glutamatergic receptors (GluRs) led to an increase in SERCaMP secretion indicating that normally ER-resident proteins are being secreted in a manner consistent with ER Ca2+ depletion. Antagonism of ER Ca2+ channels attenuated the effects of glutamate and GluR agonists on SERCaMP release. We also demonstrate that endogenous proteins containing an ER retention/retrieval sequence (ERS) are secreted in response to GluR activation supporting that neuronal activation by glutamate promotes ER exodosis. Ectopic expression of KDEL receptors attenuated the secretion of ERS-containing proteins caused by GluR agonists. Taken together, our data indicate that excessive GluR activation causes disruption of neuronal proteostasis by triggering the secretion of ER-resident proteins through ER Ca2+ depletion and describes a new facet of excitotoxicity.

Miniaturization of hiPSC-derived 3D neural cultures in stirred-tank bioreactors for parallelized preclinical assessment of rAAV

Introduction: Engineered 3D models employing human induced pluripotent stem cell (hiPSC) derivatives have the potential to recapitulate the cell diversity and structure found in the human central nervous system (CNS). Therefore, these complex cellular systems offer promising human models to address the safety and potency of advanced therapy medicinal products (ATMPs), such as gene therapies. Specifically, recombinant adeno-associated viruses (rAAVs) are currently considered highly attractive for CNS gene therapy due to their broad tropism, low toxicity, and moderate immunogenicity. To accelerate the clinical translation of rAAVs, in-depth preclinical evaluation of efficacy and safety in a human setting is primordial. The integration of hiPSC-derived CNS models in rAAV development will require, amongst other factors, robust, small-scale, high-throughput culture platforms that can feed the preclinical trials. Methods: Herein, we pioneer the miniaturization and parallelization of a 200 mL stirred-tank bioreactor-based 3D brain cell culture derived from hiPSCs. We demonstrate the applicability of the automated miniaturized Ambr® 15 Cell Culture system for the maintenance of hiPSC-derived neurospheroids (iNSpheroids), composed of neuronal and glial cells. Critical process parameters were optimized, namely, cell density and agitation mode. Results: Under optimized conditions, stable iNSpheroid cultures were attained in the microbioreactors for at least 15 days, with high cell viability and astrocytic and neuronal phenotype maintenance. This culture setup allowed the parallelization of different rAAVs, in different multiplicity of infections (MOIs), to address rAAV-host interactions at a preclinical scale. The iNSpheroids were exposed to rAAV2- and rAAV9-eGFP in the microbioreactors. Transgene expression was detected 14 days post-transduction, revealing different astrocyte/neuron tropism of the two serotypes. Discussion: We advocate that the iNSpheroid cultures in miniaturized bioreactors are reliable and reproducible screening tools for addressing rAAV transduction and tropism, compatible with preclinical demands.

Deep Learning for Cell Migration in Nonwoven Materials and Evaluating Gene Transfer Effects following AAV6-ND4 Transduction

Studying cell settlement in the three-dimensional structure of synthetic biomaterials over time is of great interest in research and clinical translation for the development of artificial tissues and organs. Tracking cells as physical objects improves our understanding of the processes of migration, homing, and cell division during colonisation of the artificial environment. In this study, the 3D environment had a direct effect on the behaviour of biological objects. Recently, deep learning-based algorithms have shown significant benefits for cell segmentation tasks and, furthermore, for biomaterial design optimisation. We analysed the primary LHON fibroblasts in an artificial 3D environment after adeno-associated virus transduction. Application of these tools to model cell homing in biomaterials and to monitor cell morphology, migration and proliferation indirectly demonstrated restoration of the normal cell phenotype after gene manipulation by AAV transduction. Following the 3Rs principles of reducing the use of living organisms in research, modeling the formation of tissues and organs by reconstructing the behaviour of different cell types on artificial materials facilitates drug testing, the study of inherited and inflammatory diseases, and wound healing. These studies on the composition and algorithms for creating biomaterials to model the formation of cell layers were inspired by the principles of biomimicry.

FGF2 gene's antisense protein, NUDT6, plays a depressogenic role by promoting inflammation and suppressing neurogenesis without altering FGF2 signalling

Fibroblast growth factor-2 (FGF2) is involved in the regulation of affective behaviour and shows antidepressant effects through the Akt and extracellular signal regulated kinase (ERK) 1/2 pathways. Nudix hydrolase 6 (NUDT6) protein is encoded from FGF2 gene's antisense strand and its role in the regulation of affective behaviour is unknown. Here, we overexpressed NUDT6 in the hippocampus and investigated its behavioural effects and the underlying molecular mechanisms affecting the behaviour. We showed that increasing hippocampal NUDT6 results in depression-like behaviour in rats without changing FGF2 levels or activating its downstream effectors, Akt and ERK1/2. Instead, NUDT6 acted by inducing inflammatory signalling, specifically by increasing S100 calcium binding protein A9 (S100A9) levels, activating nuclear factor-kappa B-p65 (NF-κB-p65), and elevating microglia numbers along with a reduction in neurogenesis. Our results suggest that NUDT6 could play a role in major depression by inducing a proinflammatory state. This is the first report of an antisense protein acting through a different mechanism of action than regulation of its sense protein. The opposite effects of NUDT6 and FGF2 on depression-like behaviour may serve as a mechanism to fine-tune affective behaviour. Our findings open up new venues for studying the differential regulation and functional interactions of sense and antisense proteins in neural function and behaviour, as well as in neuropsychiatric disorders. KEY POINTS: Hippocampal overexpression of nudix hydrolase 6 (NUDT6), the antisense protein of fibroblast growth factor-2 (FGF2), increases depression-like behaviour in rats. Hippocampal NUDT6 overexpression triggers a neuroinflammatory cascade by increasing S100 calcium binding proteinA9 (S100A9) expression and nuclear NF-κB-p65 translocation in neurons, in addition to microglial recruitment and activation. Hippocampal NUDT6 overexpression suppresses neurogenesis. NUDT6 exerts its actions without altering the levels or downstream signalling pathways of FGF2.

Establishment of a high-content imaging assay for tau aggregation in hiPSC-derived neurons differentiated from two protocols to routinely evaluate compounds and genetic perturbations

Aberrant protein aggregation is a pathological cellular hallmark of many neurodegenerative diseases, such as Alzheimer's disease (AD) and frontotemporal dementia (FTD), where the tau protein is aggregating, forming neurofibrillary tangles (NFTs), and propagating from neuron to neuron. These processes have been linked to disease progression and a decline in cognitive function. Various therapeutic approaches aim at the prevention or reduction of tau aggregates in neurons. Human induced pluripotent stem cells (hiPSCs) are a very valuable tool in neuroscience discovery, as they offer access to potentially unlimited amounts of cell types that are affected in disease, including cortical neurons, for in vitro studies. We have generated an in vitro model for tau aggregation that uses hiPSC - derived neurons expressing an aggregation prone, fluorescently tagged version of the human tau protein after lentiviral transduction. Upon addition of tau seeds in the form of recombinant sonicated paired helical filaments (sPHFs), the neurons show robust, disease-like aggregation of the tau protein. The model was developed as a plate-based high content screening assay coupled with an image analysis algorithm to evaluate the impact of small molecules or genetic perturbations on tau. We show that the assay can be used to evaluate small molecules or screen targeted compound libraries. Using siRNA-based gene knockdown, genes of interest can be evaluated, and we could show that a targeted gene library can be screened, by screening nearly 100 deubiquitinating enzymes (DUBs) in that assay. The assay uses an imaging-based readout, a relatively short timeline, quantifies the extent of tau aggregation, and also allows the assessment of cell viability. Furthermore, it can be easily adapted to different hiPSC lines or neuronal subtypes. Taken together, this complex and highly relevant approach can be routinely applied on a weekly basis in the screening funnels of several projects and generates data with a turnaround time of approximately five weeks.

