GABAergic neuron-specific whole-brain transduction by AAV-PHP.B incorporated with a new GAD65 promoter

Reversible Sterilization of Channel Catfish via Overexpression of Glutamic Acid Decarboxylase Gene

The confinement of transgenic fish is essential to prevent their escape and reproduction in natural ecosystems. Reversible transgenic sterilization is a promising approach to control the reproduction of transgenic fish. Therefore, the present study was conducted to develop a reversibly sterile channel catfish (Ictalurus punctatus) via the transgenic overexpression of the goldfish (Carassius auratus) glutamic acid decarboxylase (GAD) gene driven by the common carp (Cyprinus carpio) β-actin promoter to disrupt normal gamma-aminobutyric acid (GABA) regulation. Three generations of GAD-transgenic fish were produced. All studied generations showed repressed reproductive performance; however, this was not always statistically significant. In F1, 5.4% of the transgenic fish showed a sexual maturity score ≥ 4 (maximum = 5) at five years of age, which was lower (p = 0.07) than that of the control group (16.8%). In the spawning experiments conducted on F1 transgenic fish at six and nine years of age, 45.5% and 20.0% of fish spawned naturally, representing lower values (p = 0.09 and 0.12, respectively) than the percentages in the sibling control fish of the same age (83.3% and 66.7%, respectively). Four of six pairs of the putative infertile six-year-old fish spawned successfully after luteinizing hormone-releasing hormone analog (LHRHa) therapy. Similar outcomes were noted in the three-year-old F2 fish, with a lower spawning percentage in transgenic fish (20.0%) than in the control (66.7%). In one-year-old F2-generation transgenic fish, the observed mean serum gonadotropin-releasing hormone (GnRH) levels were 9.23 ± 2.49 and 8.14 ± 2.21 ng/mL for the females and males, respectively. In the control fish, the mean levels of GnRH were 11.04 ± 4.06 and 9.03 ± 2.36 ng/mL for the females and males, respectively, which did not differ significantly from the control (p = 0.15 and 0.27 for females and males, respectively). There was no significant difference in the estradiol levels of the female transgenic and non-transgenic fish in the one- and four-year-old F2-generation fish. The four-year-old F2-generation male transgenic fish exhibited significantly (p < 0.05) lower levels of GnRH and testosterone than the control fish. In conclusion, while overexpressing GAD repressed the reproductive abilities of channel catfish, it did not completely sterilize transgenic fish. The sterilization rate might be improved through selection in future generations.

Optimization of AAV vectors for transactivator-regulated enhanced gene expression within targeted neuronal populations

Adeno-associated virus (AAV) vectors are potential tools for cell-type-selective gene delivery to the central nervous system. Although cell-type-specific enhancers and promoters have been identified for AAV systems, there is limited information regarding the effects of AAV genomic components on the selectivity and efficiency of gene expression. Here, we offer an alternative strategy to provide specific and efficient gene delivery to a targeted neuronal population by optimizing recombinant AAV genomic components, named TAREGET (TransActivator-Regulated Enhanced Gene Expression within Targeted neuronal populations). We established this strategy in oxytocinergic neurons and showed that the TAREGET enabled sufficient gene expression to label long-projecting axons in wild-type mice. Its application to other cell types, including serotonergic and dopaminergic neurons, was also demonstrated. These results demonstrate that optimization of AAV expression cassettes can improve the specificity and efficiency of cell-type-specific gene expression and that TAREGET can renew previously established cell-type-specific promoters with improved performance.

Stabilization of V1 interneuron-motor neuron connectivity ameliorates motor phenotype in a mouse model of ALS

Loss of connectivity between spinal V1 inhibitory interneurons and motor neurons is found early in disease in the SOD1G93A mice. Such changes in premotor inputs can contribute to homeostatic imbalance of motor neurons. Here, we show that the Extended Synaptotagmin 1 (Esyt1) presynaptic organizer is downregulated in V1 interneurons. V1 restricted overexpression of Esyt1 rescues inhibitory synapses, increases motor neuron survival, and ameliorates motor phenotypes. Two gene therapy approaches overexpressing ESYT1 were investigated; one for local intraspinal delivery, and the other for systemic administration using an AAV-PHP.eB vector delivered intravenously. Improvement of motor functions is observed in both approaches, however systemic administration appears to significantly reduce onset of motor impairment in the SOD1G93A mice in absence of side effects. Altogether, we show that stabilization of V1 synapses by ESYT1 overexpression has the potential to improve motor functions in ALS, demonstrating that interneurons can be a target to attenuate ALS symptoms.

