Tropisms of AAV for subretinal delivery to the neonatal mouse retina and its application for in vivo rescue of developmental photoreceptor disorders

Gene therapy for CNS disorders: modalities, delivery and translational challenges

Gene therapy is emerging as a powerful tool to modulate abnormal gene expression, a hallmark of most CNS disorders. The transformative potentials of recently approved gene therapies for the treatment of spinal muscular atrophy (SMA), amyotrophic lateral sclerosis (ALS) and active cerebral adrenoleukodystrophy are encouraging further development of this approach. However, most attempts to translate gene therapy to the clinic have failed to make it to market. There is an urgent need not only to tailor the genes that are targeted to the pathology of interest but to also address delivery challenges and thereby maximize the utility of genetic tools. In this Review, we provide an overview of gene therapy modalities for CNS diseases, emphasizing the interconnectedness of different delivery strategies and routes of administration. Important gaps in understanding that could accelerate the clinical translatability of CNS genetic interventions are addressed, and we present lessons learned from failed clinical trials that may guide the future development of gene therapies for the treatment and management of CNS disorders.

AAV-mediated base-editing therapy ameliorates the disease phenotypes in a mouse model of retinitis pigmentosa

Base editing technology is an ideal solution for treating pathogenic single-nucleotide variations (SNVs). No gene editing therapy has yet been approved for eye diseases, such as retinitis pigmentosa (RP). Here, we show, in the rd10 mouse model, which carries an SNV identified as an RP-causing mutation in human patients, that subretinal delivery of an optimized dual adeno-associated virus system containing the adenine base editor corrects the pathogenic SNV in the neuroretina with up to 49% efficiency. Light microscopy showed that a thick and robust outer nuclear layer (photoreceptors) was preserved in the treated area compared with the thin, degenerated outer nuclear layer without treatment. Substantial electroretinogram signals were detected in treated rd10 eyes, whereas control treated eyes showed minimal signals. The water maze experiment showed that the treatment substantially improved vision-guided behavior. Together, we construct and validate a translational therapeutic solution for the treatment of RP in humans. Our findings might accelerate the development of base-editing based gene therapies.

Extracellular vesicles in the retina - putative roles in physiology and disease

The retina encompasses a network of neurons, glia and epithelial and vascular endothelia cells, all coordinating visual function. Traditionally, molecular information exchange in this tissue was thought to be orchestrated by synapses and gap junctions. Recent findings have revealed that many cell types are able to package and share molecular information via extracellular vesicles (EVs) and the technological advancements in visualisation and tracking of these delicate nanostructures has shown that the role of EVs in cell communication is pleiotropic. EVs are released under physiological conditions by many cells but they are also released during various disease stages, potentially reflecting the health status of the cells in their cargo. Little is known about the physiological role of EV release in the retina. However, administration of exogenous EVs in vivo after injury suggest a neurotrophic role, whilst photoreceptor transplantation in early stages of retina degeneration, EVs may facilitate interactions between photoreceptors and Müller glia cells. In this review, we consider some of the proposed roles for EVs in retinal physiology and discuss current evidence regarding their potential impact on ocular therapies via gene or cell replacement strategies and direct intraocular administration in the diseased eye.

Gene Augmentation for Autosomal Dominant CRX-Associated Retinopathies

The cone-rod homeobox (CRX) protein is a key transcription factor essential for photoreceptor function and survival. Mutations in human CRX gene are linked to a wide spectrum of blinding diseases ranging from mild macular dystrophy to severe Leber congenital amaurosis (LCA), cone-rod dystrophy (CRD), and retinitis pigmentosa (RP). These diseases are still incurable and mostly inherited in an autosomal dominant form. Dysfunctional mutant CRX protein interferes with the function of wild-type CRX protein, demonstrating the dominant negative effect. At present, gene augmentation is the most promising treatment strategy for hereditary diseases. This study aims to review the pathogenic mechanisms of various CRX mutations and propose two therapeutic strategies to rescue sick photoreceptors in CRX-associated retinopathies, namely, Tet-On-hCRX system and adeno-associated virus (AAV)-mediated gene augmentation. The outcome of proposed studies will guide future translational research and suggest guidelines for therapy evaluation in terms of treatment safety and efficacy.