Engineering, applications, and future perspectives of GPCR-based genetically encoded fluorescent indicators for neuromodulators

This review explores the evolving landscape of G-protein-coupled receptor (GPCR)-based genetically encoded fluorescent indicators (GEFIs), with a focus on their development, structural components, engineering strategies, and applications. We highlight the unique features of this indicator class, emphasizing the importance of both the sensing domain (GPCR structure and activation mechanism) and the reporting domain (circularly permuted fluorescent protein (cpFP) structure and fluorescence modulation). Further, we discuss indicator engineering approaches, including the selection of suitable cpFPs and expression systems. Additionally, we showcase the diversity and flexibility of their application by presenting a summary of studies where such indicators were used. Along with all the advantages, we also focus on the current limitations as well as common misconceptions that arise when using these indicators. Finally, we discuss future directions in indicator engineering, including strategies for screening with increased throughput, optimization of the ligand-binding properties, structural insights, and spectral diversity.

Reduction of Tumor Growth with RNA-Targeting Treatment of the NAB2-STAT6 Fusion Transcript in Solitary Fibrous Tumor Models

Solitary fibrous tumor (SFT) is a rare soft-tissue sarcoma. This nonhereditary cancer is the result of an environmental intrachromosomal gene fusion between NAB2 and STAT6 on chromosome 12, which fuses the activation domain of STAT6 with the repression domain of NAB2. Currently there is not an approved chemotherapy regimen for SFTs. The best response on available pharmaceuticals is a partial response or stable disease for several months. The purpose of this study is to investigate the potential of RNA-based therapies for the treatment of SFTs. Specifically, in vitro SFT cell models were engineered to harbor the characteristic NAB2-STAT6 fusion using the CRISPR/SpCas9 system. Cell migration as well as multiple cancer-related signaling pathways were increased in the engineered cells as compared to the fusion-absent parent cells. The SFT cell models were then used for evaluating the targeting efficacies of NAB2-STAT6 fusion-specific antisense oligonucleotides (ASOs) and CRISPR/CasRx systems. Our results showed that fusion specific ASO treatments caused a 58% reduction in expression of fusion transcripts and a 22% reduction in cell proliferation after 72 h in vitro. Similarly, the AAV2-mediated CRISPR/CasRx system led to a 59% reduction in fusion transcript expressions in vitro, and a 55% reduction in xenograft growth after 29 days ex vivo.

Lighting Up Neural Circuits by Viral Tracing

Neurons are highly interwoven to form intricate neural circuits that underlie the diverse functions of the brain. Dissecting the anatomical organization of neural circuits is key to deciphering how the brain processes information, produces thoughts, and instructs behaviors. Over the past decades, recombinant viral vectors have become the most commonly used tracing tools to define circuit architecture. In this review, we introduce the current categories of viral tools and their proper application in circuit tracing. We further discuss some advances in viral tracing strategy and prospective innovations of viral tools for future study.

Fluorochromized tyramide-glucose oxidase as a multiplex fluorescent tyramide signal amplification system for histochemical analysis

Tyramide signal amplification (TSA) is a highly sensitive method for histochemical analysis. Previously, we reported a TSA system, biotinyl tyramine-glucose oxidase (BT-GO), for bright-filed imaging. Here, we develop fluorochromized tyramide-glucose oxidase (FT-GO) as a multiplex fluorescent TSA system. FT-GO involves peroxidase-catalyzed deposition of fluorochromized tyramide (FT) with hydrogen peroxide produced by enzymatic reaction between glucose and glucose oxidase. We showed that FT-GO enhanced immunofluorescence signals while maintaining low background signals. Compared with indirect immunofluorescence detections, FT-GO demonstrated a more widespread distribution of monoaminergic projection systems in mouse and marmoset brains. For multiplex labeling with FT-GO, we quenched antibody-conjugated peroxidase using sodium azide. We applied FT-GO to multiplex fluorescent in situ hybridization, and succeeded in labeling neocortical interneuron subtypes by coupling with immunofluorescence. FT-GO immunofluorescence further increased the detectability of an adeno-associated virus tracer. Given its simplicity and a staining with a high signal-to-noise ratio, FT-GO would provide a versatile platform for histochemical analysis.

Augmenting neurogenesis rescues memory impairments in Alzheimer's disease by restoring the memory-storing neurons

Hippocampal neurogenesis is impaired in Alzheimer's disease (AD) patients and familial Alzheimer's disease (FAD) mouse models. However, it is unknown whether new neurons play a causative role in memory deficits. Here, we show that immature neurons were actively recruited into the engram following a hippocampus-dependent task. However, their recruitment is severely deficient in FAD. Recruited immature neurons exhibited compromised spine density and altered transcript profile. Targeted augmentation of neurogenesis in FAD mice restored the number of new neurons in the engram, the dendritic spine density, and the transcription signature of both immature and mature neurons, ultimately leading to the rescue of memory. Chemogenetic inactivation of immature neurons following enhanced neurogenesis in AD, reversed mouse performance, and diminished memory. Notably, AD-linked App, ApoE, and Adam10 were of the top differentially expressed genes in the engram. Collectively, these observations suggest that defective neurogenesis contributes to memory failure in AD.

The tyrosine capsid mutations on retrograde adeno-associated virus accelerates gene transduction efficiency

Adeno-associated virus (AAV) vector is a critical tool for gene delivery through its durable transgene expression and safety profile. Among many serotypes, AAV2-retro is typically utilized for dissecting neural circuits with its retrograde functionality. However, this vector requires a relatively long-term incubation period (over 2 weeks) to obtain enough gene expression levels presumably due to low efficiency in gene transduction. Here, we aimed to enhance transgene expression efficiency of AAV2-retro vectors by substituting multiple tyrosine residues with phenylalanines (YF mutations) in the virus capsid, which is previously reported to improve the transduction efficiency of AAV2-infected cells by evading host cell responses. We found that AAV2-retro with YF mutations (AAV2-retroYF)-mediated transgene expression was significantly enhanced in the primary culture of murine cortical neurons at 1 week after application, comparable to that of the conventional AAV2-retro at 2 week after application. Moreover, transgene expressions in the retrogradely labeled neurons mediated by AAV2-retroYF were significantly increased both in the cortico-cortical circuits and in the subcortical circuits in vivo, while the retrograde functionality of AAV2-retroYF was equally effective as that of AAV2-retro. Our data indicate that YF mutations boost AAV2-retro-mediated retrograde gene transduction in vivo and suggest that the AAV2-retroYF should be useful for efficient targeting of the projection-defined neurons, which is suited to applications for dissecting neural circuits during development as well as future clinical applications.

AAV gene therapy vectors in the TMJ

OBJECTIVES

The goal of this project was to evaluate the use of two adeno-associated viral vector serotypes, adeno-associated viral vectors (AAV)-2 and AAV-6, approved for and used for gene therapy in humans, for the delivery of therapeutic genes to the temporomandibular joint (TMJ) and the attendant sensory nerves.

