Glial promoter selectivity following AAV-delivery to the immature brain

Heterogeneity of mature oligodendrocytes in the central nervous system

Mature oligodendrocytes form myelin sheaths that are crucial for the insulation of axons and efficient signal transmission in the central nervous system. Recent evidence has challenged the classical view of the functionally static mature oligodendrocyte and revealed a gamut of dynamic functions such as the ability to modulate neuronal circuitry and provide metabolic support to axons. Despite the recognition of potential heterogeneity in mature oligodendrocyte function, a comprehensive summary of mature oligodendrocyte diversity is lacking. We delve into early 20 th -century studies by Robertson and Río-Hortega that laid the foundation for the modern identification of regional and morphological heterogeneity in mature oligodendrocytes. Indeed, recent morphologic and functional studies call into question the long-assumed homogeneity of mature oligodendrocyte function through the identification of distinct subtypes with varying myelination preferences. Furthermore, modern molecular investigations, employing techniques such as single cell/nucleus RNA sequencing, consistently unveil at least six mature oligodendrocyte subpopulations in the human central nervous system that are highly transcriptomically diverse and vary with central nervous system region. Age and disease related mature oligodendrocyte variation denotes the impact of pathological conditions such as multiple sclerosis, Alzheimer's disease, and psychiatric disorders. Nevertheless, caution is warranted when subclassifying mature oligodendrocytes because of the simplification needed to make conclusions about cell identity from temporally confined investigations. Future studies leveraging advanced techniques like spatial transcriptomics and single-cell proteomics promise a more nuanced understanding of mature oligodendrocyte heterogeneity. Such research avenues that precisely evaluate mature oligodendrocyte heterogeneity with care to understand the mitigating influence of species, sex, central nervous system region, age, and disease, hold promise for the development of therapeutic interventions targeting varied central nervous system pathology.

Evaluation of safety and early efficacy of AAV gene therapy in mouse models of vanishing white matter disease

Leukoencephalopathy with vanishing white matter (VWM) is a progressive incurable white matter disease that most commonly occurs in childhood and presents with ataxia, spasticity, neurological degeneration, seizures, and premature death. A distinctive feature is episodes of rapid neurological deterioration provoked by stressors such as infection, seizures, or trauma. VWM is caused by autosomal recessive mutations in one of five genes that encode the eukaryotic initiation factor 2B complex, which is necessary for protein translation and regulation of the integrated stress response. The majority of mutations are in EIF2B5. Astrocytic dysfunction is central to pathophysiology, thereby constituting a potential therapeutic target. Herein we characterize two VWM murine models and investigate astrocyte-targeted adeno-associated virus serotype 9 (AAV9)-mediated EIF2B5 gene supplementation therapy as a therapeutic option for VWM. Our results demonstrate significant rescue in body weight, motor function, gait normalization, life extension, and finally, evidence that gene supplementation attenuates demyelination. Last, the greatest rescue results from a vector using a modified glial fibrillary acidic protein (GFAP) promoter-AAV9-gfaABC(1)D-EIF2B5-thereby supporting that astrocytic targeting is critical for disease correction. In conclusion, we demonstrate safety and early efficacy through treatment with a translatable astrocyte-targeted gene supplementation therapy for a disease that has no cure.

Targeting 14-3-3θ-mediated TDP-43 pathology in amyotrophic lateral sclerosis and frontotemporal dementia mice

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are characterized by cytoplasmic deposition of the nuclear TAR-binding protein 43 (TDP-43). Although cytoplasmic re-localization of TDP-43 is a key event in the pathogenesis of ALS/FTD, the underlying mechanisms remain unknown. Here, we identified a non-canonical interaction between 14-3-3θ and TDP-43, which regulates nuclear-cytoplasmic shuttling. Neuronal 14-3-3θ levels were increased in sporadic ALS and FTD with TDP-43 pathology. Pathogenic TDP-43 showed increased interaction with 14-3-3θ, resulting in cytoplasmic accumulation, insolubility, phosphorylation, and fragmentation of TDP-43, resembling pathological changes in disease. Harnessing this increased affinity of 14-3-3θ for pathogenic TDP-43, we devised a gene therapy vector targeting TDP-43 pathology, which mitigated functional deficits and neurodegeneration in different ALS/FTD mouse models expressing mutant or non-mutant TDP-43, including when already symptomatic at the time of treatment. Our study identified 14-3-3θ as a mediator of cytoplasmic TDP-43 localization with implications for ALS/FTD pathogenesis and therapy.

Cellular and molecular mechanisms of aspartoacylase and its role in Canavan disease

Canavan disease is an autosomal recessive and lethal neurological disorder, characterized by the spongy degeneration of the white matter in the brain. The disease is caused by a deficiency of the cytosolic aspartoacylase (ASPA) enzyme, which catalyzes the hydrolysis of N-acetyl-aspartate (NAA), an abundant brain metabolite, into aspartate and acetate. On the physiological level, the mechanism of pathogenicity remains somewhat obscure, with multiple, not mutually exclusive, suggested hypotheses. At the molecular level, recent studies have shown that most disease linked ASPA gene variants lead to a structural destabilization and subsequent proteasomal degradation of the ASPA protein variants, and accordingly Canavan disease should in general be considered a protein misfolding disorder. Here, we comprehensively summarize the molecular and cell biology of ASPA, with a particular focus on disease-linked gene variants and the pathophysiology of Canavan disease. We highlight the importance of high-throughput technologies and computational prediction tools for making genotype-phenotype predictions as we await the results of ongoing trials with gene therapy for Canavan disease.

Gene therapy of adeno-associated virus (AAV) vectors in preclinical models of ischemic stroke

Stroke has been associated with devastating clinical outcomes, with current treatment strategies proving largely ineffective. Therefore, there is a need to explore alternative treatment options for addressing post-stroke functional deficits. Gene therapy utilizing adeno-associated viruses (AAVs) as a critical gene vector delivering genes to the central nervous system (CNS) gene delivery has emerged as a promising approach for treating various CNS diseases. This review aims to provide an overview of the biological characteristics of AAV vectors and the therapeutic advancements observed in preclinical models of ischemic stroke. The study further investigates the potential of manipulating AAV vectors in preclinical applications, emphasizing the challenges and prospects in the selection of viral vectors, drug delivery strategies, immune reactions, and clinical translation.