AAV vectors: The Rubik's cube of human gene therapy

Defective genes account for ∼80% of the total of more than 7,000 diseases known to date. Gene therapy brings the promise of a one-time treatment option that will fix the errors in patient genetic coding. Recombinant viruses are highly efficient vehicles for in vivo gene delivery. Adeno-associated virus (AAV) vectors offer unique advantages, such as tissue tropism, specificity in transduction, eliciting of a relatively low immune responses, no incorporation into the host chromosome, and long-lasting delivered gene expression, making them the most popular viral gene delivery system in clinical trials, with three AAV-based gene therapy drugs already approved by the US Food and Drug Administration (FDA) or European Medicines Agency (EMA). Despite the success of AAV vectors, their usage in particular scenarios is still limited due to remaining challenges, such as poor transduction efficiency in certain tissues, low organ specificity, pre-existing humoral immunity to AAV capsids, and vector dose-dependent toxicity in patients. In the present review, we address the different approaches to improve AAV vectors for gene therapy with a focus on AAV capsid selection and engineering, strategies to overcome anti-AAV immune response, and vector genome design, ending with a glimpse at vector production methods and the current state of recombinant AAV (rAAV) at the clinical level.

AAV-PHP.eB transduces both the inner and outer retina with high efficacy in mice

Recombinant adeno-associated virus (AAV) vectors are one of the main gene delivery vehicles used in retinal gene therapy approaches; however, there is a need to further improve the efficacy, tropism, and safety of these vectors. In this study, using a CMV-EGFP expression cassette, we characterize the retinal utility of AAV-PHP.eB, a serotype recently developed by in vivo directed evolution, which can cross the blood-brain barrier and target neurons with high efficacy in mice. Systemic and intravitreal delivery of AAV-PHP.eB resulted in the high transduction efficacy of retinal ganglion and horizontal cells, with systemic delivery providing pan-retinal coverage of the mouse retina. Subretinal delivery transduced photoreceptors and retinal pigment epithelium cells robustly. EGFP expression (number of transduced cells and mRNA levels) were similar when the retinas were transduced systemically or intravitreally with AAV-PHP.eB or intravitreally with AAV2/2. Notably, in photoreceptors, EGFP fluorescence intensities and mRNA levels were 50–70 times higher, when subretinal injections with AAV-PHP.eB were compared to AAV2/8. Our results demonstrate the pan-retinal transduction of ganglion cells and extremely efficient transduction of photoreceptor and retinal pigment epithelium cells as the most valuable features of AAV-PHP.eB in the mouse retina.

The carboxyl-terminal region of SDCCAG8 comprises a functional module essential for cilia formation as well as organ development and homeostasis

In humans, ciliary dysfunction causes ciliopathies, which present as multiple organ defects, including developmental and sensory abnormalities. Sdccag8 is a centrosomal/basal body protein essential for proper cilia formation. Gene mutations in SDCCAG8 have been found in patients with ciliopathies manifesting a broad spectrum of symptoms, including hypogonadism. Among these mutations, several that are predicted to truncate the SDCCAG8 carboxyl (C) terminus are also associated with such symptoms; however, the underlying mechanisms are poorly understood. In the present study, we identified the Sdccag8 C-terminal region (Sdccag8-C) as a module that interacts with the ciliopathy proteins, Ick/Cilk1 and Mak, which were shown to be essential for the regulation of ciliary protein trafficking and cilia length in mammals in our previous studies. We found that Sdccag8-C is essential for Sdccag8 localization to centrosomes and cilia formation in cultured cells. We then generated a mouse mutant in which Sdccag8-C was truncated (Sdccag8^ΔC/ΔC mice) using a CRISPR-mediated stop codon knock-in strategy. In Sdccag8^ΔC/ΔC mice, we observed abnormalities in cilia formation and ciliopathy-like organ phenotypes, including cleft palate, polydactyly, retinal degeneration, and cystic kidney, which partially overlapped with those previously observed in Ick- and Mak-deficient mice. Furthermore, Sdccag8^ΔC/ΔC mice exhibited a defect in spermatogenesis, which was a previously uncharacterized phenotype of Sdccag8 dysfunction. Together, these results shed light on the molecular and pathological mechanisms underlying ciliopathies observed in patients with SDCCAG8 mutations and may advance our understanding of protein–protein interaction networks involved in cilia development.

Broad locations of antigenic regions for anti-TRPM1 autoantibodies in paraneoplastic retinopathy with retinal ON bipolar cell dysfunction

PURPOSE

Cancer-associated retinal ON bipolar cell dysfunction (CARBD), which includes melanoma-associated retinopathy (MAR), has been reported to be caused by autoantibodies against the molecules expressed in ON bipolar cells, including TRPM1. The purpose of this study was to determine the antigenic regions of the autoantibodies against TRPM1 in the sera of CARBD patients, in whom we previously detected anti-TRPM1 autoantibodies.