METHODS

Young adult wild-type C57BL/6 mice were intra-articularly inoculated with AAV-2 and AAV-6 encoding the reporter gene gfp, the expression of which was assessed in the TMJ as well as along nerves innervating the TMJ.

RESULTS

AAV-2 and AAV-6 serotypes were characterized by varying levels of tissue tropism demonstrating different efficacy of infection for articular chondrocytes, meniscal fibroblasts, and trigeminal neurons. Specifically, AAV-2 infected both neurons and articular chondrocytes/meniscal fibroblasts, whereas AAV-6 showed selectivity primarily for neurons.

CONCLUSIONS

The results of this study are clinically significant in the successful application of gene therapy vectors for TMJ disorders, as this new knowledge will allow for appropriate targeting of specific therapeutic genes to selective tissues (neurons vs. chondrocytes/fibroblasts) as needed by using specific viral vector serotypes.

Glial cells as integrators of peripheral and central signals in the regulation of energy homeostasis

Communication between the periphery and the brain is key for maintaining energy homeostasis. To do so, peripheral signals from the circulation reach the brain via the circumventricular organs (CVOs), which are characterized by fenestrated vessels lacking the protective blood-brain barrier (BBB). Glial cells, by virtue of their plasticity and their ideal location at the interface of blood vessels and neurons, participate in the integration and transmission of peripheral information to neuronal networks in the brain for the neuroendocrine control of whole-body metabolism. Metabolic diseases, such as obesity and type 2 diabetes, can disrupt the brain-to-periphery communication mediated by glial cells, highlighting the relevance of these cell types in the pathophysiology of such complications. An improved understanding of how glial cells integrate and respond to metabolic and humoral signals has become a priority for the discovery of promising therapeutic strategies to treat metabolic disorders. This Review highlights the role of glial cells in the exchange of metabolic signals between the periphery and the brain that are relevant for the regulation of whole-body energy homeostasis.

Caffeine and MDMA (Ecstasy) Exacerbate ER Stress Triggered by Hyperthermia

Drugs of abuse can cause local and systemic hyperthermia, a known trigger of endoplasmic reticulum (ER) stress and the unfolded protein response (UPR). Another trigger of ER stress and UPR is ER calcium depletion, which causes ER exodosis, the secretion of ER-resident proteins. In rodent models, club drugs such as 3,4-methylenedioxymethamphetamine (MDMA, ‘ecstasy’) can create hyperthermic conditions in the brain and cause toxicity that is affected by the environmental temperature and the presence of other drugs, such as caffeine. In human studies, MDMA stimulated an acute, dose-dependent increase in core body temperature, but an examination of caffeine and MDMA in combination remains a topic for clinical research. Here we examine the secretion of ER-resident proteins and activation of the UPR under combined exposure to MDMA and caffeine in a cellular model of hyperthermia. We show that hyperthermia triggers the secretion of normally ER-resident proteins, and that this aberrant protein secretion is potentiated by the presence of MDMA, caffeine, or a combination of the two drugs. Hyperthermia activates the UPR but the addition of MDMA or caffeine does not alter the canonical UPR gene expression despite the drug effects on ER exodosis of UPR-related proteins. One exception was increased BiP/GRP78 mRNA levels in MDMA-treated cells exposed to hyperthermia. These findings suggest that club drug use under hyperthermic conditions exacerbates disruption of ER proteostasis, contributing to cellular toxicity.

Non-Integrating Lentiviral Vectors in Clinical Applications: A Glance Through

Lentiviral vectors (LVs) play an important role in gene therapy and have proven successful in clinical trials. LVs are capable of integrating specific genetic materials into the target cells and allow for long-term expression of the cDNA of interest. The use of non-integrating LVs (NILVs) reduces insertional mutagenesis and the risk of malignant cell transformation over integrating lentiviral vectors. NILVs enable transient expression or sustained episomal expression, especially in non-dividing cells. Important modifications have been made to the basic human immunodeficiency virus (HIV) structures to improve the safety and efficacy of LVs. NILV-aided transient expression has led to more pre-clinical studies on primary immunodeficiencies, cytotoxic cancer therapies, and hemoglobinopathies. Recently, the third generation of self-inactivating LVs was applied in clinical trials for recombinant protein production, vaccines, gene therapy, cell imaging, and induced pluripotent stem cell (iPSC) generation. This review discusses the basic lentiviral biology and the four systems used for generating NILV designs. Mutations or modifications in LVs and their safety are addressed with reference to pre-clinical studies. The detailed application of NILVs in promising pre-clinical studies is also discussed.

Feasibility of Targeted Delivery of AAV5-GFP into the Cerebellum of Nonhuman Primates Following a Single Convection-Enhanced Delivery Infusion

In this study, we built upon our previous work to demonstrate the distribution and transport of AAV5-green fluorescent protein (GFP) following a single convection-enhanced delivery infusion into the nonhuman primate cerebellum, with no untoward side effects noted. Dosing under magnetic resonance imaging guidance revealed a sixfold larger volume of distribution compared with the volume of infusion, with no evidence of reflux underscoring the convective properties of the cerebellum and step design of the cannula. Postmortem tissue analysis, 4 weeks post-adeno-associated viral (AAV) delivery, revealed the robust presence of the transgene in situ, with GFP detection in secondary regions not directly targeted by the infusion, denoting distal transport of the vector. Irrespective of tropism, a twofold larger area of transgene expression was found and was corroborated against the presence of contrast on T1-weighted images. Different levels of transduction were detected between animals, which were negatively correlated with the level of antibody titer against the GFP construct, whereby the higher the antibody titer, the lower the level of transgene expression. These findings support the use of the posterior fossa as a potential target site for direct delivery of gene-based therapeutics for cerebellar diseases.

GJB2 gene therapy and conditional deletion reveal developmental stage-dependent effects on inner ear structure and function

Pathogenic variants in GJB2, the gene encoding connexin 26, are the most common cause of autosomal-recessive hereditary deafness. Despite this high prevalence, pathogenic mechanisms leading to GJB2-related deafness are not well understood, and cures are absent. Humans with GJB2-related deafness retain at least some auditory hair cells and neurons, and their deafness is usually stable. In contrast, mice with conditional loss of Gjb2 in supporting cells exhibit extensive loss of hair cells and neurons and rapidly progress to profound deafness, precluding the application of therapies that require intact cochlear cells. In an attempt to design a less severe Gjb2 animal model, we generated mice with inducible Sox10iCre ^ERT2 -mediated loss of Gjb2. Tamoxifen injection led to reduced connexin 26 expression and impaired function, but cochlear hair cells and neurons survived for 2 months, allowing phenotypic rescue attempts within this time. AAV-mediated gene transfer of GJB2 in mature mutant ears did not demonstrate threshold improvement and in some animals exacerbated hearing loss and resulted in hair cell loss. We conclude that Sox10iCre ^ERT2 ;Gjb2 ^flox/flox mice are valuable for studying the biology of connexin 26 in the cochlea. In particular, these mice may be useful for evaluating gene therapy vectors and development of therapies for GJB2-related deafness.