Engineering of the AAV-Compatible Hair Cell-Specific Small-Size Myo15 Promoter for Gene Therapy in the Inner Ear

Adeno-associated virus (AAV)-mediated gene therapy is widely applied to treat numerous hereditary diseases in animal models and humans. The specific expression of AAV-delivered transgenes driven by cell type-specific promoters should further increase the safety of gene therapy. However, current methods for screening cell type-specific promoters are labor-intensive and time-consuming. Herein, we designed a "multiple vectors in one AAV" strategy for promoter construction in vivo. Through this strategy, we truncated a native promoter for Myo15 expression in hair cells (HCs) in the inner ear, from 1,611 bp down to 1,157 bp, and further down to 956 bp. Under the control of these 2 promoters, green fluorescent protein packaged in AAV-PHP.eB was exclusively expressed in the HCs. The transcription initiation ability of the 2 promoters was further verified by intein-mediated otoferlin recombination in a dual-AAV therapeutic system. Driven by these 2 promoters, human otoferlin was selectively expressed in HCs, resulting in the restoration of hearing in treated Otof -/- mice for at least 52 weeks. In summary, we developed an efficient screening strategy for cell type-specific promoter engineering and created 2 truncated Myo15 promoters that not only restored hereditary deafness in animal models but also show great potential for treating human patients in future.

Improving cell-specific recombination using AAV vectors in the murine CNS by capsid and expression cassette optimization

The production of cell-type- and age-specific genetically modified mice is a powerful approach for unraveling unknown gene functions. Here, we present a simple and timesaving method that enables adeno-associated virus (AAV)-mediated cell-type- and age-specific recombination in floxed mice. To achieve astrocyte-specific recombination in floxed Ai14 reporter mice, we intravenously injected blood-brain barrier-penetrating AAV-PHP.eB vectors expressing Cre recombinase (Cre) using the astrocyte-specific mouse glial fibrillary acidic protein (mGfaABC1D) promoter. However, we observed nonspecific neuron-predominant transduction despite the use of an astrocyte-specific promoter. We speculated that subtle but continuous Cre expression in nonastrocytic cells triggers recombination, and that excess production of Cre in astrocytes inhibits recombination by forming Cre-DNA aggregates. Here, we resolved this paradoxical event by dividing a single AAV into two mGfaABC1D-promoter-driven AAV vectors, one expressing codon-optimized flippase (FlpO) and another expressing flippase recognition target-flanked rapidly degrading Cre (dCre), together with switching the neuron-tropic PHP.eB capsid to astrocyte-tropic AAV-F. Moreover, we found that the FlpO-dCre system with a target cell-tropic capsid can also function in neuron-targeting recombination in floxed mice.

Non-severe thermal burn injuries induce long-lasting downregulation of gene expression in cortical excitatory neurons and microglia

Burn injuries are devastating traumas, often leading to life-long consequences that extend beyond the observable burn scar. In the context of the nervous system, burn injury patients commonly develop chronic neurological disorders and have been suggested to have impaired motor cortex function, but the long-lasting impact on neurons and glia in the brain is unknown. Using a mouse model of non-severe burn injury, excitatory and inhibitory neurons in the primary motor cortex were labelled with fluorescent proteins using adeno-associated viruses (AAVs). A total of 5 weeks following the burn injury, virus labelled excitatory and inhibitory neurons were isolated using fluorescence-activated cell sorting (FACS). In addition, microglia and astrocytes from the remaining cortical tissue caudal to the motor cortex were immunolabelled and isolated with FACS. Whole transcriptome RNA-sequencing was used to identify any long-lasting changes to gene expression in the different cell types. RNA-seq analysis showed changes to the expression of a small number of genes with known functions in excitatory neurons and microglia, but not in inhibitory neurons or astrocytes. Specifically, genes related to GABA-A receptors in excitatory neurons and several cellular functions in microglia were found to be downregulated in burn injured mice. These findings suggest that non-severe burn injuries lead to long lasting transcriptomic changes in the brain, but only in specific cell types. Our findings provide a broad overview of the long-lasting impact of burn injuries on the central nervous system which may help identify potential therapeutic targets to prevent neurological dysfunction in burn patients.