METHODS

The antigenic regions against TRPM1 in the sera of eight CARBD patients were examined by Western blots using HEK293T cells transfected with the plasmids expressing FLAG-tagged TRPM1 fragments. The clinical course of these patients was also documented.

RESULTS

The clinical course differed among the patients. The electroretinograms (ERGs) and symptoms were improved in three patients, deteriorated in one patient, remained unchanged for a long time in one patient, and were not followable in three patients. Seven of the eight sera possessed multiple antigenic regions: two sera contained at least four antigen recognition regions, and three sera had at least three regions. The antigen regions were spread over the entire TRPM1 protein: five sera in the N-terminal intracellular domain, six sera in the transmembrane-containing region, and six sera in the C-terminal intracellular domain. No significant relationship was observed between the location of the antigen epitope and the patients' clinical course.

CONCLUSIONS

The antigenic regions of anti-TRPM1 autoantibodies in CARBD patients were present not only in the N-terminal intracellular domain, which was reported in an earlier report, but also in the transmembrane-containing region and in the C-terminal intracellular domain. In addition, the antigenic regions for TRPM1 were found to vary among the CARBD patients examined, and most of the sera had multiple antigenic regions.

In utero intraocular AAV injection for early gene expression in the developing rodent retina

The visual system is the best system to study activity-dependent sensory circuit development. The connections from the retina to the dorsal lateral geniculate nucleus, the retinogeniculate connections, undergo extensive remodeling during early postnatal life. Thus, techniques that allow the expression of transgenes early in the developing retina are essential to study visual system development. Here, we describe a protocol to express genes-of-interest in the developing mouse retina via in utero intraocular adeno-associated virus injections. For complete details on the use and execution of this protocol, please refer to Yasuda et al. (2021).

Engineered Capsids for Efficient Gene Delivery to the Retina and Cornea

Adeno-associated viral (AAV) vectors represent an ideal vehicle for human gene transfer. One advantage to the AAV vector system is the availability of multiple naturally occurring serotypes that provide selective tropisms for various target cells. Strategies to enhance the properties of the natural AAV isolates have been developed and can be divided into two approaches, rational design or directed evolution. The rational design approach utilizes knowledge of AAV capsids to make targeted changes to the capsid to alter transduction efficiency or specificity, while the directed evolution approach does not require a priori knowledge of capsid structure and includes random mutagenesis, capsid shuffling, or random peptide insertion. In this study, we describe the generation of novel variants for both AAV2 and AAV5 using a rational design approach and knowledge of AAV receptor binding, surface charge, and AAV capsid protein posttranslational modifications. The novel AAV2 and AAV5 variants demonstrate improved transduction properties in both the mouse retina and cornea. The translational fidelity of the novel AAV2 variant was confirmed in the context of the nonhuman primate (NHP) retina, whereas a NHP tissue explant model was established to allow the rapid assessment of translational fidelity between species for the AAV5 variants. The capsid-modified AAV2 and AAV5 variants described in this study have novel attributes that will add to the efficacy and specificity of their potential use in gene therapy for a range of human ocular diseases.

Overexpression of BMP4 protects retinal ganglion cells in a mouse model of experimental glaucoma

PURPOSE

Activation of bone morphogenetic protein (BMP) 4 signaling promotes the survival of retinal ganglion cell (RGC) after acute injury. Chordin-like 1 (CHRDL1) is an endogenous BMP antagonist. In this study, we researched whether CHRDL1 was involved in BMP4 signaling and regulation of RGC degeneration in a mouse model of glaucoma.

METHODS

Magnetic microbeads were intracameral injected to induce experimental glaucoma in a mouse model. A recombinant adeno-associated virus (rAAV) system was designed for overexpression of BMP4 or CHRDL1 in mouse retina. Immunohistochemistry and hematoxylin-eosin (HE) stains were performed to identify changes in retinal morphology. Electroretinogram (ERG) recordings were used to assess changes in visual function.

RESULTS

The mRNA expression levels of Bmp4 and its downstream BMPRIa, small mothers against decapentaplegic 1 (Smad1), were significantly upregulated in retinas with glaucoma. RGC survival was significantly enhanced in the beads + AAV-BMP4 group and significantly reduced in the beads + AAV-CHRDL1 group, compared with the beads + AAV-EGFP group. Similar results were observed in retinal explant culture in vitro. Consistent with these findings, the photopic negative response (PhNR)responses in ERG, which indicate RGC function, were restored in mice overexpressing BMP4, whereas a-wave and b-wave responses were not. Activation of CHRLD1 inhibited Smad1/5/8 phosphorylation and exacerbated RGC damage. The expression of Glial fibrillary acidic protein (GFAP) was decreased significantly in beads + AAV-BMP4 group.