Optogenetics in Alzheimer's Disease: Focus on Astrocytes

Alzheimer's disease (AD) is the most common form of dementia, resulting in disability and mortality. The global incidence of AD is consistently surging. Although numerous therapeutic agents with promising potential have been developed, none have successfully treated AD to date. Consequently, the pursuit of novel methodologies to address neurodegenerative processes in AD remains a paramount endeavor. A particularly promising avenue in this search is optogenetics, enabling the manipulation of neuronal activity. In recent years, research attention has pivoted from neurons to glial cells. This review aims to consider the potential of the optogenetic correction of astrocyte metabolism as a promising strategy for correcting AD-related disorders. The initial segment of the review centers on the role of astrocytes in the genesis of neurodegeneration. Astrocytes have been implicated in several pathological processes associated with AD, encompassing the clearance of β-amyloid, neuroinflammation, excitotoxicity, oxidative stress, and lipid metabolism (along with a critical role in apolipoprotein E function). The effect of astrocyte-neuronal interactions will also be scrutinized. Furthermore, the review delves into a number of studies indicating that changes in cellular calcium (Ca2+) signaling are one of the causes of neurodegeneration. The review's latter section presents insights into the application of various optogenetic tools to manipulate astrocytic function as a means to counteract neurodegenerative changes.

Mitofusin 1 overexpression rescues the abnormal mitochondrial dynamics caused by the Mitofusin 2 K357T mutation in vitro

BACKGROUND AND AIMS

Mitofusin 1 (MFN1) and MFN2 are outer mitochondrial membrane fusogenic proteins regulating mitochondrial network morphology. MFN2 mutations cause Charcot-Marie-Tooth type 2A (CMT2A), an axonal neuropathy characterized by mitochondrial fusion defects, which in the case of a GTPase domain mutant, were rescued following wild-type MFN1/2 (MFN1/2WT ) overexpression. In this study, we compared the therapeutic efficiency between MFN1WT and MFN2WT overexpression in correcting mitochondrial defects induced by the novel MFN2K357T mutation located in the highly conserved R3 region.

METHODS

Constructs expressing either MFN2K357T , MFN2WT , or MFN1WT under the ubiquitous chicken β-actin hybrid (CBh) promoter were generated. Flag or myc tag was used for their detection. Differentiated SH-SY5Y cells were single transfected with MFN1WT , MFN2WT , or MFN2K357T , as well as double transfected with MFN2K357T /MFN2WT or MFN2K357T /MFN1WT .

RESULTS

SH-SY5Y cells transfected with MFN2K357T exhibited severe perinuclear mitochondrial clustering with axon-like processes devoid of mitochondria. Single transfection with MFN1WT resulted in a more interconnected mitochondrial network than transfection with MFN2WT , accompanied by mitochondrial clusters. Double transfection of MFN2K357T with either MFN1WT or MFN2WT resolved the mutant-induced mitochondrial clusters and led to detectable mitochondria throughout the axon-like processes. MFN1WT showed higher efficacy than MFN2WT in rescuing these defects.

INTERPRETATION

These results further demonstrate the higher potential of MFN1WT over MFN2WT overexpression to rescue CMT2A-induced mitochondrial network abnormalities due to mutations outside the GTPase domain. This higher phenotypic rescue conferred by MFN1WT , possibly due to its higher mitochondrial fusogenic ability, may be applied to different CMT2A cases regardless of the MFN2 mutation type.

Developing Hypoimmunogenic Human iPSC-Derived Oligodendrocyte Progenitor Cells as an Off-The-Shelf Cell Therapy for Myelin Disorders

Demyelinating disorders are among the most common and debilitating diseases in neurology. Canavan disease (CD) is a lethal demyelinating disease caused by mutation of the aspartoacylase (ASPA) gene, which leads to the accumulation of its substrate N-acetyl-l-aspartate (NAA), and consequently demyelination and vacuolation in the brain. In this study, hypoimmunogenic human induced pluripotent stem cell (iPSC)-derived oligodendrocyte progenitor cells (OPC) are developed from a healthy donor as an "off-the-shelf" cell therapy. Hypoimmunogenic iPSCs are generated through CRISPR/Cas9 editing of the human leukocyte antigen (HLA) molecules in healthy donor-derived iPSCs and differentiated into OPCs. The OPCs are engrafted into the brains of CD (nur7) mice and exhibit widespread distribution in the brain. The engrafted OPCs mature into oligodendrocytes that express the endogenous wildtype ASPA gene. Consequently, the transplanted mice exhibit elevated human ASPA expression and enzymatic activity and reduced NAA level in the brain. The transplanted OPCs are able to rescue major pathological features of CD, including defective myelination, extensive vacuolation, and motor function deficits. Moreover, the hypoimmunogenic OPCs exhibit low immunogenicity both in vitro and in vivo. The hypoimmunogenic OPCs can be used as "off-the-shelf" universal donor cells to treat various CD patients and many other demyelinating disorders, especially autoimmune demyelinating diseases, such as multiple sclerosis.

Recessive aminoacyl-tRNA synthetase disorders: lessons learned from in vivo disease models

Protein synthesis is a fundamental process that underpins almost every aspect of cellular functioning. Intriguingly, despite their common function, recessive mutations in aminoacyl-tRNA synthetases (ARSs), the family of enzymes that pair tRNA molecules with amino acids prior to translation on the ribosome, cause a diverse range of multi-system disorders that affect specific groups of tissues. Neurological development is impaired in most ARS-associated disorders. In addition to central nervous system defects, diseases caused by recessive mutations in cytosolic ARSs commonly affect the liver and lungs. Patients with biallelic mutations in mitochondrial ARSs often present with encephalopathies, with variable involvement of peripheral systems. Many of these disorders cause severe disability, and as understanding of their pathogenesis is currently limited, there are no effective treatments available. To address this, accurate in vivo models for most of the recessive ARS diseases are urgently needed. Here, we discuss approaches that have been taken to model recessive ARS diseases in vivo, highlighting some of the challenges that have arisen in this process, as well as key results obtained from these models. Further development and refinement of animal models is essential to facilitate a better understanding of the pathophysiology underlying recessive ARS diseases, and ultimately to enable development and testing of effective therapies.