An enhancer-AAV approach selectively targeting dentate granule cells of the mouse hippocampus

The mammalian brain contains a diverse array of cell types, including dozens of neuronal subtypes with distinct anatomical and functional characteristics. The brain leverages these neuron-type specializations to perform diverse circuit operations and thus execute different behaviors properly. Through the use of Cre lines, access to specific neuron types has improved over past decades. Despite their extraordinary utility, development and cross-breeding of Cre lines is time consuming and expensive, presenting a significant barrier to entry for investigators. Furthermore, cell-based therapeutics developed in Cre mice are not clinically translatable. Recently, several adeno-associated virus (AAV) vectors utilizing neuron-type-specific regulatory transcriptional sequences (enhancer-AAVs) were developed that overcome these limitations. Using a publicly available RNA sequencing (RNA-seq) dataset, we evaluated the potential of several candidate enhancers for neuron-type-specific targeting in the hippocampus. Here, we demonstrate that a previously identified enhancer-AAV selectively targets dentate granule cells over other excitatory neuron types in the hippocampus of wild-type adult mice.

Excitatory subtypes of the lateral amygdala neurons are differentially involved in regulation of synaptic plasticity and excitation/inhibition balance in aversive learning in mice

The amygdala plays a crucial role in aversive learning. In Pavlovian fear conditioning, sensory information about an emotionally neutral conditioned stimulus (CS) and an innately aversive unconditioned stimulus is associated with the lateral amygdala (LA), and the CS acquires the ability to elicit conditioned responses. Aversive learning induces synaptic plasticity in LA excitatory neurons from CS pathways, such as the medial geniculate nucleus (MGN) of the thalamus. Although LA excitatory cells have traditionally been classified based on their firing patterns, the relationship between the subtypes and functional properties remains largely unknown. In this study, we classified excitatory cells into two subtypes based on whether the after-depolarized potential (ADP) amplitude is expressed in non-ADP cells and ADP cells. Their electrophysiological properties were significantly different. We examined subtype-specific synaptic plasticity in the MGN-LA pathway following aversive learning using optogenetics and found significant experience-dependent plasticity in feed-forward inhibitory responses in fear-conditioned mice compared with control mice. Following aversive learning, the inhibition/excitation (I/E) balance in ADP cells drastically changed, whereas that in non-ADP cells tended to change in the reverse direction. These results suggest that the two LA subtypes are differentially regulated in relation to synaptic plasticity and I/E balance during aversive learning.

State-dependent modulation of positive and negative affective valences by a parabrachial nucleus-to-ventral tegmental area pathway in mice

Appropriately responding to various sensory signals in the environment is essential for animal survival. Accordingly, animal behaviors are closely related to external and internal states, which include the positive and negative emotional values of sensory signals triggered by environmental factors. While the lateral parabrachial nucleus (LPB) plays a key role in nociception and supports negative valences, it also transmits signals including positive valences. However, the downstream neuronal mechanisms of positive and negative valences have not been fully explored. In the present study, we investigated the ventral tegmental area (VTA) as a projection target for LPB neurons. Optogenetic activation of LPB-VTA terminals in male mice elicits positive reinforcement in an operant task and induces both avoidance and attraction in a place-conditioning task. Inhibition of glutamic acid decarboxylase (GAD) 65-expressing cells in the VTA promotes avoidance behavior induced by photoactivation of the LPB-VTA pathway. These findings indicate that the LPB-VTA pathway is one of the LPB outputs for the transmission of positive and negative valence signals, at least in part, with GABAergic modification in VTA.