Level of hM4D(Gi) DREADD Expression Determines Inhibitory and Neurotoxic Effects in the Hippocampus

Selective neuromodulation using designer receptors exclusively activated by designer drugs (DREADDs) has become an increasingly important research tool, as well as an emerging therapeutic approach. However, the safety profile of DREADD expression is unknown. Here, different titers of adeno-associated viral (AAV) vector were administered in an attempt to vary total expression levels of the inhibitory DREADD hM4D(Gi) in excitatory hippocampal neurons. Male Sprague Dawley rats were injected with AAV2/7 encoding DREADD-mCherry, DREADD, or mCherry. Pronounced neuronal loss and neuroinflammatory reactions were observed after transduction with the high titer DREADD AAV, which also resulted in the highest DREADD expression levels. No such effects were observed in the mCherry control group, despite an equally high titer, nor in conditions where lower viral vector titers were injected. In the high titer DREADD conditions, dentate gyrus (DG) evoked potentials were inhibited on clozapine-induced activation of hM4D(Gi), while in low titer conditions DG evoked potentials were enhanced. Recordings of single neuronal activity nevertheless indicated a reduction in spontaneous firing of granule cell layer neurons. Our results indicate that prolonged, high levels of DREADD expression can have neurotoxic effects and that chemogenetic suppression of excitatory hippocampal neurons can paradoxically enhance DG evoked potentials.

Higher Transduction Efficiency of AAV5 to Neural Stem Cells and Immature Neurons in Gerbil Dentate Gyrus Compared to AAV2 and rh10

The safety and high efficiency of adeno-associated virus (AAV) vectors has facilitated their wide scale use to deliver therapeutic genes for experimental and clinical purposes in diseases affecting the central nervous system (CNS). AAV1, 2, 5, 8, 9, and rh10 are the most commonly used serotypes for CNS applications. Most AAVs are known to transduce genes predominantly into neurons. However, the precise tropism of AAVs in the dentate gyrus (DG), the region where persistent neurogenesis occurs in the adult brain, is not fully understood. We stereotaxically injected 1.5 × 10¹⁰ viral genomes of AAV2, 5, or rh10 carrying green fluorescent protein (GFP) into the right side of gerbil hippocampus, and performed immunofluorescent analysis using differentiation stage-specific markers one week after injection. We found that AAV5 showed a significantly larger number of double positive cells for GFP and Sox2 in the DG, compared to the AAV2 and rh10 groups. On the other hand, AAVrh10 presented a substantially larger number of double positive cells for GFP and NeuN in the DG, compared to AAV2 and AAV5. Our findings indicated that AAV5 showed high transduction efficiency to neural stem cells and precursor cells, while AAVrh10 showed much higher efficiency to mature neurons in the DG.

Use of CRISPR/Cas9-mediated disruption of CNS cell type genes to profile transduction of AAV by neonatal intracerebroventricular delivery in mice

Adeno-associated virus (AAV) transduction efficiency and tropism are conventionally determined by high expression of a fluorescent reporter gene. Emerging data has suggested that such conventional methods may underestimate AAV transduction for cells in which reporter expression from AAV vectors is undetectable. To explore an alternative method that captures AAV transduction in cells in which low expression of a cargo is sufficient for the intended activity, we sought after CRISPR/Cas9-mediated gene disruption. In this study, we use AAV to deliver CRISPR/guide RNA designed to abolish the genes NeuN, GFAP, or MOG expressed specifically in neurons, astrocytes, or oligodendrocytes respectively in the central nervous system (CNS) of mice. Abrogated expression of these cell-type-specific genes can be measured biochemically in CNS subregions and provides quantitative assessment of AAV transduction in these CNS cell types. By using this method, we compared CNS transduction of AAV9, AAV-PHP.B, and AAV-PHP.eB delivered via intracerebroventricular injection (ICV) in neonatal mice. We found both AAV-PHP.B and AAV-PHP.eB resulted in marked disruption of the NeuN gene by CRISPR/Cas9, significantly greater than AAV9 in several brain regions and spinal cord. In contrast, only modest disruption of the GFAP gene and the MOG gene was observed by all three AAV variants. Since the procedure of ICV circumvents the blood-brain barrier, our data suggests that, independent of their ability to cross the blood-brain barrier, AAV-PHP.B variants also exhibit remarkably improved neuronal transduction in the CNS. We anticipate this approach will facilitate profiling of AAV cellular tropism in murine CNS.

Genetic Constructs for the Control of Astrocytes' Activity

In the current review, we aim to discuss the principles and the perspectives of using the genetic constructs based on AAV vectors to regulate astrocytes’ activity. Practical applications of optogenetic approaches utilizing different genetically encoded opsins to control astroglia activity were evaluated. The diversity of astrocytic cell-types complicates the rational design of an ideal viral vector for particular experimental goals. Therefore, efficient and sufficient targeting of astrocytes is a multiparametric process that requires a combination of specific AAV serotypes naturally predisposed to transduce astroglia with astrocyte-specific promoters in the AAV cassette. Inadequate combinations may result in off-target neuronal transduction to different degrees. Potentially, these constraints may be bypassed with the latest strategies of generating novel synthetic AAV serotypes with specified properties by rational engineering of AAV capsids or using directed evolution approach by searching within a more specific promoter or its replacement with the unique enhancer sequences characterized using modern molecular techniques (ChIP-seq, scATAC-seq, snATAC-seq) to drive the selective transgene expression in the target population of cells or desired brain regions. Realizing these strategies to restrict expression and to efficiently target astrocytic populations in specific brain regions or across the brain has great potential to enable future studies.

AAV Targeting of Glial Cell Types in the Central and Peripheral Nervous System and Relevance to Human Gene Therapy

Different glial cell types are found throughout the central (CNS) and peripheral nervous system (PNS), where they have important functions. These cell types are also involved in nervous system pathology, playing roles in neurodegenerative disease and following trauma in the brain and spinal cord (astrocytes, microglia, oligodendrocytes), nerve degeneration and development of pain in peripheral nerves (Schwann cells, satellite cells), retinal diseases (Müller glia) and gut dysbiosis (enteric glia). These cell type have all been proposed as potential targets for treating these conditions. One approach to target these cell types is the use of gene therapy to modify gene expression. Adeno-associated virus (AAV) vectors have been shown to be safe and effective in targeting cells in the nervous system and have been used in a number of clinical trials. To date, a number of studies have tested the use of different AAV serotypes and cell-specific promoters to increase glial cell tropism and expression. However, true glial-cell specific targeting for a particular glial cell type remains elusive. This review provides an overview of research into developing glial specific gene therapy and discusses some of the issues that still need to be addressed to make glial cell gene therapy a clinical reality.

A comparison of AAV-vector production methods for gene therapy and preclinical assessment

Adeno Associated Virus (AAV)-mediated gene expression in the brain is widely applied in the preclinical setting to investigate the therapeutic potential of specific molecular targets, characterize various cellular functions, and model central nervous system (CNS) diseases. In therapeutic applications in the clinical setting, gene therapy offers several advantages over traditional pharmacological based therapies, including the ability to directly manipulate disease mechanisms, selectively target disease-afflicted regions, and achieve long-term therapeutic protein expression in the absence of repeated administration of pharmacological agents. Next to the gold-standard iodixanol-based AAV vector production, we recently published a protocol for AAV production based on chloroform-precipitation, which allows for fast in-house production of small quantities of AAV vector without the need for specialized equipment. To validate our recent protocol, we present here a direct side-by-side comparison between vectors produced with either method in a series of in vitro and in vivo assays with a focus on transgene expression, cell loss, and neuroinflammatory responses in the brain. We do not find differences in transduction efficiency nor in any other parameter in our in vivo and in vitro panel of assessment. These results suggest that our novel protocol enables most standardly equipped laboratories to produce small batches of high quality and high titer AAV vectors for their experimental needs.