Intravital Imaging of Fluorescent Protein Expression in Mice with a Closed-Skull Traumatic Brain Injury and Cranial Window Using a Two-Photon Microscope

The goal of this protocol is to demonstrate how to longitudinally visualize the expression and localization of a protein of interest within specific cell types of an animal's brain, upon exposure to exogenous stimuli. Here, the administration of a closed-skull traumatic brain injury (TBI) and simultaneous implantation of a cranial window for subsequent longitudinal intravital imaging in mice is shown. Mice are intracranially injected with an adeno-associated virus (AAV) expressing enhanced green fluorescent protein (EGFP) under a neuronal specific promoter. After 2 to 4 weeks, the mice are subjected to a repetitive TBI using a weight drop device over the AAV injection location. Within the same surgical session, the mice are implanted with a metal headpost and then a glass cranial window over the TBI impacting site. The expression and cellular localization of EGFP is examined using a two-photon microscope in the same brain region exposed to trauma over the course of months.

Preclinical studies in Krabbe disease: A model for the investigation of novel combination therapies for lysosomal storage diseases

Krabbe disease (KD) is a lysosomal storage disease (LSD) caused by mutations in the galc gene. There are over 50 monogenetic LSDs, which largely impede the normal development of children and often lead to premature death. At present, there are no cures for LSDs and the available treatments are generally insufficient, short acting, and not without co-morbidities or long-term side effects. The last 30 years have seen significant advances in our understanding of LSD pathology as well as treatment options. Two gene therapy-based clinical trials, NCT04693598 and NCT04771416, for KD were recently started based on those advances. This review will discuss how our knowledge of KD got to where it is today, focusing on preclinical investigations, and how what was discovered may prove beneficial for the treatment of other LSDs.

Dual-function AAV gene therapy reverses late-stage Canavan disease pathology in mice

The leukodystrophy Canavan disease is a fatal white matter disorder caused by loss-of-function mutations of the aspartoacylase-encoding ASPA gene. There are no effective treatments available and experimental gene therapy trials have failed to provide sufficient amelioration from Canavan disease symptoms. Preclinical studies suggest that Canavan disease-like pathology can be addressed by either ASPA gene replacement therapy or by lowering the expression of the N-acetyl-L-aspartate synthesizing enzyme NAT8L. Both approaches individually prevent or even reverse pathological aspects in Canavan disease mice. Here, we combined both strategies and assessed whether intracranial adeno-associated virus-mediated gene delivery to a Canavan disease mouse model at 12 weeks allows for reversal of existing pathology. This was enabled by a single vector dual-function approach. In vitro and in vivo biopotency assessment revealed significant knockdown of neuronal Nat8l paired with robust ectopic aspartoacylase expression. Following nomination of the most efficient cassette designs, we performed proof-of-concept studies in post-symptomatic Aspa-null mice. Late-stage gene therapy resulted in a decrease of brain vacuoles and long-term reversal of all pathological hallmarks, including loss of body weight, locomotor impairments, elevated N-acetyl-L-aspartate levels, astrogliosis, and demyelination. These data suggest feasibility of a dual-function vector combination therapy, directed at replacing aspartoacylase with concomitantly suppressing N-acetyl-L-aspartate production, which holds potential to permanently alleviate Canavan disease symptoms and expands the therapeutic window towards a treatment option for adult subjects.

CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes

Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

A Role for PGC-1a in the Control of Abnormal Mitochondrial Dynamics in Alzheimer’s Disease

Emerging evidence suggests that the proper control of mitochondrial dynamics provides a window for therapeutic intervention for Alzheimer’s disease (AD) progression. The transcriptional coactivator peroxisome proliferator activated receptor gamma coactivator 1 (PGC-1a) has been shown to regulate mitochondrial biogenesis in neurons. Thus far, the roles of PGC-1a in Alzheimer’s disease and its potential value for restoring mitochondrial dysfunction remain largely unknown. In the present study, we explored the impacts of PGC-1a on AD pathology and neurobehavioral dysfunction and its potential mechanisms with a particular focus on mitochondrial dynamics. Paralleling AD-related pathological deposits, neuronal apoptosis, abnormal mitochondrial dynamics and lowered membrane potential, a remarkable reduction in the expression of PGC-1a was shown in the cortex of APP/PS1 mice at 6 months of age. By infusing AAV-Ppargc1α into the lateral parietal association (LPtA) cortex of the APP/PS1 brain, we found that PGC-1a ameliorated AD-like behavioral abnormalities, such as deficits in spatial reference memory, working memory and sensorimotor gating. Notably, overexpressed PGC-1a in LPtA rescued mitochondrial swelling and damage in neurons, likely through correcting the altered balance in mitochondrial fission–fusion and its abnormal distribution. Our findings support the notion that abnormal mitochondrial dynamics is likely an important mechanism that leading to mitochondrial dysfunction and AD-related pathological and cognitive impairments, and they indicate the potential value of PGC-1a for restoring mitochondrial dynamics as an innovative therapeutic target for AD.

The Polycomb group gene rnf2 is essential for central and enteric neural system development in zebrafish

The development of central nervous system (CNS) and enteric nervous system (ENS) is under precise and strict control in vertebrates. Whether and how the Polycomb repressive complex 1 (PRC1) is involved in it remain unclear. To investigate the role of PRC1 in the nervous system development, using CRISPR/Cas9 technology, we have generated mutant zebrafish lines for the rnf2 gene which encodes Ring1b, the enzymatic component of the PRC1 complex. We show that rnf2 loss of function leads to abnormal migration and differentiation of neural crest and neural precursor cells. rnf2 mutant embryos exhibit aganglionosis, in which the hindgut is devoid of neurons. In particular, the formation of 5-HT serotonin neurons and myelinating glial cells is defective. Furthermore, ectopic expression of ENS marker genes is observed in forebrain of rnf2 mutant embryos. These findings suggest that the rnf2 gene plays an important role in the migration and differentiation of neural precursor cells, and its absence leads to abnormal development of ENS and CNS in zebrafish.

FOXG1 as a Potential Therapeutic Target for Alzheimer's Disease with a Particular Focus on Cell Cycle Regulation

BACKGROUND

Several recent findings have revealed that targeting of cell cycle reentry and (or) progression may provide an opportunity for the therapeutic intervention of Alzheimer's disease (AD). FOXG1 has been shown to play important roles in pattern formation, cell proliferation, and cell specification. Thus far, the roles of FoxG1 and its involvement in AD are largely unknown.

OBJECTIVE

Our study aimed to explore the intervention effect of FOXG1 on AD pathology and its potential mechanism with a particular focus on cell cycle regulation.

METHODS

We investigated the association of Foxg1 gene variants with AD-like behavioral deficits, p21 expression, neuronal apoptosis, and amyloid-β (Aβ) aggregate formation; we further determined whether targeting FOXG1-regulated cell cycle has therapeutic potential in AD.