NeuroActivityToolkit-Toolbox for Quantitative Analysis of Miniature Fluorescent Microscopy Data

The visualization of neuronal activity in vivo is an urgent task in modern neuroscience. It allows neurobiologists to obtain a large amount of information about neuronal network architecture and connections between neurons. The miniscope technique might help to determine changes that occurred in the network due to external stimuli and various conditions: processes of learning, stress, epileptic seizures and neurodegenerative diseases. Furthermore, using the miniscope method, functional changes in the early stages of such disorders could be detected. The miniscope has become a modern approach for recording hundreds to thousands of neurons simultaneously in a certain brain area of a freely behaving animal. Nevertheless, the analysis and interpretation of the large recorded data is still a nontrivial task. There are a few well-working algorithms for miniscope data preprocessing and calcium trace extraction. However, software for further high-level quantitative analysis of neuronal calcium signals is not publicly available. NeuroActivityToolkit is a toolbox that provides diverse statistical metrics calculation, reflecting the neuronal network properties such as the number of neuronal activations per minute, amount of simultaneously co-active neurons, etc. In addition, the module for analyzing neuronal pairwise correlations is implemented. Moreover, one can visualize and characterize neuronal network states and detect changes in 2D coordinates using PCA analysis. This toolbox, which is deposited in a public software repository, is accompanied by a detailed tutorial and is highly valuable for the statistical interpretation of miniscope data in a wide range of experimental tasks.

Spatial transcriptomics for profiling the tropism of viral vectors in tissues

A barrier to advancing engineered adeno-associated viral vectors (AAVs) for precision access to cell subtypes is a lack of high-throughput, high-resolution assays to characterize in vivo transduction profiles. In this study, we developed an ultrasensitive, sequential fluorescence in situ hybridization (USeqFISH) method for spatial transcriptomic profiling of endogenous and viral RNA with a short barcode in intact tissue volumes by integrating hydrogel-based tissue clearing, enhanced signal amplification and multiplexing using sequential labeling. Using USeqFISH, we investigated the transduction and cell subtype tropisms across mouse brain regions of six systemic AAVs, including AAV-PHP.AX, a new variant that transduces robustly and efficiently across neurons and astrocytes. Here we reveal distinct cell subtype biases of each AAV variant, including a bias of AAV-PHP.N toward excitatory neurons. USeqFISH also enables profiling of pooled regulatory cargos, as we show for a 13-variant pool of microRNA target sites in AAV genomes. Lastly, we demonstrate potential applications of USeqFISH for in situ AAV profiling and multimodal single-cell analysis in non-human primates.

Anatomical identification of a corticocortical top-down recipient inhibitory circuitry by enhancer-restricted transsynaptic tracing

Despite the importance of postsynaptic inhibitory circuitry targeted by mid/long-range projections (e.g., top-down projections) in cognitive functions, its anatomical properties, such as laminar profile and neuron type, are poorly understood owing to the lack of efficient tracing methods. To this end, we developed a method that combines conventional adeno-associated virus (AAV)-mediated transsynaptic tracing with a distal-less homeobox (Dlx) enhancer-restricted expression system to label postsynaptic inhibitory neurons. We called this method "Dlx enhancer-restricted Interneuron-SpECific transsynaptic Tracing" (DISECT). We applied DISECT to a top-down corticocortical circuit from the secondary motor cortex (M2) to the primary somatosensory cortex (S1) in wild-type mice. First, we injected AAV1-Cre into the M2, which enabled Cre recombinase expression in M2-input recipient S1 neurons. Second, we injected AAV1-hDlx-flex-green fluorescent protein (GFP) into the S1 to transduce GFP into the postsynaptic inhibitory neurons in a Cre-dependent manner. We succeeded in exclusively labeling the recipient inhibitory neurons in the S1. Laminar profile analysis of the neurons labeled via DISECT indicated that the M2-input recipient inhibitory neurons were distributed in the superficial and deep layers of the S1. This laminar distribution was aligned with the laminar density of axons projecting from the M2. We further classified the labeled neuron types using immunohistochemistry and in situ hybridization. This post hoc classification revealed that the dominant top-down M2-input recipient neuron types were somatostatin-expressing neurons in the superficial layers and parvalbumin-expressing neurons in the deep layers. These results demonstrate that DISECT enables the investigation of multiple anatomical properties of the postsynaptic inhibitory circuitry.