Self-complementarity in adeno-associated virus enhances transduction and gene expression in mouse cochlear tissues

Sensorineural hearing loss is one of the most common disabilities worldwide. Such prevalence necessitates effective tools for studying the molecular workings of cochlear cells. One prominent and effective vector for expressing genes of interest in research models is adeno-associated virus (AAV). However, AAV efficacy in transducing cochlear cells can vary for a number of reasons including serotype, species, and methodology, and oftentimes requires high multiplicity of infection which can damage the sensory cells. Reports in other systems suggest multiple approaches can be used to enhance AAV transduction including self-complementary vector design and pharmacological inhibition of degradation. Here we produced AAV to drive green fluorescent protein (GFP) expression in explanted neonatal mouse cochleae. Treatment with eeyarestatin I, tyrphostin 23, or lipofectamine 2000 did not result in increased transduction, however, self-complementary vector design resulted in significantly more GFP positive cells when compared to single-stranded controls. Similarly, self-complementary AAV2 vectors demonstrated enhanced transduction efficiency compared to single stranded AAV2 when injected via the posterior semicircular canal, in vivo. Self-complementary vectors for AAV1, 8, and 9 serotypes also demonstrated robust GFP transduction in cochlear cells in vivo, though these were not directly compared to single stranded vectors. These findings suggest that second-strand synthesis may be a rate limiting step in AAV transduction of cochlear tissues and that self-complementary AAV can be used to effectively target large numbers of cochlear cells in vitro and in vivo.

Functional interrogation of neural circuits with virally transmitted optogenetic tools

The vertebrate brain comprises a plethora of cell types connected by intertwined pathways. Optogenetics enriches the neuroscientific tool set for disentangling these neuronal circuits in a manner which exceeds the spatio-temporal precision of previously existing techniques. Technically, optogenetics can be divided in three types of optical and genetic combinations: (1) it is primarily understood as the manipulation of the activity of genetically modified cells (typically neurons) with light, i.e. optical actuators. (2) A second combination refers to visualizing the activity of genetically modified cells (again typically neurons), i.e. optical sensors. (3) A completely different interpretation of optogenetics refers to the light activated expression of a genetically induced construct. Here, we focus on the first two types of optogenetics, i.e. the optical actuators and sensors in an attempt to give an overview into the topic. We first cover methods to express opsins into neurons and introduce strategies of targeting specific neuronal populations in different animal species. We then summarize combinations of optogenetics with behavioral read out and neuronal imaging. Finally, we give an overview of the current state-of-the-art and an outlook on future perspectives.

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

The Influence of Murine Genetic Background in Adeno-Associated Virus Transduction of the Mouse Brain

Adeno-associated virus (AAV) vectors have become an important tool for delivering therapeutic genes for a wide range of neurological diseases. AAV serotypes possess differential cellular tropism in the central nervous system. Although several AAV serotypes or mutants have been reported to transduce the brain efficiently, conflicting data occur across studies with the use of various rodent strains from different genetic backgrounds. Herein, we performed a systematic comparison of the brain transduction properties among five AAV serotypes (AAV2, 5, 7, 8, and 9) in two common rodent strains (C57BL/6J and FVB/N), following local intrastriatal injection of AAV vectors encoding enhanced green fluorescent protein (EGFP) driven by the CBh promoter. Important differences were found regarding overall cellular tropism and transduction efficiency, including contralateral transduction among the AAV serotypes and between the mouse strains. We have further found loss of NeuN-immunoreactivity and microglial activation from AAV transduction in the different mouse strains. The important strain-specific differences from our study suggest that the genetic background of the mouse may affect AAV serotype transduction properties in the brain. These data can provide valuable information about how to choose an effective AAV vector for clinical application and interpret the data obtained from preclinical studies and clinical trials.

Viral vectors for neuronal cell type-specific visualization and manipulations

Characterizing neuronal cell types demands efficient strategies for specific labeling and manipulation of individual subtypes to dissect their connectivity and functions. Recombinant viral technology offers a powerful toolbox for targeted transgene expression in specific neuronal populations. In order to achieve cell type-specific targeting, exciting progress has been made to: alter viral tropisms, design rational delivery strategies, and drive selective expression patterns with engineered DNA sequences in viral genomes. For the latter case, emerging single-cell genomic analyses provide rich databases. In this review, we will summarize current status, and point out challenges, of using viral vectors for neuronal cell type-specific visualization and manipulations. With concerted efforts, progress will continue to be made toward developing viral vectors for the vast array of neuronal subtypes in the mammalian nervous system.

Epstein-Barr virus-derived vector suitable for long-term expression in neurons

Exogenous gene expression is a fundamental and indispensable technique for testing gene function in neurons. Several ways to express exogenous genes in neurons are available, but each method has pros and cons. The lentivirus vector is useful for high efficiency gene transfer to neurons and stabilizes gene expression via genome integration, but this integration may destroy the host genome. The Epstein-Barr virus (EBV)-derived vector (EB vector) is an accessible and useful vector in human cell lines because the vector is not integrated into the host genome but stays in the nucleus as an episome. However, there has been no report on this process in rodent neurons.

We examined the usefulness of the EB vector for testing gene function in neurons. We found that EB vector-derived exogenous proteins such as green fluorescent protein (GFP) and GFP-tagged actin were easily detectable even after three weeks of transfection. Second, a tetracycline-induced gene expression system in the EB vector was active after three weeks of transfection, indicating that plasmids were retained in neurons for up to three weeks. Third, we determined that only Family of repeat element of the plasmid vector is essential for its long-term presence in neurons. These results show that the modified EB vector is a useful tool for examining gene function in neurons.

KDEL Receptors Are Differentially Regulated to Maintain the ER Proteome under Calcium Deficiency

Retention of critical endoplasmic reticulum (ER) luminal proteins needed to carry out diverse functions (e.g., protein synthesis and folding, lipid metabolism) is mediated through a carboxy-terminal ER retention sequence (ERS) and its interaction with KDEL receptors. Here, we demonstrate that depleting ER calcium causes mass departure of ERS-containing proteins from cells by overwhelming KDEL receptors. In addition, we provide evidence that KDELR2 and KDELR3, but not KDELR1, are unfolded protein response (UPR) genes upregulated as an adaptive response to counteract the loss of ERS-containing proteins, suggesting previously unknown isoform-specific functions of the KDEL receptors. Overall, our findings establish that decreases in ER calcium change the composition of the ER luminal proteome and secretome, which can impact cellular functions and cell viability. The redistribution of the ER proteome from inside the cell to the outside has implications for dissecting the complex relationship of ER homeostasis with diverse disease pathologies.

Adeno-Associated Viral Vectors in Neuroscience Research

Adeno-associated viral vectors (AAVs) are increasingly useful preclinical tools in neuroscience research studies for interrogating cellular and neurocircuit functions and mapping brain connectivity. Clinically, AAVs are showing increasing promise as viable candidates for treating multiple neurological diseases. Here, we briefly review the utility of AAVs in mapping neurocircuits, manipulating neuronal function and gene expression, and activity labeling in preclinical research studies as well as AAV-based gene therapies for diseases of the nervous system. This review highlights the vast potential that AAVs have for transformative research and therapeutics in the neurosciences.