CONCLUSIONS

BMP4 promoted RGC survival and visual function in an experimental glaucoma model. Activation of CHRDL1 exaggerated RGC degeneration by inhibiting the BMP4/Smad1/5/8 pathway. The mechanism of BMP4/Smad1/5/8 pathway may be related to the inhibition of glial cell activation. Our studies suggested that BMP4 and CHRLD1 might serve as therapeutic targets in glaucoma.

Delivering AAV to the Central Nervous and Sensory Systems

As gene therapy enters mainstream medicine, it is more important than ever to have a grasp of exactly how to leverage it for maximum benefit. The development of new targeting strategies and tools makes treating patients with genetic diseases possible. Many Mendelian disorders are amenable to gene replacement or correction. These often affect post-mitotic tissues, meaning that a single stably expressing therapy can be applied. Recent years have seen the development of a large number of novel viral vectors for delivering specific therapies. These new vectors - predominately recombinant adeno-associated virus (AAV) variants - target nervous tissues with differing efficiencies. This review gives an overview of current gene therapies in the brain, ear, and eye, and describes the optimal approaches, depending on cell type and transgene. Overall, this work aims to serve as a primer for gene therapy in the central nervous and sensory systems.

Gene Therapy of Dominant CRX-Leber Congenital Amaurosis using Patient Stem Cell-Derived Retinal Organoids

Mutations in the photoreceptor transcription factor gene cone-rod homeobox (CRX) lead to distinct retinopathy phenotypes, including early-onset vision impairment in dominant Leber congenital amaurosis (LCA). Using induced pluripotent stem cells (iPSCs) from a patient with CRX-I138fs48 mutation, we established an in vitro model of CRX-LCA in retinal organoids that showed defective photoreceptor maturation by histology and gene profiling, with diminished expression of visual opsins. Adeno-associated virus (AAV)-mediated CRX gene augmentation therapy partially restored photoreceptor phenotype and expression of phototransduction-related genes as determined by single-cell RNA-sequencing. Retinal organoids derived from iPSCs of a second dominant CRX-LCA patient carrying K88N mutation revealed the loss of opsin expression as a common phenotype, which was alleviated by AAV-mediated augmentation of CRX. Our studies provide a proof-of-concept for developing gene therapy of dominant CRX-LCA and other CRX retinopathies.

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Leber congenital amaurosis caused by CRX mutations is modeled in retinal organoids

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Patient iPSCs-derived organoids show impaired expression of visual opsins

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AAV-mediated CRX delivery partially restores expression of phototransduction genes

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Gene therapy is applicable to mutations in DNA-binding and transactivation domains

Intracranial delivery of AAV9 gene therapy partially prevents retinal degeneration and visual deficits in CLN6-Batten disease mice

Batten disease is a family of rare, fatal, neuropediatric diseases presenting with memory/learning decline, blindness, and loss of motor function. Recently, we reported the use of an AAV9-mediated gene therapy that prevents disease progression in a mouse model of CLN6-Batten disease (Cln6 ^nclf ), restoring lifespans in treated animals. Despite the success of our viral-mediated gene therapy, the dosing strategy was optimized for delivery to the brain parenchyma and may limit the therapeutic potential to other disease-relevant tissues, such as the eye. Here, we examine whether cerebrospinal fluid (CSF) delivery of scAAV9.CB.CLN6 is sufficient to ameliorate visual deficits in Cln6 ^nclf mice. We show that intracerebroventricular (i.c.v.) delivery of scAAV9.CB.CLN6 completely prevents hallmark Batten disease pathology in the visual processing centers of the brain, preserving neurons of the superior colliculus, thalamus, and cerebral cortex. Importantly, i.c.v.-delivered scAAV9.CB.CLN6 also expresses in many cells throughout the central retina, preserving many photoreceptors typically lost in Cln6 ^nclf mice. Lastly, scAAV9.CB.CLN6 treatment partially preserved visual acuity in Cln6 ^nclf mice as measured by optokinetic response. Taken together, we report the first instance of CSF-delivered viral gene reaching and rescuing pathology in both the brain parenchyma and retinal neurons, thereby partially slowing visual deterioration.