RESULTS

Paralleling AD-like behavioral abnormalities, neuronal apoptosis, and Aβ deposits, a significant reduction in the expression of FOXG1 was observed in APP/PS1 mice at 6 months of age. Using the APP/PS1;Foxg1fl/fl-CreAAV mouse line, we found that FOXG1 potentially antagonized cell cycle reentry by negatively regulating the levels of p21-activated kinase (PAK3). By reducing p21cip1-mediated arrest at the G2 stage and regulating cyclin A1- and cyclin B-dependent progression patterns of the cell cycle, FOXG1 blocked neuronal apoptosis and Aβ deposition.

CONCLUSION

These results indicate that FOXG1 contributes to the regulation of the neuronal cell cycle, thereby affecting brain abnormalities in AD. An elevation of the FOXG1 level, either pharmacologically or through other means, could present a therapeutic strategy for AD.

The oligodendrocyte-enriched orphan G protein-coupled receptor Gpr62 is dispensable for central nervous system myelination

BACKGROUND

Myelination is a highly regulated process in the vertebrate central nervous system (CNS) whereby oligodendrocytes wrap axons with multiple layers of insulating myelin in order to allow rapid electrical conduction. Establishing the proper pattern of myelin in neural circuits requires communicative axo-glial interactions, however, the molecular interactions that occur between oligodendrocytes and axons during developmental myelination and myelin maintenance remain to be fully elucidated. Our previous work identified G protein-coupled receptor 62 (Gpr62), an uncharacterized orphan g-protein coupled receptor, as being selectively expressed by mature oligodendrocytes within the CNS, suggesting a potential role in myelination or axoglial interactions. However, no studies to date have assessed the functional requirement for Gpr62 in oligodendrocyte development or CNS myelination.

METHODS

To address this, we generated a knockout mouse strain lacking the Gpr62 gene. We assessed CNS myelination during both postnatal development and adulthood using immunohistochemistry, electron microscopy and western blot. In addition, we utilized AAV-mediated expression of a tagged Gpr62 in oligodendrocytes to determine the subcellular localization of the protein in vivo.

RESULTS

We find that virally expressed Gpr62 protein is selectively expressed on the adaxonal myelin layer, suggestive of a potential role for Gpr62 in axo-myelinic signaling. Nevertheless, Gpr62 knockout mice display normal oligodendrocyte numbers and apparently normal myelination within the CNS during both postnatal development and adulthood.

CONCLUSIONS

We conclude that in spite of being well-placed to mediate neuronal-oligodendrocyte communications, Gpr62 is overall dispensable for CNS myelination.

GFAP at 50

Fifty years have passed since the discovery of glial fibrillary acidic protein (GFAP) by Lawrence Eng and colleagues. Now recognized as a member of the intermediate filament family of proteins, it has become a subject for study in fields as diverse as structural biology, cell biology, gene expression, basic neuroscience, clinical genetics and gene therapy. This review covers each of these areas, presenting an overview of current understanding and controversies regarding GFAP with the goal of stimulating continued study of this fascinating protein.

Rapid initiation of cell cycle reentry processes protects neurons from amyloid-β toxicity

Neurons are postmitotic cells. Reactivation of the cell cycle by neurons has been reported in Alzheimer's disease (AD) brains and models. This gave rise to the hypothesis that reentering the cell cycle renders neurons vulnerable and thus contributes to AD pathogenesis. Here, we use the fluorescent ubiquitination-based cell cycle indicator (FUCCI) technology to monitor the cell cycle in live neurons. We found transient, self-limited cell cycle reentry activity in naive neurons, suggesting that their postmitotic state is a dynamic process. Furthermore, we observed a diverse response to oligomeric amyloid-β (oAβ) challenge; neurons without cell cycle reentry activity would undergo cell death without activating the FUCCI reporter, while neurons undergoing cell cycle reentry activity at the time of the oAβ challenge could maintain and increase FUCCI reporter signal and evade cell death. Accordingly, we observed marked neuronal FUCCI positivity in the brains of human mutant Aβ precursor protein transgenic (APP23) mice together with increased neuronal expression of the endogenous cell cycle control protein geminin in the brains of 3-mo-old APP23 mice and human AD brains. Taken together, our data challenge the current view on cell cycle in neurons and AD, suggesting that pathways active during early cell cycle reentry in neurons protect from Aβ toxicity.

Challenges and Opportunities of Targeting Astrocytes to Halt Neurodegenerative Disorders

Neurodegenerative diseases are a heterogeneous group of disorders whose incidence is likely to duplicate in the next 30 years along with the progressive aging of the western population. Non-cell-specific therapeutics or therapeutics designed to tackle aberrant pathways within neurons failed to slow down or halt neurodegeneration. Yet, in the last few years, our knowledge of the importance of glial cells to maintain the central nervous system homeostasis in health conditions has increased exponentially, along with our awareness of their fundamental and multifaced role in pathological conditions. Among glial cells, astrocytes emerge as promising therapeutic targets in various neurodegenerative disorders. In this review, we present the latest evidence showing the astonishing level of specialization that astrocytes display to fulfill the demands of their neuronal partners as well as their plasticity upon injury. Then, we discuss the controversies that fuel the current debate on these cells. We tackle evidence of a potential beneficial effect of cell therapy, achieved by transplanting astrocytes or their precursors. Afterwards, we introduce the different strategies proposed to modulate astrocyte functions in neurodegeneration, ranging from lifestyle changes to environmental cues. Finally, we discuss the challenges and the recent advancements to develop astrocyte-specific delivery systems.

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.

Diversity of Reactive Astrogliosis in CNS Pathology: Heterogeneity or Plasticity?

Astrocytes are essential for the development and homeostatic maintenance of the central nervous system (CNS). They are also critical players in the CNS injury response during which they undergo a process referred to as "reactive astrogliosis." Diversity in astrocyte morphology and gene expression, as revealed by transcriptional analysis, is well-recognized and has been reported in several CNS pathologies, including ischemic stroke, CNS demyelination, and traumatic injury. This diversity appears unique to the specific pathology, with significant variance across temporal, topographical, age, and sex-specific variables. Despite this, there is limited functional data corroborating this diversity. Furthermore, as reactive astrocytes display significant environmental-dependent plasticity and fate-mapping data on astrocyte subsets in the adult CNS is limited, it remains unclear whether this diversity represents heterogeneity or plasticity. As astrocytes are important for neuronal survival and CNS function post-injury, establishing to what extent this diversity reflects distinct established heterogeneous astrocyte subpopulations vs. environmentally dependent plasticity within established astrocyte subsets will be critical for guiding therapeutic development. To that end, we review the current state of knowledge on astrocyte diversity in the context of three representative CNS pathologies: ischemic stroke, demyelination, and traumatic injury, with the goal of identifying key limitations in our current knowledge and suggesting future areas of research needed to address them. We suggest that the majority of identified astrocyte diversity in CNS pathologies to date represents plasticity in response to dynamically changing post-injury environments as opposed to heterogeneity, an important consideration for the understanding of disease pathogenesis and the development of therapeutic interventions.