Long-term adult human brain slice cultures as a model system to study human CNS circuitry and disease

Most of our knowledge on human CNS circuitry and related disorders originates from model organisms. How well such data translate to the human CNS remains largely to be determined. Human brain slice cultures derived from neurosurgical resections may offer novel avenues to approach this translational gap. We now demonstrate robust preservation of the complex neuronal cytoarchitecture and electrophysiological properties of human pyramidal neurons in long-term brain slice cultures. Further experiments delineate the optimal conditions for efficient viral transduction of cultures, enabling ‘high throughput’ fluorescence-mediated 3D reconstruction of genetically targeted neurons at comparable quality to state-of-the-art biocytin fillings, and demonstrate feasibility of long term live cell imaging of human cells in vitro. This model system has implications toward a broad spectrum of translational studies, regarding the validation of data obtained in non-human model systems, for therapeutic screening and genetic dissection of human CNS circuitry.

Development of Efficient AAV2/DJ-Based Viral Vectors to Selectively Downregulate the Expression of Neuronal or Astrocytic Target Proteins in the Rat Central Nervous System

Viral vectors have become very popular to overexpress or downregulate proteins of interest in different cell types. They conveniently allow the precise targeting of well-defined tissue areas, which is particularly useful in complex organs like the brain. In theory, each vector should have its own cell specificity that can be obtained by using different strategies (e.g., using a cell-specific promoter). For the moment, there is few vectors that have been developed to alternatively target, using the same capsid, neurons and astrocytes in the central nervous system. There is even fewer examples of adeno-associated viral vectors able to efficiently transduce cells both in vitro and in vivo. The development of viral vectors allowing the cell-specific downregulation of a protein in cultured cells of the central nervous system as well as in vivo within a large brain area would be highly desirable to address several important questions in neurobiology. Here we report that the use of the AAV2/DJ viral vector associated to an hybrid CMV/chicken β-actin promoter (CBA) or to a modified form of the glial fibrillary acidic protein promoter (G1B3) allows a specific transduction of neurons or astrocytes in more than half of the barrel field within the rat somatosensory cortex. Moreover, the use of the miR30E-shRNA technology led to an efficient downregulation of two proteins of interest related to metabolism both in vitro and in vivo. Our results demonstrate that it is possible to downregulate the expression of different protein isoforms in a cell-specific manner using a common serotype. It is proposed that such an approach could be extended to other cell types and used to target several proteins of interest within the same brain area.

Neuronal Activation Stimulates Cytomegalovirus Promoter-Driven Transgene Expression

The cytomegalovirus (CMV) immediate early promoter has been extensively developed and exploited for transgene expression in vitro and in vivo, including human clinical trials. The CMV promoter has long been considered a stable, constitutive, and ubiquitous promoter for transgene expression. Using two different CMV-based promoters, we found an increase in CMV-driven transgene expression in the rodent brain and in primary neuronal cultures in response to methamphetamine, glutamate, kainic acid, and activation of G protein-coupled receptor signaling using designer receptors exclusively activated by designer drugs (DREADDs). In contrast, promoters derived from human synapsin 1 (hSYN1) gene or elongation factor 1α (EF1α) did not exhibit altered transgene expression in response to the same neuronal stimulations. Overall, our results suggest that the long-standing assertion that the CMV promoter confers constitutive expression in neurons should be reevaluated, and future studies should empirically determine the activity of the CMV promoter in a given application.

Neuron-Specific Genome Modification in the Adult Rat Brain Using CRISPR-Cas9 Transgenic Rats

Historically, the rat has been the preferred animal model for behavioral studies. Limitations in genome modification have, however, caused a lag in their use compared to the bevy of available transgenic mice. Here, we have developed several transgenic tools, including viral vectors and transgenic rats, for targeted genome modification in specific adult rat neurons using CRISPR-Cas9 technology. Starting from wild-type rats, knockout of tyrosine hydroxylase was achieved with adeno-associated viral (AAV) vectors expressing Cas9 or guide RNAs (gRNAs). We subsequently created an AAV vector for Cre-dependent gRNA expression as well as three new transgenic rat lines to specifically target CRISPR-Cas9 components to dopaminergic neurons. One rat represents the first knockin rat model made by germline gene targeting in spermatogonial stem cells. The rats described herein serve as a versatile platform for making cell-specific and sequence-specific genome modifications in the adult brain and potentially other Cre-expressing tissues of the rat.

Long-term RNAi knockdown of α-synuclein in the adult rat substantia nigra without neurodegeneration

α-Synuclein plays a central role in the pathogenesis of Parkinson's disease (PD); interventions that decrease its expression appear neuroprotective in PD models. Successful translation of these observations into effective therapies will be dependent on the safety of suppressing α-synuclein expression in the adult brain. We investigated long-term α-synuclein knockdown in the adult rat CNS. 8-month old animals received either AAV-sh[Snca] (an RNA interference vector targeting the Snca mRNA transcript) or AAV-sh[Ctrl] (a control vector) unilaterally into the substantia nigra. No signs of systemic toxicity or motor dysfunction were observed in either experimental group over 12 months. Viral transgene expression persisted to 12 months post-inoculation, at which point Snca mRNA expression in substantia nigra dopaminergic neurons of animals that received AAV-sh[Snca] was decreased by ≈90%, and α-synuclein immunoreactivity by >70% relative to the control side. Stereological quantification of Nissl-labeled neurons showed no evidence of neurodegeneration in the substantia nigra 12 months after inoculation with either vector, and we observed abundant dopaminergic neurons with minimal α-synuclein immunoreactivity that appeared otherwise unremarkable in the AAV-sh[Snca] group. Despite the absence of neurodegeneration, some loss of TH expression was evident in nigral neurons after transduction with either vector, presumably a non-specific consequence of vector delivery, cellular transduction, or expression of shRNA or GFP. We conclude that long-term α-synuclein knockdown in the substantia nigra does not cause significant functional deficits in the ascending dopaminergic projection, or neurodegeneration. These findings are encouraging that it may be feasible to target α-synuclein expression therapeutically in PD.

Downregulation of tyrosine hydroxylase phenotype after AAV injection above substantia nigra: Caution in experimental models of Parkinson's disease

Adeno-associated virus (AAV) vector-mediated delivery of human α-synuclein (α-syn) gene in rat substantia nigra (SN) results in increased expression of α-syn protein in the SN and striatum which can progressively degenerate dopaminergic neurons. Therefore, this model is thought to recapitulate the neurodegeneration in Parkinson's disease. Here, using AAV to deliver α-syn above the SN in male and female rats resulted in clear expression of human α-syn in the SN and striatum. The protein was associated with moderate behavioral deficits and some loss of tyrosine hydroxylase (TH) in the nigrostriatal areas. However, the immunohistochemistry results were highly variable and showed little to no correlation with behavior and the amount of α-syn present. Expression of green fluorescent protein (GFP) was used as a control to monitor gene delivery and expression efficacy. AAV-GFP resulted in a similar or greater TH loss compared to AAV-α-syn and therefore an additional vector that does not express a protein was tested. Vectors with double-floxed inverse open reading frame (DIO ORF) encoding fluorescent proteins that generate RNA that is not translated also resulted in TH downregulation in the SN but showed no significant behavioral deficits. These results demonstrate that although expression of wild-type human α-syn can cause neurodegeneration, the variability and lack of correlation with outcome measures are drawbacks with the model. Furthermore, design and control selection should be considered carefully because of conflicting conclusions due to AAV downregulation of TH, and we recommend caution with having highly regulated TH as the only marker for the dopamine system.