Ocular delivery of CRISPR/Cas genome editing components for treatment of eye diseases

A variety of inherited or multifactorial ocular diseases call for novel treatment paradigms. The newly developed genome editing technology, CRISPR, has shown great promise in treating these diseases, but delivery of the CRISPR/Cas components to target ocular tissues and cells requires appropriate use of vectors and routes of administration to ensure safety, efficacy and specificity. Although adeno-associated viral (AAV) vectors are thus far the most commonly used tool for ocular gene delivery, sustained expression of CRISPR/Cas components may cause immune reactions and an increased risk of off-target editing. In this review, we summarize the ocular administration routes and discuss the advantages and disadvantages of viral and non-viral vectors for delivery of CRISPR/Cas components to the eye. We review the existing studies of CRISPR/Cas genome editing for ocular diseases and discuss the major challenges of the technology in ocular applications. We also discuss the most recently developed CRISPR tools such as base editing and prime editing which may be used for future ocular applications.

Retinal Tropism and Transduction of Adeno-Associated Virus Varies by Serotype and Route of Delivery (Intravitreal, Subretinal, or Suprachoroidal) in Rats

Viral-mediated gene augmentation offers tremendous promise for the treatment of inherited retinal diseases. The development of effective gene therapy requires an understanding of the vector's tissue-specific behavior, which may vary depending on serotype, route of delivery, or target species. Using an ex vivo organotypic explant system, we previously demonstrated that retinal tropism and transduction of adeno-associated virus type 2 (AAV2) vary significantly depending on serotype in human eyes. However, the ex vivo system has limited ability to assess route of ocular delivery, and relatively little literature exists on tropic differences between serotypes and routes of delivery in vivo. In this study, we demonstrate that retinal tropism and transduction efficiency of five different AAV2 serotypes (AAV2/1, AAV2/2, AAV2/6, AAV2/8, and AAV2/9) expressing enhanced green fluorescent protein driven by a cytomegalovirus promoter vary greatly depending on serotype and route of delivery (intravitreal, subretinal, or suprachoroidal) in rats. With subretinal delivery, all serotypes successfully transduced the retinal pigmented epithelium and outer nuclear layer (ONL), with AAV2/1 displaying the highest transduction efficiency and AAV2/2 and AAV2/6 showing lower ONL transduction. There was minimal transduction of the inner retina through subretinal delivery for any serotype. Tropism by suprachoroidal delivery mirrored that of subretinal delivery for all AAV serotypes but resulted in a wider distribution and greater ONL transduction. With intravitreal delivery, retinal transduction was seen primarily in the inner retina (retinal nerve fiber, ganglion cell, and inner nuclear layers) for AAV2/1 and AAV2/6, with AAV2/6 showing the highest transduction. When compared with data from human explant models, there are substantial differences in tropism and transduction that are important to consider when using rats as preclinical models for the development of ocular gene therapies for humans.

Recombinant Adeno-Associated Viral Vectors (rAAV)-Vector Elements in Ocular Gene Therapy Clinical Trials and Transgene Expression and Bioactivity Assays

Inherited retinal dystrophies and optic neuropathies cause chronic disabling loss of visual function. The development of recombinant adeno-associated viral vectors (rAAV) gene therapies in all disease fields have been promising, but the translation to the clinic has been slow. The safety and efficacy profiles of rAAV are linked to the dose of applied vectors. DNA changes in the rAAV gene cassette affect potency, the expression pattern (cell-specificity), and the production yield. Here, we present a library of rAAV vectors and elements that provide a workflow to design novel vectors. We first performed a meta-analysis on recombinant rAAV elements in clinical trials (2007-2020) for ocular gene therapies. We analyzed 33 unique rAAV gene cassettes used in 57 ocular clinical trials. The rAAV gene therapy vectors used six unique capsid variants, 16 different promoters, and six unique polyadenylation sequences. Further, we compiled a list of promoters, enhancers, and other sequences used in current rAAV gene cassettes in preclinical studies. Then, we give an update on pro-viral plasmid backbones used to produce the gene therapy vectors, inverted terminal repeats, production yield, and rAAV safety considerations. Finally, we assess rAAV transgene and bioactivity assays applied to cells or organoids in vitro, explants ex vivo, and clinical studies.

Rescue of Defective Electroretinographic Responses in Dp71-Null Mice With AAV-Mediated Reexpression of Dp71

Purpose

To study the potential effect of a gene therapy, designed to rescue the expression of dystrophin Dp71 in the retinas of Dp71-null mice, on retinal physiology.