Direct and Indirect Nociceptive Input from the Trigeminal Dorsal Horn to Pain-Modulating Neurons in the Rostral Ventromedial Medulla

The brain is able to amplify or suppress nociceptive signals by means of descending projections to the spinal and trigeminal dorsal horns from the rostral ventromedial medulla (RVM). Two physiologically defined cell classes within RVM, "ON-cells" and "OFF-cells," respectively facilitate and inhibit nociceptive transmission. However, sensory pathways through which nociceptive input drives changes in RVM cell activity are only now being defined. We recently showed that indirect inputs from the dorsal horn via the parabrachial complex (PB) convey nociceptive information to RVM. The purpose of the present study was to determine whether there are also direct dorsal horn inputs to RVM pain-modulating neurons. We focused on the trigeminal dorsal horn, which conveys sensory input from the face and head, and used a combination of single-cell recording with optogenetic activation and inhibition of projections to RVM and PB from the trigeminal interpolaris-caudalis transition zone (Vi/Vc) in male and female rats. We determined that a direct projection from ventral Vi/Vc to RVM carries nociceptive information to RVM pain-modulating neurons. This projection included a GABAergic component, which could contribute to nociceptive inhibition of OFF-cells. This approach also revealed a parallel, indirect, relay of trigeminal information to RVM via PB. Activation of the indirect pathway through PB produced a more sustained response in RVM compared with activation of the direct projection from Vi/Vc. These data demonstrate that a direct trigeminal output conveys nociceptive information to RVM pain-modulating neurons with a parallel indirect pathway through the parabrachial complex.SIGNIFICANCE STATEMENT Rostral ventromedial medulla (RVM) pain-modulating neurons respond to noxious stimulation, which implies that they receive input from pain-transmission circuits. However, the traditional view has been that there is no direct input to RVM pain-modulating neurons from the dorsal horn, and that nociceptive information is carried by indirect pathways. Indeed, we recently showed that noxious information can reach RVM pain-modulating neurons via the parabrachial complex (PB). Using in vivo electrophysiology and optogenetics, the present study identified a direct relay of nociceptive information from the trigeminal dorsal horn to physiologically identified pain-modulating neurons in RVM. Combined tracing and electrophysiology data revealed that the direct projection includes GABAergic neurons. Direct and indirect pathways may play distinct functional roles in recruiting pain-modulating neurons.

Overexpression of wild type glycine alpha 1 subunit rescues ethanol sensitivity in accumbal receptors and reduces binge drinking in mice

The nucleus accumbens (nAc) is a critical region in the brain reward system since it integrates abundant synaptic inputs contributing to the control of neuronal excitability in the circuit. The presence of inhibitory α1 glycine receptor (GlyRs) subunits, sensitive to ethanol, has been recently reported in accumbal neurons suggesting that they are protective against excessive binge consumption. In the present study, we used viral vectors (AAV) to overexpress mutant and WT α1 subunits in accumbal neurons in D1 Cre and α1 KI mice. Injection of a Cre-inducible AAV carrying an ethanol insensitive α1 subunit in D1 Cre neurons was unable to affect sensitivity to ethanol in GlyRs or affect ethanol drinking. On the other hand, using an AAV that transduced WT α1 GlyRs in GABAergic neurons in the nAc of high-ethanol consuming mice caused a reduction in ethanol intake as reflected by lowered drinking in the dark and reduced blood ethanol concentration. As expected, the AAV increased the glycine current density by 5-fold without changing the expression of GABAA receptors. Examination of the ethanol sensitivity in isolated accumbal neurons indicated that the GlyRs phenotype changed from an ethanol resistant to an ethanol sensitive type. These results support the conclusion that increased inhibition in the nAc can control excessive ethanol consumption and that selective targeting of GlyRs by pharmacotherapy might provide a mechanistic procedure to reduce ethanol binge.

A novel enhancer-AAV approach selectively targeting dentate granule cells

The mammalian brain contains the most diverse array of cell types of any organ, including dozens of neuronal subtypes with distinct anatomical and functional characteristics. The brain leverages these neuron-type-specializations to perform diverse circuit operations and thus execute different behaviors properly. Through the use of Cre lines, access to specific neuron types has steadily improved over past decades. Despite their extraordinary utility, development and cross-breeding of Cre lines is time-consuming and expensive, presenting a significant barrier to entry for many investigators. Furthermore, cell-based therapeutics developed in Cre mice are not clinically translatable. Recently, several AAV vectors utilizing neuron-type-specific regulatory transcriptional sequences (enhancer-AAVs) were developed which overcome these limitations. Using a publicly available RNAseq dataset, we evaluated the potential of several candidate enhancers for neuron-type-specific targeting in the hippocampus. Here we identified a promising enhancer-AAV for targeting dentate granule cells and validated its selectivity in wild-type adult mice.