Tetrode Recording from the Hippocampus of Behaving Mice Coupled with Four-Point-Irradiation Closed-Loop Optogenetics: A Technique to Study the Contribution of Hippocampal SWR Events to Learning

With the advent of optogenetics, it became possible to change the activity of a targeted population of neurons in a temporally controlled manner. To combine the advantages of 60-channel in vivo tetrode recording and laser-based optogenetics, we have developed a closed-loop recording system that allows for the actual electrophysiological signal to be used as a trigger for the laser light mediating the optogenetic intervention. We have optimized the weight, size, and shape of the corresponding implant to make it compatible with the size, force, and movements of a behaving mouse, and we have shown that the system can efficiently block sharp wave ripple (SWR) events using those events themselves as a trigger. To demonstrate the full potential of the optogenetic recording system we present a pilot study addressing the contribution of SWR events to learning in a complex behavioral task.

Global Transcriptional Response to CRISPR/Cas9-AAV6-Based Genome Editing in CD34+ Hematopoietic Stem and Progenitor Cells

Genome-editing technologies are currently being translated to the clinic. However, cellular effects of the editing machinery have yet to be fully elucidated. Here, we performed global microarray-based gene expression measurements on human CD34⁺ hematopoietic stem and progenitor cells that underwent editing. We probed effects of the entire editing process as well as each component individually, including electroporation, Cas9 (mRNA or protein) with chemically modified sgRNA, and AAV6 transduction. We identified differentially expressed genes relative to control treatments, which displayed enrichment for particular biological processes. All editing machinery components elicited immune, stress, and apoptotic responses. Cas9 mRNA invoked the greatest amount of transcriptional change, eliciting a distinct viral response and global transcriptional downregulation, particularly of metabolic and cell cycle processes. Electroporation also induced significant transcriptional change, with notable downregulation of metabolic processes. Surprisingly, AAV6 evoked no detectable viral response. We also found Cas9/sgRNA ribonucleoprotein treatment to be well tolerated, in spite of eliciting a DNA damage signature. Overall, this data establishes a benchmark for cellular tolerance of CRISPR/Cas9-AAV6-based genome editing, ensuring that the clinical protocol is as safe and efficient as possible.

Pre-α-pro-GDNF and Pre-β-pro-GDNF Isoforms Are Neuroprotective in the 6-hydroxydopamine Rat Model of Parkinson's Disease

Glial cell line-derived neurotrophic factor (GDNF) is one of the most studied neurotrophic factors. GDNF has two splice isoforms, full-length pre-α-pro-GDNF (α-GDNF) and pre-β-pro-GDNF (β-GDNF), which has a 26 amino acid deletion in the pro-region. Thus far, studies have focused solely on the α-GDNF isoform, and nothing is known about the in vivo effects of the shorter β-GDNF variant. Here we compare for the first time the effects of overexpressed α-GDNF and β-GDNF in non-lesioned rat striatum and the partial 6-hydroxydopamine lesion model of Parkinson's disease. GDNF isoforms were overexpressed with their native pre-pro-sequences in the striatum using an adeno-associated virus (AAV) vector, and the effects on motor performance and dopaminergic phenotype of the nigrostriatal pathway were assessed. In the non-lesioned striatum, both isoforms increased the density of dopamine transporter-positive fibers at 3 weeks after viral vector delivery. Although both isoforms increased the activity of the animals in cylinder assay, only α-GDNF enhanced the use of contralateral paw. Four weeks later, the striatal tyrosine hydroxylase (TH)-immunoreactivity was decreased in both α-GDNF and β-GDNF treated animals. In the neuroprotection assay, both GDNF splice isoforms increased the number of TH-immunoreactive cells in the substantia nigra but did not promote behavioral recovery based on amphetamine-induced rotation or cylinder assays. Thus, the shorter GDNF isoform, β-GDNF, and the full-length α-isoform have comparable neuroprotective efficacy on dopamine neurons of the nigrostriatal circuitry.

Assessment of AAV Vector Tropisms for Mouse and Human Pluripotent Stem Cell-Derived RPE and Photoreceptor Cells

Adeno-associated viral vectors are showing great promise as gene therapy vectors for a wide range of retinal disorders. To date, evaluation of therapeutic approaches has depended almost exclusively on the use of animal models. With recent advances in human stem cell technology, stem cell-derived retina now offers the possibility to assess efficacy in human organoids in vitro. Here we test six adeno-associated virus (AAV) serotypes [AAV2/2, AAV2/9, AAV2/8, AAV2/8T(Y733F), AAV2/5, and ShH10] to determine their efficiency in transducing mouse and human pluripotent stem cell-derived retinal pigment epithelium (RPE) and photoreceptor cells in vitro. All the serotypes tested were capable of transducing RPE and photoreceptor cells in vitro. AAV ShH10 and AAV2/5 are the most efficient vectors at transducing both mouse and human RPE, while AAV2/8 and ShH10 achieved similarly robust transduction of human embryonic stem cell-derived cone photoreceptors. Furthermore, we show that human embryonic stem cell-derived photoreceptors can be used to establish promoter specificity in human cells in vitro. The results of this study will aid capsid selection and vector design for preclinical evaluation of gene therapy approaches, such as gene editing, that require the use of human cells and tissues.

Gene Therapy for Neuropathic Pain through siRNA-IRF5 Gene Delivery with Homing Peptides to Microglia

Astrocyte- and microglia-targeting peptides were identified and isolated using phage display technology. A series of procedures, including three cycles of both in vivo and in vitro biopanning, was performed separately in astrocytes and in M1 or M2 microglia, yielding 50–58 phage plaques in each cell type. Analyses of the sequences of this collection identified one candidate homing peptide targeting astrocytes (AS1[C-LNSSQPS-C]) and two candidate homing peptides targeting microglia (MG1[C-HHSSSAR-C] and MG2[C-NTGSPYE-C]). To determine peptide specificity for the target cell in vitro, each peptide was synthesized and introduced into the primary cultures of astrocytes or microglia. Those peptides could bind to the target cells and be selectively taken up by the corresponding cell, namely, astrocytes, M1 microglia, or M2 microglia. To confirm cell-specific gene delivery to M1 microglia, the complexes between peptide MG1 and siRNA-interferon regulatory factor 5 were prepared and intrathecally injected into a mouse model of neuropathic pain. The complexes successfully suppressed hyperalgesia with high efficiency in this neuropathic pain model. Here, we describe a novel gene therapy for the treatment neuropathic pain, which has a high potential to be of clinical relevance. This strategy will ensure the targeted delivery of therapeutic genes while minimizing side effects to non-target tissues or cells.

New Developments in Genetic rat models of Parkinson's Disease

Preclinical research on Parkinson's disease has relied heavily on mouse and rat animal models. Initially, PD animal models were generated primarily by chemical neurotoxins that induce acute loss of dopaminergic neurons in the substantia nigra. On the discovery of genetic mutations causally linked to PD, mice were used more than rats to generate laboratory animals bearing PD-linked mutations because mutagenesis was more difficult in rats. Recent advances in technology for mammalian genome engineering and optimization of viral expression vectors have increased the use of genetic rat models of PD. Emerging research tools include "knockout" rats with disruption of genes in which mutations have been causally linked to PD, including LRRK2, α-synuclein, Parkin, PINK1, and DJ-1. Rats have also been increasingly used for transgenic and viral-mediated overexpression of genes relevant to PD, particularly α-synuclein. It may not be realistic to obtain a single animal model that completely reproduces every feature of a human disease as complex as PD. Nevertheless, compared with mice with the same mutations, many genetic rat animal models of PD better reproduce key aspects of PD including progressive loss of dopaminergic neurons in the substantia nigra, locomotor behavior deficits, and age-dependent formation of abnormal α-synuclein protein aggregates. Here we briefly review new developments in genetic rat models of PD that may have greater potential for identifying underlying mechanisms, for discovering novel therapeutic targets, and for developing greatly needed treatments to slow or halt disease progression. © 2018 International Parkinson and Movement Disorder Society.