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.

Emerging Concepts in Vector Development for Glial Gene Therapy: Implications for Leukodystrophies

Central Nervous System (CNS) homeostasis and function rely on intercellular synchronization of metabolic pathways. Developmental and neurochemical imbalances arising from mutations are frequently associated with devastating and often intractable neurological dysfunction. In the absence of pharmacological treatment options, but with knowledge of the genetic cause underlying the pathophysiology, gene therapy holds promise for disease control. Consideration of leukodystrophies provide a case in point; we review cell type – specific expression pattern of the disease – causing genes and reflect on genetic and cellular treatment approaches including ex vivo hematopoietic stem cell gene therapies and in vivo approaches using adeno-associated virus (AAV) vectors. We link recent advances in vectorology to glial targeting directed towards gene therapies for specific leukodystrophies and related developmental or neurometabolic disorders affecting the CNS white matter and frame strategies for therapy development in future.

Widespread, Specific, and Efficient Transgene Expression in Oligodendrocytes After Intracerebral and Intracerebroventricular Delivery of Viral Vectors in Rodent Brain

Several neurodegenerative disorders are characterized by oligodendroglial pathology and myelin loss. Oligodendrogliopathies are a group of rare diseases for which there currently is no therapy. Gene delivery through viral vectors to oligodendrocytes is a potential strategy to deliver therapeutic molecules to oligodendrocytes for disease modification. However, targeting oligodendroglial cells in vivo is challenging due to their widespread distribution in white and gray matter. In this study, we aimed to address several of these difficulties by designing and testing different oligodendroglial targeting vectors in rat and mouse brain, utilizing different promoters, serotypes, and delivery routes. We found that different oligodendroglial promoters (myelin basic protein [MBP], cytomegalovirus-enhanced MBP, and myelin-associated glycoprotein [MAG]) vary considerably in their ability to drive oligodendroglial transgene expression and different viral vector serotypes (rAAV2/7, rAAV2/8, and rAAV2/9) exhibit varying efficacies in transducing oligodendrocytes. Different administration routes through intracerebral or intraventricular injection allow widespread targeting of mature oligodendrocytes. Delivery of rAAV2/9-MAG-GFP into the cerebrospinal fluid results in GFP expression along the entire rostrocaudal axis of the spinal cord. Collectively, these results show that oligodendrocytes can be targeted with high specificity and widespread expression, which will be useful for gene therapeutic interventions or disease modeling purposes.

Canavan Disease as a Model for Gene Therapy-Mediated Myelin Repair

In recent years, the scientific and therapeutic fields for rare, genetic central nervous system (CNS) diseases such as leukodystrophies, or white matter disorders, have expanded significantly in part due to technological advancements in cellular and clinical screenings as well as remedial therapies using novel techniques such as gene therapy. However, treatments aimed at normalizing the pathological changes associated with leukodystrophies have especially been complicated due to the innate and variable effects of glial abnormalities, which can cause large-scale functional deficits in developmental myelination and thus lead to downstream neuronal impairment. Emerging research in the past two decades have depicted glial cells, particularly oligodendrocytes and astrocytes, as key, regulatory modulators in constructing and maintaining myelin function and neuronal viability. Given the significance of myelin formation in the developing brain, myelin repair in a time-dependent fashion is critical in restoring homeostatic functionality to the CNS of patients diagnosed with white matter disorders. Using Canavan Disease (CD) as a leukodystrophy model, here we review the hypothetical roles of N-acetylaspartate (NAA), one of the brain's most abundant amino acid derivatives, in Canavan disease's CNS myelinating pathology, as well as discuss the possible functions astrocytes serve in both CD and other leukodystrophies' time-sensitive disease correction. Through this analysis, we also highlight the potential remyelinating benefits of gene therapy for other leukodystrophies in which alternative CNS cell targeting for white matter disorders may be an applicable path for reparative treatment.

AAV9-mediated gene delivery of MCT1 to oligodendrocytes does not provide a therapeutic benefit in a mouse model of ALS

Oligodendrocyte dysfunction has been implicated in the pathophysiology of amyotrophic lateral sclerosis (ALS), a neurodegenerative disorder characterized by progressive motor neuron loss. The failure of trophic support provided by oligodendrocytes is associated with a concomitant reduction in oligodendroglial monocarboxylate transporter 1 (MCT1) expression and is detrimental for the long-term survival of motor neuron axons. Therefore, we established an adeno-associated virus 9 (AAV9)-based platform by which MCT1 was targeted mostly to white matter oligodendrocytes to investigate whether this approach could provide a therapeutic benefit in the SOD1^G93A mouse model of ALS. Despite good oligodendrocyte transduction and AAV-mediated MCT1 transgene expression, the disease outcome of SOD1^G93A mice was not altered. Our study further increases our current understanding about the complex nature of oligodendrocyte pathology in ALS and provides valuable insights into the future development of therapeutic strategies to efficiently modulate these cells.

A Hypomorphic Dars1 D367Y Model Recapitulates Key Aspects of the Leukodystrophy HBSL