Advances in AAV technology for delivering genetically encoded cargo to the nonhuman primate nervous system

Modern neuroscience approaches including optogenetics, calcium imaging, and other genetic manipulations have facilitated our ability to dissect specific circuits in rodent models to study their role in neurological disease. These approaches regularly use viral vectors to deliver genetic cargo (e.g., opsins) to specific tissues and genetically-engineered rodents to achieve cell-type specificity. However, the translatability of these rodent models, cross-species validation of identified targets, and translational efficacy of potential therapeutics in larger animal models like nonhuman primates remains difficult due to the lack of efficient primate viral vectors. A refined understanding of the nonhuman primate nervous system promises to deliver insights that can guide the development of treatments for neurological and neurodegenerative diseases. Here, we outline recent advances in the development of adeno-associated viral vectors for optimized use in nonhuman primates. These tools promise to help open new avenues for study in translational neuroscience and further our understanding of the primate brain.

In silico prediction and in vivo testing of promoters targeting GABAergic inhibitory neurons

Impairment of GABAergic inhibitory neuronal function is linked to epilepsy and other neurological and psychiatric disorders. Recombinant adeno-associated virus (rAAV)-based gene therapy targeting GABAergic neurons is a promising treatment for GABA-associated disorders. However, there is a need to develop rAAV-compatible gene-regulatory elements capable of selectively driving expression in GABAergic neurons throughout the brain. Here, we designed several novel GABAergic gene promoters. In silico analyses, including evolutionarily conserved DNA sequence alignments and transcription factor binding site searches among GABAergic neuronal genes, were carried out to reveal novel sequences for use as rAAV-compatible promoters. rAAVs (serotype 9) were injected into the CSF of neonatal mice and into the brain parenchyma of adult mice to assess promoter specificity. In mice injected neonatally, transgene expression was detected in multiple brain regions with very high neuronal specificity and moderate-to-high GABAergic neuronal selectivity. The GABA promoters differed greatly in their levels of expression and, in some brain regions, showed strikingly different patterns of GABAergic neuron transduction. This study is the first report of rAAV vectors that are functional in multiple brain regions using promoters designed by in silico analyses from multiple GABAergic genes. These novel GABA-targeting vectors may be useful tools to advance gene therapy for GABA-associated disorders.

Bringing to light the physiological and pathological firing patterns of human induced pluripotent stem cell-derived neurons using optical recordings

Human induced pluripotent stem cells (hiPSCs) are a promising approach to study neurological and neuropsychiatric diseases. Most methods to record the activity of these cells have major drawbacks as they are invasive or they do not allow single cell resolution. Genetically encoded voltage indicators (GEVIs) open the path to high throughput visualization of undisturbed neuronal activity. However, conventional GEVIs perturb membrane integrity through inserting multiple copies of transmembrane domains into the plasma membrane. To circumvent large add-ons to the plasma membrane, we used a minimally invasive novel hybrid dark quencher GEVI to record the physiological and pathological firing patterns of hiPSCs-derived sensory neurons from patients with inherited erythromelalgia, a chronic pain condition associated with recurrent attacks of redness and swelling in the distal extremities. We observed considerable differences in action potential firing patterns between patient and control neurons that were previously overlooked with other recording methods. Our system also performed well in hiPSC-derived forebrain neurons where it detected spontaneous synchronous bursting behavior, thus opening the path to future applications in other cell types and disease models including Parkinson's disease, Alzheimer's disease, epilepsy, and schizophrenia, conditions associated with disturbances of neuronal activity and synchrony.

Development of microglia-targeting adeno-associated viral vectors as tools to study microglial behavior in vivo