Cellular selectivity of AAV serotypes for gene delivery in neurons and astrocytes by neonatal intracerebroventricular injection

The non-pathogenic parvovirus, adeno-associated virus (AAV), is an efficient vector for transgene expression in vivo and shows promise for treatment of brain disorders in clinical trials. Currently, there are more than 100 AAV serotypes identified that differ in the binding capacity of capsid proteins to specific cell surface receptors that can transduce different cell types and brain regions in the CNS. In the current study, multiple AAV serotypes expressing a GFP reporter (AAV1, AAV2/1, AAVDJ, AAV8, AAVDJ8, AAV9, AAVDJ9) were screened for their infectivity in both primary murine astrocyte and neuronal cell cultures. AAV2/1, AAVDJ8 and AAV9 were selected for further investigation of their tropism throughout different brain regions and cell types. Each AAV was administered to P0-neonatal mice via intracerebroventricular injections (ICV). Brains were then systematically analyzed for GFP expression at 3 or 6 weeks post-infection in various regions, including the olfactory bulb, striatum, cortex, hippocampus, substantia nigra (SN) and cerebellum. Cell counting data revealed that AAV2/1 infections were more prevalent in the cortical layers but penetrated to the midbrain less than AAVDJ8 and AAV9. Additionally, there were differences in the persistence of viral transgene expression amongst the three serotypes examined in vivo at 3 and 6 weeks post-infection. Because AAV-mediated transgene expression is of interest in neurodegenerative diseases such as Parkinson's Disease, we examined the SN with microscopy techniques, such as CLARITY tissue transmutation, to identify AAV serotypes that resulted in optimal transgene expression in either astrocytes or dopaminergic neurons. AAVDJ8 displayed more tropism in astrocytes compared to AAV9 in the SN region. We conclude that ICV injection results in lasting expression of virally encoded transgene when using AAV vectors and that specific AAV serotypes are required to selectively deliver transgenes of interest to different brain regions in both astrocytes and neurons.

Glial fibrillary acidic protein promoter determines transgene expression in satellite glial cells following intraganglionic adeno-associated virus delivery in adult rats

Recombinant adeno-associated viral (AAV)-mediated therapeutic gene transfer to dorsal root ganglia (DRG) is an effective and safe tool for treating chronic pain. However, AAV with various constitutively active promoters leads to transgene expression predominantly to neurons, while glial cells are refractory to AAV transduction in the peripheral nervous system. The present study evaluated whether in vivo satellite glial cell (SGC) transduction in the DRG can be enhanced by the SGC-specific GFAP promoter and by using shH10 and shH19, which are engineered capsid variants with Müller glia-prone transduction. Titer-matched AAV6 (as control), AAVshH10, and AAVshH19, all encoding the EGFP driven by the constitutively active CMV promoter, as well as AAV6-EGFP and AAVshH10-EGFP driven by a GFAP promoter (AAV6-GFAP-EGFP and AAVshH10-GFAP-EGFP), were injected into DRG of adult male rats. Neurotropism of gene expression was determined and compared by immunohistochemistry. Results showed that injection of AAV6- and AAVshH10-GFAP-EGFP induces robust EGFP expression selectively in SGCs, whereas injection of either AAVshH10-CMV-EGFP or AAVshH19-CMV-EGFP into DRG resulted in a similar in vivo transduction profile to AAV6-CMV-EGFP, all showing efficient transduction of sensory neurons without significant transduction of glial cell populations. Coinjection of AAV6-CMV-mCherry and AAV6-GFAP-EGFP induces transgene expression in neurons and SGCs separately. This report, together with our prior studies, demonstrates that the GFAP promoter rather than capsid tropism determines selective gene expression in SGCs following intraganglionic AAV delivery in adult rats. A dual AAV system, one with GFAP promoter and the other with CMV promoter, can efficiently express transgenes selectively in neurons versus SGCs.

Effects of Endogenous Oxytocin Receptor Signaling in Nucleus Tractus Solitarius on Satiation-Mediated Feeding and Thermogenic Control in Male Rats

Central oxytocin receptor (OT-R) signaling reduces food intake and increases energy expenditure, but the central sites and mechanisms mediating these effects are unresolved. We showed previously that pharmacological activation of OT-R in hindbrain/nucleus tractus solitarius (NTS) amplifies the intake-inhibitory effects of gastrointestinal (GI) satiation signals. Unexplored were the energetic effects of hindbrain OT-R agonism and the physiological relevance of NTS OT-R signaling on food intake and energy expenditure control. Using a virally mediated OT-R knockdown (KD) strategy and a range of behavioral paradigms, this study examined the role of endogenous NTS OT-R signaling on satiation-mediated food intake inhibition and thermogenic control. Results showed that, compared with controls, NTS OT-R KD rats consumed larger meals, were less responsive to the intake-inhibitory effects of a self-ingested preload, and consumed more chow following a 24-hour fast. These data indicate that NTS OT-R signaling is necessary for normal satiation control. Whereas both control and NTS OT-R KD rats increased core temperature following high-fat diet maintenance (relative to chow maintenance), the percent increase in core temperature was greater in control compared with NTS OT-R KD rats during the light cycle. Hindbrain oxytocin agonist delivery increased core temperature in both control and NTS OT-R KD rats and the percent increase relative to vehicle treatment was not significantly different between groups. Together, data reveal a critical role for endogenous NTS OT-R signaling in mediating the intake-inhibitory effects of endogenous GI satiation signals and in diet-induced thermogenesis.

Generation and validation of novel adeno-associated viral vectors for the analysis of Ca2+ homeostasis in motor neurons

A finely tuned Ca²⁺ homeostasis in restricted cell domains is of fundamental importance for neurons, where transient Ca²⁺ oscillations direct the proper coordination of electro-chemical signals and overall neuronal metabolism. Once such a precise regulation is unbalanced, however, neuronal functions and viability are severely compromised. Accordingly, disturbed Ca²⁺ metabolism has often been claimed as a major contributor to different neurodegenerative disorders, such as amyotrophic lateral sclerosis that is characterised by selective motor neuron (MN) damage. This notion highlights the need for probes for the specific and precise analysis of local Ca²⁺ dynamics in MNs. Here, we generated and functionally validated adeno-associated viral vectors for the expression of gene-encoded fluorescent Ca²⁺ indicators targeted to different cell domains, under the transcriptional control of a MN-specific promoter. We demonstrated that the probes are specifically expressed, and allow reliable local Ca²⁺ measurements, in MNs from murine primary spinal cord cultures, and can also be expressed in spinal cord MNs in vivo, upon systemic administration to newborn mice. Preliminary analyses using these novel vectors have shown larger cytosolic Ca²⁺ responses following stimulation of AMPA receptors in the cytosol of primary cultured MNs from a murine genetic model of ALS compared to the healthy counterpart.