Methods

We recorded electroretinograms (ERGs) in Dp71-null and wild-type littermate mice. In dark-adapted eyes, responses to flashes of several strengths were measured. In addition, flash responses on a 25-candela/square meters background were measured. On- and Off-mediated responses to sawtooth stimuli and responses to photopic sine-wave modulation (3–30 Hz) were also recorded. After establishing the ERG phenotype, the ShH10-GFP adeno-associated virus (AAV), which has been previously shown to target specifically Müller glial cells (MGCs), was delivered intravitreously with or without (sham therapy) the Dp71 coding sequence under control of a CBA promoter. ERG recordings were repeated three months after treatment. Real-time quantitative PCR and Western blotting analyses were performed in order to quantify Dp71 expression in the retinas.

Results

Dp71-null mice displayed reduced b-waves in dark- and light-adapted flash ERGs and smaller response amplitudes to photopic rapid-on sawtooth modulation and to sine-wave stimuli. Three months after intravitreal injections of the ShH10-GFP-2A-Dp71 AAV vector, ERG responses were completely recovered in treated eyes of Dp71-null mice. The functional rescue was associated with an overexpression of Dp71 in treated retinas.

Conclusions

The present results show successful functional recovery accompanying the reexpression of Dp71. In addition, this experimental model sheds light on MGCs influencing ERG components, since previous reports showed that aquaporin 4 and Kir4.1 channels were mislocated in MGCs of Dp71-null mice, while their distribution could be normalized following intravitreal delivery of the same ShH10-GFP-2A-Dp71 vector.

AAV-Mediated Gene Delivery to 3D Retinal Organoids Derived from Human Induced Pluripotent Stem Cells

Human induced pluripotent stem cells (hiPSCs) promise a great number of future applications to investigate retinal development, pathophysiology and cell therapies for retinal degenerative diseases. Specific approaches to genetically modulate hiPSC would be valuable for all of these applications. Vectors based on adeno-associated virus (AAV) have shown the ability for gene delivery to retinal organoids derived from hiPSCs. Thus far, little work has been carried out to investigate mechanisms of AAV-mediated gene delivery and the potential advantages of engineered AAVs to genetically modify retinal organoids. In this study, we compared the early transduction efficiency of several recombinant and engineered AAVs in hiPSC-derived RPE cells and retinal organoids in relation to the availability of their cell-surface receptors and as a function of time. The genetic variant AAV2-7m8 had a superior transduction efficiency when applied at day 44 of differentiation on retinal organoids and provided long-lasting expressions for at least 4 weeks after infection without compromising cell viability. All of the capsids we tested transduced the hiPSC-RPE cells, with the AAV2-7m8 variant being the most efficient. Transduction efficiency was correlated with the presence of primary cell-surface receptors on the hiPS-derived organoids. Our study explores some of the mechanisms of cell attachment of AAVs and reports long-term gene expression resulting from gene delivery in retinal organoids.

miR-124 dosage regulates prefrontal cortex function by dopaminergic modulation

MicroRNA-124 (miR-124) is evolutionarily highly conserved among species and one of the most abundantly expressed miRNAs in the developing and mature central nervous system (CNS). Previous studies reported that miR-124 plays a role in CNS development, such as neuronal differentiation, maturation, and survival. However, the role of miR-124 in normal brain function has not yet been revealed. Here, we subjected miR-124-1^+/− mice, to a comprehensive behavioral battery. We found that miR-124-1^+/− mice showed impaired prepulse inhibition (PPI), methamphetamine-induced hyperactivity, and social deficits. Whole cell recordings using prefrontal cortex (PFC) slices showed enhanced synaptic transmission in layer 5 pyramidal cells in the miR-124-1^+/− PFC. Based on the results of behavioral and electrophysiological analysis, we focused on genes involved in the dopaminergic system and identified a significant increase of Drd2 expression level in the miR-124-1^+/− PFC. Overexpression or knockdown of Drd2 in the control or miR-124-1^+/− PFC demonstrates that aberrant Drd2 signaling leads to impaired PPI. Furthermore, we identified that expression of glucocorticoid receptor gene Nr3c1, which enhances Drd2 expression, increased in the miR-124-1^+/− PFC. Taken together, the current study suggests that miR-124 dosage modulates PFC function through repressing the Drd2 pathway, suggesting a critical role of miR-124 in normal PFC function.

AAV cis-regulatory sequences are correlated with ocular toxicity

Adeno-associated viral (AAV) gene therapy is becoming an important therapeutic modality, especially for ocular diseases, due to its efficiency of gene delivery and relative lack of pathogenicity. However, AAV sometimes can cause inflammation and toxicity. We explored such effects using injections into the mouse eye. We found a strong correlation of toxicity and inflammation with the use of promoters that were broadly active, or specifically active in the retinal pigment epithelium. AAVs with photoreceptor-specific promoters were found to be nontoxic at all doses tested. These studies reveal that safer vectors can be designed if assays for relevant and specific cell types are developed and tested with a range of vectors with different genomic elements.