Here we describe the microglia-targeting adeno-associated viral (AAV) vectors containing a 1.7-kb putative promoter region of microglia/macrophage-specific ionized calcium-binding adaptor molecule 1 (Iba1), along with repeated miRNA target sites for microRNA (miR)-9 and miR-129-2-3p. The 1.7-kb genomic sequence upstream of the start codon in exon 1 of the Iba1 (Aif1) gene, functions as microglia preferential promoter in the striatum and cerebellum. Furthermore, ectopic transgene expression in non-microglial cells is markedly suppressed upon adding two sets of 4-repeated miRNA target sites for miR-9 and miR-129-2-3p, which are expressed exclusively in non-microglial cells and sponged AAV-derived mRNAs. Our vectors transduced ramified microglia in healthy tissues and reactive microglia in lipopolysaccharide-treated mice and a mouse model of neurodegenerative disease. Moreover, live fluorescent imaging allowed the monitoring of microglial motility and intracellular Ca2+ mobilization. Thus, microglia-targeting AAV vectors are valuable for studying microglial pathophysiology and therapies, particularly in the striatum and cerebellum.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Selective transduction of cerebellar Purkinje and granule neurons using delivery of AAV-PHP.eB and AAVrh10 vectors at axonal terminal locations

Adeno-associated virus (AAV)-based brain gene therapies require precision without off-targeting of unaffected neurons to avoid side effects. The cerebellum and its cell populations, including granule and Purkinje cells, are vulnerable to neurodegeneration; hence, conditions to deliver the therapy to specific cell populations selectively remain challenging. We have investigated a system consisting of the AAV serotypes, targeted injections, and transduction modes (direct or retrograde) for targeted delivery of AAV to cerebellar cell populations. We selected the AAV-PHP.eB and AAVrh10 serotypes valued for their retrograde features, and we thoroughly examined their cerebellar transduction pattern when injected into lobules and deep cerebellar nuclei. We found that AAVrh10 is suitable for the transduction of neurons in the mode highly dependent on placing the virus at axonal terminals. The strategy secures selective transduction for granule cells. The AAV-PHP.eB can transduce Purkinje cells and is very selective for the cell type when injected into the DCN at axonal PC terminals. Therefore, both serotypes can be used in a retrograde mode for selective transduction of major neuronal types in the cerebellum. Moreover, our in vivo transduction strategies are suitable for pre-clinical protocol development for gene delivery to granule cells by AAVrh10 and Purkinje cells by AAV-PHPeB.

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.

Viral strategies for targeting spinal neuronal subtypes in adult wild-type rodents

Targeting specific subtypes of interneurons in the spinal cord is primarily restricted to a small group of genetic model animals. Since the development of new transgenic model animals can be expensive and labor intensive, it is often difficult to generalize these findings and verify them in other model organisms, such as the rat, ferret or monkey, that may be more beneficial in certain experimental investigations. Nevertheless, endogenous enhancers and promoters delivered using an adeno-associated virus (AAV) have been successful in providing expression in specific subtypes of neurons in the forebrain of wildtype animals, and therefore may introduce a shortcut. GABAergic interneurons, for instance, have successfully been targeted using the mDlx promoter, which has recently been developed and is now widely used in wild type animals. Here, we test the specificity and efficiency of the mDlx enhancer for robust targeting of inhibitory interneurons in the lumbar spinal cord of wild-type rats using AAV serotype 2 (AAV2). Since this has rarely been done in the spinal cord, we also test the expression and specificity of the CamKIIa and hSynapsin promoters using serotype 9. We found that AAV2-mDlx does in fact target many neurons that contain an enzyme for catalyzing GABA, the GAD-65, with high specificity and a small fraction of neurons containing an isoform, GAD-67. Expression was also seen in some motor neurons although with low correlation. Viral injections using the CamKIIa enhancer via AAV9 infected in some glutamatergic neurons, but also GABAergic neurons, whereas hSynapsin via AAV9 targets almost all the neurons in the lumbar spinal cord.

Bidirectional Optogenetic Modulation of the Subthalamic Nucleus in a Rodent Model of Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by a range of motor symptoms. Treatments are focused on dopamine replacement therapy or deep brain stimulation (DBS). The subthalamic nucleus (STN) is a common target for DBS treatment of PD. However, the function of the STN in normal conditions and pathology is poorly understood. Here, we show in rats that optogenetic modulation of STN neuronal activity exerts bidirectional control of motor function, where inhibition of the STN increases movement and STN activation decreases movement. We also examined the effect of bidirectional optogenetic manipulation STN neuronal activity under dopamine depleted condition using the bilateral rodent 6-hydroxydopamine (6-OHDA) model of Parkinson's disease. Optogenetic inhibition of the STN in the absence of dopamine had no impact on motor control yet STN excitation led to pronounced abnormal involuntary movement. Administration of levodopa rescued the abnormal involuntary movements induced by STN excitation. Although dopamine and STN dysfunction are well established in PD pathology, here we demonstrate simultaneous STN over activity and loss of dopamine lead to motor deficits. Moreover, we show the dysfunction of the STN is dependent on dopamine. This study provides evidence that the loss of dopamine and the over activity of the STN are key features of PD motor deficits. These results provide insight into the STN pathology in PD and therapeutic mechanism of targeting the STN for the treatment for PD.