Hypomyelination with brain stem and spinal cord involvement and leg spasticity (HBSL) is a leukodystrophy caused by missense mutations of the aspartyl-tRNA synthetase-encoding gene DARS1. The clinical picture includes the regression of acquired motor milestones, spasticity, ataxia, seizures, nystagmus, and intellectual disabilities. Morphologically, HBSL is characterized by a distinct pattern of hypomyelination in the central nervous system including the anterior brainstem, the cerebellar peduncles and the supratentorial white matter as well as the dorsal columns and the lateral corticospinal tracts of the spinal cord. Adequate HBSL animal models are lacking. Dars1 knockout mice are embryonic lethal precluding examination of the etiology. To address this, we introduced the HBSL-causing Dars1 D367Y point mutation into the mouse genome. Surprisingly, mice carrying this mutation homozygously were phenotypically normal. As hypomorphic mutations are more severe in trans to a deletion, we crossed Dars1 D367Y/D367Y mice with Dars1-null carriers. The resulting Dars1 D367Y/- offspring displayed a strong developmental delay compared to control Dars1 D367Y/+ littermates, starting during embryogenesis. Only a small fraction of Dars1 D367Y/- mice were born, and half of these mice died with hydrocephalus during the first 3 weeks of life. Of the few Dars1 D367Y/- mice that were born at term, 25% displayed microphthalmia. Throughout postnatal life, Dars1 D367Y/- mice remained smaller and lighter than their Dars1 D367Y/+ littermates. Despite this early developmental deficit, once they made it through early adolescence Dars1 D367Y/- mice were phenotypically inconspicuous for most of their adult life, until they developed late onset motor deficits as well as vacuolization and demyelination of the spinal cord white matter. Expression levels of the major myelin proteins were reduced in Dars1 D367Y/- mice compared to controls. Taken together, Dars1 D367Y/- mice model aspects of the clinical picture of the corresponding missense mutation in HBSL. This model will enable studies of late onset deficits, which is precluded in Dars1 knockout mice, and can be leveraged to test potential HBSL therapeutics including DARS1 gene replacement therapy.

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.

Tracing goes viral: Viruses that introduce expression of fluorescent proteins in chemically-specific neurons

Over the last century, there has been great progress in understanding how the brain works. In particular, the last two decades have been crucial in gaining more awareness over the complex functioning of neurotransmitter systems. The use of viral vectors in neuroscience has been pivotal for such development. Exploiting the properties of viral particles, modifying them according to the research needs, and making them target chemically-specific neurons, techniques such as optogenetics and chemogenetics have been developed, which could lead to a giant step toward gene therapy for brain disorders. In this review, we aim to provide an overview of some of the most widely used viral techniques in neuroscience. We will discuss advantages and disadvantages of these methods. In particular, attention is dedicated to the pivotal role played by the introduction of adeno-associated virus and the retrograde tracer canine-associated-2 Cre virus in order to achieve optimal visualization, and interrogation, of chemically-specific neuronal populations and their projections.

Cerebellar Astrocyte Transduction as Gene Therapy for Megalencephalic Leukoencephalopathy

Megalencephalic leukoencephalopathy with subcortical cysts (MLC) is a rare genetic disorder belonging to the group of vacuolating leukodystrophies. It is characterized by megalencephaly, loss of motor functions, epilepsy, and mild mental decline. In brain biopsies of MLC patients, vacuoles were observed in myelin and in astrocytes surrounding blood vessels. There is no therapy for MLC patients, only supportive treatment. We show here a preclinical gene therapy approach for MLC using the Mlc1 knock-out mouse. An adeno-associated virus coding for human MLC1 under the control of the glial fibrillary acidic protein promoter was injected in the cerebellar subarachnoid space of Mlc1 knock-out and wild-type animals at 2 months of age, before the onset of the disease, as a preventive approach. We also tested a therapeutic strategy by injecting the animals at 5 months, once the histopathological abnormalities are starting, or at 15 months, when they have progressed to a more severe pathology. MLC1 expression in the cerebellum restored the adhesion molecule GlialCAM and the chloride channel ClC-2 localization in Bergmann glia, which both are mislocalized in Mlc1 knock-out model. More importantly, myelin vacuolation was extremely reduced in treated mice at all ages and correlated with the amount of expressed MLC1 in Bergmann glia, indicating not only the preventive potential of this strategy but also its therapeutic capacity. In summary, here we provide the first therapeutic approach for patients affected with MLC. This work may have also implications to treat other diseases affecting motor function such as ataxias.

Protective Functions of Reactive Astrocytes Following Central Nervous System Insult

Astrocytes play important roles in numerous central nervous system disorders including autoimmune inflammatory, hypoxic, and degenerative diseases such as Multiple Sclerosis, ischemic stroke, and Alzheimer’s disease. Depending on the spatial and temporal context, activated astrocytes may contribute to the pathogenesis, progression, and recovery of disease. Recent progress in the dissection of transcriptional responses to varying forms of central nervous system insult has shed light on the mechanisms that govern the complexity of reactive astrocyte functions. While a large body of research focuses on the pathogenic effects of reactive astrocytes, little is known about how they limit inflammation and contribute to tissue regeneration. However, these protective astrocyte pathways might be of relevance for the understanding of the underlying pathology in disease and may lead to novel targeted approaches to treat autoimmune inflammatory and degenerative disorders of the central nervous system. In this review article, we have revisited the emerging concept of protective astrocyte functions and discuss their role in the recovery from inflammatory and ischemic disease as well as their role in degenerative disorders. Focusing on soluble astrocyte derived mediators, we aggregate the existing knowledge on astrocyte functions in the maintenance of homeostasis as well as their reparative and tissue-protective function after acute lesions and in neurodegenerative disorders. Finally, we give an outlook of how these mediators may guide future therapeutic strategies to tackle yet untreatable disorders of the central nervous system.

Viral-Mediated Gene Replacement Therapy in the Developing Central Nervous System: Current Status and Future Directions

The past few years have witnessed rapid developments in viral-mediated gene replacement therapy for pediatric central nervous system neurogenetic disorders. Here, we provide pediatric neurologists with an up-to-date, comprehensive overview of these developments and note emerging trends for future research. This review presents the different types of viral vectors used in viral-mediated gene replacement therapy; the fundamental properties of viral-mediated gene replacement therapy; the challenges associated with the use of this therapy in the central nervous system; the pathway for therapy development, from translational basic science studies to clinical trials; and an overview of the therapies that have reached clinical trials in patients. Current viral platforms under investigation include adenovirus vectors, adeno-associated viral vectors, lentiviral/retroviral vectors, and herpes simplex virus type 1 vectors. This review also presents an in-depth analysis of numerous studies that investigated these viral platforms in cultured cells and in transgenic animal models for pediatric neurogenetic disorders. Viral vectors have been applied to clinical trials for many different pediatric neurogenetic disorders, including Canavan disease, metachromatic leukodystrophy, neuronal ceroid lipofuscinosis, mucopolysaccharidosis III, spinal muscular atrophy, and aromatic l-amino acid decarboxylase deficiency. Of these diseases, only spinal muscular atrophy has a viral-mediated gene replacement therapy approved for marketing. Despite significant progress in therapy development, many challenges remain. Surmounting these challenges is critical to advancing the current status of viral-mediated gene replacement therapy for pediatric central nervous system neurogenetic disorders.