Adeno-associated viral vectors (AAVs) have become popular for gene therapy, given their many advantages, including their reduced inflammatory profile compared with that of other viruses. However, even in areas of immune privilege such as the eye, AAV vectors are capable of eliciting host-cell responses. To investigate the effects of such responses on several ocular cell types, we tested multiple AAV genome structures and capsid types using subretinal injections in mice. Assays of morphology, inflammation, and physiology were performed. Pathological effects on photoreceptors and the retinal pigment epithelium (RPE) were observed. Müller glia and microglia were activated, and the proinflammatory cytokines TNF-α and IL-1β were up-regulated. There was a strong correlation between cis-regulatory sequences and toxicity. AAVs with any one of three broadly active promoters, or an RPE-specific promoter, were toxic, while AAVs with four different photoreceptor-specific promoters were not toxic at the highest doses tested. There was little correlation between toxicity and transgene, capsid type, preparation method, or cellular contaminants within a preparation. The toxic effect was dose-dependent, with the RPE being more sensitive than photoreceptors. Our results suggest that ocular AAV toxicity is associated with certain AAV cis-regulatory sequences and/or their activity and that retinal damage occurs due to responses by the RPE and/or microglia. By applying multiple, sensitive assays of toxicity, AAV vectors can be designed so that they can be used safely at high dose, potentially providing greater therapeutic efficacy.

Adeno-Associated Viral Vector 2 and 9 Transduction Is Enhanced in Streptozotocin-Induced Diabetic Mouse Retina

Adeno-associated viruses (AAVs) are currently the most popular vector platform technology for ocular gene therapy. While transduction efficiency and tropism of intravitreally administered AAV has been fairly well established in various retinal conditions, its transduction pattern in diabetic retinas has not previously been characterized. Here, we describe the transduction efficiencies of four different AAV serotypes, AAV2, 5, 8, and 9, in streptozotocin (STZ)-induced diabetic mouse retinas after intravitreal injections, which differed according to the duration of diabetic induction. STZ was intraperitoneally injected into C57/B6 diabetic mice subjected to unilateral intravitreal injection of AAV2, AAV5, AAV8, and AAV9 packaged with EGFP. Significantly enhanced AAV2 and AAV9 transduction was observed in 2-month-old diabetic mouse retinas compared to the 2-week-old diabetic mouse retinas and nondiabetic, vector uninjected or injected retinas. Intravitreal injection of AAV5 or AAV8 serotype in 2-month- and 2-week-old diabetic mouse retinas did not show any significant vector transduction enhancement compared to the nondiabetic control retinas. The tropism of AAV2 and AAV9 in diabetic mouse retinas differed. AAV2 was transduced into various retinal cells, including Müller cells, microglia, retinal ganglion cells (RGCs), bipolar cells, horizontal cells, and amacrine cells, whereas AAV9 was effectively transduced only into RGC and horizontal cells. The expression levels of receptors and co-receptors for AAV2 and AAV9 were significantly increased in 2-month-old diabetic mouse retinas. The results of our study demonstrated that AAV2 and AAV9 may be the vector of choice in treating diabetic retinopathy (DR) with gene therapy, and DR-related retinal changes may improve AAV vector transduction efficiency.

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

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

Rod Outer Segment Development Influences AAV-Mediated Photoreceptor Transduction After Subretinal Injection

Vectors based on the adeno-associated virus (AAV) are currently the preferred tools for delivering genes to photoreceptors (PR) in small and large animals. AAVs have been applied successfully in various models of PR dystrophies. However, unknown barriers still limit AAV's efficient application in several forms of severe PR degenerations due to insufficient transgene expression and/or treated cells at the time of injection. Optimizations of PR gene therapy strategies will likely benefit from the identification of the cellular factors that influence PR transduction. Interestingly, recent studies have shown that the AAV transduction profile of PRs differs significantly between neonatal and adult mouse retinas after subretinal injection. This phenomenon may provide clues to identify host factors that influence the efficiency of AAV-mediated PR transduction. This study demonstrates that rod outer segments are critical modulators of efficient AAV-mediated rod transduction. During retinal development, rod transduction correlated temporally and spatially with the differentiation order of PRs when vectors were introduced subretinally but not when introduced intravitreally. All subretinally injected vectors had an initial preference to transduce cones in the absence of formed rod outer segments and then displayed a preference for rods as the cells matured, independently of the expression cassette or AAV serotype. Consistent with this observation, altered development of rod outer segments was associated with a strong reduction of rod transduction and an increase in the percentage of transduced cones by 2- to 2.8-fold. A similar increase of cone transduction was observed in the adult retinal degeneration 1 (rd1) retina compared to wild-type mice. These results suggest that the loss of rod outer segments in diseased retinas could markedly affect gene transfer efficiency of AAV vectors by limiting the ability of AAVs to infect dying rods efficiently. This information could be exploited for the development of more efficient AAV-based PR gene delivery procedures.