Adeno-Associated Virus Toolkit to Target Diverse Brain Cells

Recombinant adeno-associated viruses (AAVs) are commonly used gene delivery vehicles for neuroscience research. They have two engineerable features: the capsid (outer protein shell) and cargo (encapsulated genome). These features can be modified to enhance cell type or tissue tropism and control transgene expression, respectively. Several engineered AAV capsids with unique tropisms have been identified, including variants with enhanced central nervous system transduction, cell type specificity, and retrograde transport in neurons. Pairing these AAVs with modern gene regulatory elements and state-of-the-art reporter, sensor, and effector cargo enables highly specific transgene expression for anatomical and functional analyses of brain cells and circuits. Here, we discuss recent advances that provide a comprehensive (capsid and cargo) AAV toolkit for genetic access to molecularly defined brain cell types. Expected final online publication date for the Annual Review of Neuroscience, Volume 45 is July 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

AAV Vector-Mediated Antibody Delivery (A-MAD) in the Central Nervous System

In the last four decades, monoclonal antibodies and their derivatives have emerged as a powerful class of therapeutics, largely due to their exquisite targeting specificity. Several clinical areas, most notably oncology and autoimmune disorders, have seen the successful introduction of monoclonal-based therapeutics. However, their adoption for treatment of Central Nervous System diseases has been comparatively slow, largely due to issues of efficient delivery resulting from limited permeability of the Blood Brain Barrier. Nevertheless, CNS diseases are becoming increasingly prevalent as societies age, accounting for ~6.5 million fatalities worldwide per year. Therefore, harnessing the full therapeutic potential of monoclonal antibodies (and their derivatives) in this clinical area has become a priority. Adeno-associated virus-based vectors (AAVs) are a potential solution to this problem. Preclinical studies have shown that AAV vector-mediated antibody delivery provides protection against a broad range of peripheral diseases, such as the human immunodeficiency virus (HIV), influenza and malaria. The parallel identification and optimization of AAV vector platforms which cross the Blood Brain Barrier with high efficiency, widely transducing the Central Nervous System and allowing high levels of local transgene production, has now opened a number of interesting scenarios for the development of AAV vector-mediated antibody delivery strategies to target Central Nervous System proteinopathies.

Cortical Hyperexcitability in the Driver’s Seat in ALS

Amyotrophic lateral sclerosis (ALS) is a fatal disease characterized by the degeneration of cortical and spinal motor neurons. With no effective treatment available to date, patients face progressive paralysis and eventually succumb to the disease due to respiratory failure within only a few years. Recent research has revealed the multifaceted nature of the mechanisms and cell types involved in motor neuron degeneration, thereby opening up new therapeutic avenues. Intriguingly, two key features present in both ALS patients and rodent models of the disease are cortical hyperexcitability and hyperconnectivity, the mechanisms of which are still not fully understood. We here recapitulate current findings arguing for cell autonomous and non-cell autonomous mechanisms causing cortical excitation and inhibition imbalance, which is involved in the degeneration of motor neurons in ALS. Moreover, we will highlight recent evidence that strongly indicates a cardinal role for the motor cortex as a main driver and source of the disease, thus arguing for a corticofugal trajectory of the pathology.

GABAergic Gene Regulatory Elements Used in Adeno-Associated Viral Vectors

Several neurological and psychiatric disorders have been associated with impairments in GABAergic inhibitory neurons in the brain. Thus, in the current era of accelerated development of molecular medicine and biologically-based drugs, there is a need to identify gene regulatory sequences that can be utilized for selectively manipulating the expression of nucleic acids and proteins in GABAergic neurons. This is particularly important for the use of viral vectors in gene therapy. In this Mini Review, we discuss the use of various gene regulatory elements for targeting GABAergic neurons, with an emphasis on adeno-associated viral vectors, the most widely used class of viral vectors for treating brain diseases.