Intramuscular Delivery of Gene Therapy for Targeting the Nervous System

Virus-mediated gene therapy has the potential to deliver exogenous genetic material into specific cell types to promote survival and counteract disease. This is particularly enticing for neuronal conditions, as the nervous system is renowned for its intransigence to therapeutic targeting. Administration of gene therapy viruses into skeletal muscle, where distal terminals of motor and sensory neurons reside, has been shown to result in extensive transduction of cells within the spinal cord, brainstem, and sensory ganglia. This route is minimally invasive and therefore clinically relevant for gene therapy targeting to peripheral nerve soma. For successful transgene expression, viruses administered into muscle must undergo a series of processes, including host cell interaction and internalization, intracellular sorting, long-range retrograde axonal transport, endosomal liberation, and nuclear import. In this review article, we outline key characteristics of major gene therapy viruses—adenovirus, adeno-associated virus (AAV), and lentivirus—and summarize the mechanisms regulating important steps in the virus journey from binding at peripheral nerve terminals to nuclear delivery. Additionally, we describe how neuropathology can negatively influence these pathways, and conclude by discussing opportunities to optimize the intramuscular administration route to maximize gene delivery and thus therapeutic potential.

The role of oligodendrocyte gap junctions in neuroinflammation

Gap junctions (GJs) provide channels for direct cell-to-cell connectivity serving the homeostasis in several organs of vertebrates including the central (CNS) and peripheral (PNS) nervous systems. GJs are composed of connexins (Cx), which show a highly distinct cellular and subcellular expression pattern. Oligodendrocytes, the myelinating cells of the CNS, are characterized by extensive GJ connectivity with each other as well as with astrocytes. The main oligodendrocyte connexins forming these GJ channels are Cx47 and Cx32. The importance of these channels has been highlighted by the discovery of human diseases caused by mutations in oligodendrocyte connexins, manifesting with leukodystrophy or transient encephalopathy. Experimental models have provided further evidence that oligodendrocyte GJs are essential for CNS myelination and homeostasis, while a strong inflammatory component has been recognized in the absence of oligodendrocyte connexins. Further studies revealed that connexins are also disrupted in multiple sclerosis (MS) brain, and in experimental models of induced inflammatory demyelination. Moreover, induced demyelination was more severe and associated with higher degree of CNS inflammation in models with oligodendrocyte GJ deficiency, suggesting that disrupted connexin expression in oligodendrocytes is not only a consequence but can also drive a pro-inflammatory environment in acquired demyelinating disorders such as MS. In this review, we summarize the current insights from human disorders as well as from genetic and acquired models of demyelination related to oligodendrocyte connexins, with the remaining challenges and perspectives.

Gene therapy approaches targeting Schwann cells for demyelinating neuropathies

Charcot-Marie-Tooth disease (CMT) encompasses numerous genetically heterogeneous inherited neuropathies, which together are one of the commonest neurogenetic disorders. Axonal CMT types result from mutations in neuronally expressed genes, whereas demyelinating CMT forms mostly result from mutations in genes expressed by myelinating Schwann cells. The demyelinating forms are the most common, and may be caused by dominant mutations and gene dosage effects (as in CMT1), as well as by recessive mutations and loss of function mechanisms (as in CMT4). The discovery of causative genes and increasing insights into molecular mechanisms through the study of experimental disease models has provided the basis for the development of gene therapy approaches. For demyelinating CMT, gene silencing or gene replacement strategies need to be targeted to Schwann cells. Progress in gene replacement for two different CMT forms, including CMT1X caused by GJB1 gene mutations, and CMT4C, caused by SH3TC2 gene mutations, has been made through the use of a myelin-specific promoter to restrict expression in Schwann cells, and by lumbar intrathecal delivery of lentiviral viral vectors to achieve more widespread biodistribution in the peripheral nervous system. This review summarizes the molecular-genetic mechanisms of selected demyelinating CMT neuropathies and the progress made so far, as well as the remaining challenges in the path towards a gene therapy to treat these disorders through the use of optimal gene therapy tools including clinically translatable delivery methods and adeno-associated viral (AAV) vectors.

Adeno-Associated Virus Technologies and Methods for Targeted Neuronal Manipulation

Cell-type-specific expression of molecular tools and sensors is critical to construct circuit diagrams and to investigate the activity and function of neurons within the nervous system. Strategies for targeted manipulation include combinations of classical genetic tools such as Cre/loxP and Flp/FRT, use of cis-regulatory elements, targeted knock-in transgenic mice, and gene delivery by AAV and other viral vectors. The combination of these complex technologies with the goal of precise neuronal targeting is a challenge in the lab. This report will discuss the theoretical and practical aspects of combining current technologies and establish best practices for achieving targeted manipulation of specific cell types. Novel applications and tools, as well as areas for development, will be envisioned and discussed.

Gene Therapy for ALS-A Perspective

Amyotrophic lateral sclerosis (ALS) is a fatal motor neuron disease (MND) with no cure. Recent advances in gene therapy open a new perspective to treat this disorder-particularly for the characterized genetic forms. Gene therapy approaches, involving the delivery of antisense oligonucleotides into the central nervous system (CNS) are being tested in clinical trials for patients with mutations in SOD1 or C9orf72 genes. Viral vectors can be used to deliver therapeutic sequences to stably transduce motor neurons in the CNS. Vectors derived from adeno-associated virus (AAV), can efficiently target genes and have been tested in several pre-clinical settings with promising outcomes. Recently, the Food and Drug Administration (FDA) approved Zolgensma, an AAV-mediated treatment for another MND-the infant form of spinal muscular atrophy. Given the accelerated progress in gene therapy, it is potentially a promising avenue to develop an efficient and safe cure for ALS.

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.

The adeno-associated virus rh10 vector is an effective gene transfer system for chronic spinal cord injury

Treatment options for chronic spinal cord injury (SCI) remain limited due to unfavourable changes in the microenvironment. Gene therapy can overcome these barriers through continuous delivery of therapeutic gene products to the target tissue. In particular, adeno-associated virus (AAV) vectors are potential candidates for use in chronic SCI, considering their safety and stable gene expression in vivo. Given that different AAV serotypes display different cellular tropisms, it is extremely important to select an optimal serotype for establishing a gene transfer system during the chronic phase of SCI. Therefore, we generated multiple AAV serotypes expressing ffLuc-cp156, a fusion protein of firefly luciferase and Venus, a variant of yellow fluorescent protein with fast and efficient maturation, as a reporter, and we performed intraparenchymal injection in a chronic SCI mouse model. Among the various serotypes tested, AAVrh10 displayed the highest photon count on bioluminescence imaging. Immunohistological analysis revealed that AAVrh10 showed favourable tropism for neurons, astrocytes, and oligodendrocytes. Additionally, with AAVrh10, the area expressing Venus was larger in the injury epicentre and extended to the surrounding tissue. Furthermore, the fluorescence intensity was significantly higher with AAVrh10 than with the other vectors. These results indicate that AAVrh10 may be an appropriate serotype for gene delivery to the chronically injured spinal cord. This promising tool may be applied for research and development related to the treatment of chronic SCI.

Gene Therapy Tools for Brain Diseases

Neurological disorders affecting the central nervous system (CNS) are still incompletely understood. Many of these disorders lack a cure and are seeking more specific and effective treatments. In fact, in spite of advancements in knowledge of the CNS function, the treatment of neurological disorders with modern medical and surgical approaches remains difficult for many reasons, such as the complexity of the CNS, the limited regenerative capacity of the tissue, and the difficulty in conveying conventional drugs to the organ due to the blood-brain barrier. Gene therapy, allowing the delivery of genetic materials that encodes potential therapeutic molecules, represents an attractive option. Gene therapy can result in a stable or inducible expression of transgene(s), and can allow a nearly specific expression in target cells. In this review, we will discuss the most commonly used tools for the delivery of genetic material in the CNS, including viral and non-viral vectors; their main applications; their advantages and disadvantages. We will discuss mechanisms of genetic regulation through cell-specific and inducible promoters, which allow to express gene products only in specific cells and to control their transcriptional activation. In addition, we will describe the applications to CNS diseases of post-transcriptional regulation systems (RNA interference); of systems allowing spatial or temporal control of expression [optogenetics and Designer Receptors Exclusively Activated by Designer Drugs (DREADDs)]; and of gene editing technologies (CRISPR/Cas9, Zinc finger proteins). Particular attention will be reserved to viral vectors derived from herpes simplex type 1, a potential tool for the delivery and expression of multiple transgene cassettes simultaneously.

Gene suppressing therapy for Pelizaeus-Merzbacher disease using artificial microRNA

Copy number increase or decrease of certain dosage-sensitive genes may cause genetic diseases with distinct phenotypes, conceptually termed genomic disorders. The most common cause of Pelizaeus-Merzbacher disease (PMD), an X-linked hypomyelinating leukodystrophy, is genomic duplication encompassing the entire proteolipid protein 1 (PLP1) gene. Although the exact molecular and cellular mechanisms underlying PLP1 duplication, which causes severe hypomyelination in the central nervous system, remain largely elusive, PLP1 overexpression is likely the fundamental cause of this devastating disease. Here, we investigated if adeno-associated virus-mediated (AAV-mediated) gene-specific suppression may serve as a potential cure for PMD by correcting quantitative aberrations in gene products. We developed an oligodendrocyte-specific Plp1 gene suppression therapy using artificial microRNA under the control of human CNP promoter in a self-complementary AAV (scAAV) platform. A single direct brain injection achieved widespread oligodendrocyte-specific Plp1 suppression in the white matter of WT mice. AAV treatment in Plp1-transgenic mice, a PLP1 duplication model, ameliorated cytoplasmic accumulation of Plp1, preserved mature oligodendrocytes from degradation, restored myelin structure and gene expression, and improved survival and neurological phenotypes. Together, our results provide evidence that AAV-mediated gene suppression therapy can serve as a potential cure for PMD resulting from PLP1 duplication and possibly for other genomic disorders.

Adeno-associated virus-based Alzheimer's disease mouse models and potential new therapeutic avenues

Alzheimer's disease (AD) is a highly prevalent neurodegenerative condition that presents with cognitive decline. The current understanding of underlying disease mechanisms remains incomplete. Genetically modified mouse models have been instrumental in deciphering pathomechanisms in AD. While these models were typically generated by classical transgenesis and genome editing, the use of adeno-associated viruses (AAVs) to model and investigate AD in mice, as well as to develop novel gene-therapy approaches, is emerging. Here, we reviewed literature that used AAVs to study and model AD and discuss potential gene therapy strategies. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

The Application of Adeno-Associated Viral Vector Gene Therapy to the Treatment of Fragile X Syndrome

Viral vector-mediated gene therapy has grown by leaps and bounds over the past several years. Although the reasons for this progress are varied, a deeper understanding of the basic biology of the viruses, the identification of new and improved versions of viral vectors, and simply the vast experience gained by extensive testing in both animal models of disease and in clinical trials, have been key factors. Several studies have investigated the efficacy of adeno-associated viral (AAV) vectors in the mouse model of fragile X syndrome where AAVs have been used to express fragile X mental retardation protein (FMRP), which is missing or highly reduced in the disorder. These studies have demonstrated a range of efficacies in different tests from full correction, to partial rescue, to no effect. Here we provide a backdrop of recent advances in AAV gene therapy as applied to central nervous system disorders, outline the salient features of the fragile X studies, and discuss several key issues for moving forward. Collectively, the findings to date from the mouse studies on fragile X syndrome, and data from clinical trials testing AAVs in other neurological conditions, indicate that AAV-mediated gene therapy could be a viable strategy for treating fragile X syndrome.

Expression of the DNA-Binding Factor TOX Promotes the Encephalitogenic Potential of Microbe-Induced Autoreactive CD8+ T Cells

Infections are thought to trigger CD8⁺ cytotoxic T lymphocyte (CTL) responses during autoimmunity. However, the transcriptional programs governing the tissue-destructive potential of CTLs remain poorly defined. In a model of central nervous system (CNS) inflammation, we found that infection with lymphocytic choriomeningitis virus (LCMV), but not Listeria monocytogenes (Lm), drove autoimmunity. The DNA-binding factor TOX was induced in CTLs during LCMV infection and was essential for their encephalitogenic properties, and its expression was inhibited by interleukin-12 during Lm infection. TOX repressed the activity of several transcription factors (including Id2, TCF-1, and Notch) that are known to drive CTL differentiation. TOX also reduced immune checkpoint sensitivity by restraining the expression of the inhibitory checkpoint receptor CD244 on the surface of CTLs, leading to increased CTL-mediated damage in the CNS. Our results identify TOX as a transcriptional regulator of tissue-destructive CTLs in autoimmunity, offering a potential mechanistic link to microbial triggers.