Assessment of Adeno-Associated Virus Serotype Tropism in Human Retinal Explants

Advances in the discovery of the causes of monogenic retinal disorders, combined with technologies for the delivery of DNA to the retina, offer enormous opportunities for the treatment of previously untreatable blinding diseases. However, for gene augmentation to be most effective, vectors that have the correct cell-type specificity are needed. While animal models are very useful, they often exhibit differences in retinal cell surface receptors compared to the human retina. This study evaluated the use of an ex vivo organotypic explant system to test the transduction efficiency and tropism of seven different adeno-associated virus type 2 (AAV2) serotypes in the human retina and retinal pigment epithelium-choroid-AAV2/1, AAV2/2, AAV2/4, AAV2/5, AAV2/6, AAV2/8, and AAV2/9-all driving expression of GFP under control of the cytomegalovirus promoter. After 7 days in culture, it was found that AAV2/4 and AAV2/5 were particularly efficient at transducing photoreceptor cells and that AAV2/5 was highly specific to the outer nuclear layer, whereas AAV2/8 displayed consistently low transduction of photoreceptors. To validate the authenticity of the organotypic culture system, the transduction of the same set of AAVs was also compared in a pig model, in which sub-retinal injections in vivo were compared to cultured and transduced organotypic cultures ex vivo. This study shows how different AAV serotypes behave in the human retina and provides insight for further investigation of each of these serotypes for gene augmentation-based treatment of inherited retinal degeneration.

Tropism of engineered and evolved recombinant AAV serotypes in the rd1 mouse and ex vivo primate retina

There is much debate on the adeno-associated virus (AAV) serotype that best targets specific retinal cell types and the route of surgical delivery-intravitreal or subretinal. This study compared three of the most efficacious AAV vectors known to date in a mouse model of retinal degeneration (rd1 mouse) and macaque and human retinal explants. Green fluorescent protein (GFP) driven by a ubiquitous promoter was packaged into three AAV capsids: AAV2/8(Y733F), AAV2/2(quad Y-F) and AAV2/2(7m8). Overall, AAV2/2(7m8) transduced the largest area of retina and resulted in the highest level of GFP expression, followed by AAV2/2(quad Y-F) and AAV2/8(Y733F). AAV2/2(7m8) and AAV2/2(quad Y-F) both resulted in similar patterns of transduction whether they were injected intravitreally or subretinally. AAV2/8(Y733F) transduced a significantly smaller area of retina when injected intravitreally compared with subretinally. Retinal ganglion cells, horizontal cells and retinal pigment epithelium expressed relatively high levels of GFP in the mouse retina, whereas amacrine cells expressed low levels of GFP and bipolar cells were infrequently transduced. Cone cells were the most frequently transduced cell type in macaque retina explants, whereas Müller cells were the predominant transduced cell type in human retinal explants. Of the AAV serotypes tested, AAV2/2(7m8) was the most effective at transducing a range of cell types in degenerate mouse retina and macaque and human retinal explants.

Using Patient-Specific Induced Pluripotent Stem Cells and Wild-Type Mice to Develop a Gene Augmentation-Based Strategy to Treat CLN3-Associated Retinal Degeneration

Juvenile neuronal ceroid lipofuscinosis (JNCL) is a childhood neurodegenerative disease with early-onset, severe central vision loss. Affected children develop seizures and CNS degeneration accompanied by severe motor and cognitive deficits. There is no cure for JNCL, and patients usually die during the second or third decade of life. In this study, independent lines of induced pluripotent stem cells (iPSCs) were generated from two patients with molecularly confirmed mutations in CLN3, the gene mutated in JNCL. Clinical-grade adeno-associated adenovirus serotype 2 (AAV2) carrying the full-length coding sequence of human CLN3 was generated in a U.S. Food and Drug Administration-registered cGMP facility. AAV2-CLN3 was efficacious in restoring full-length CLN3 transcript and protein in patient-specific fibroblasts and iPSC-derived retinal neurons. When injected into the subretinal space of wild-type mice, purified AAV2-CLN3 did not show any evidence of retinal toxicity. This study provides proof-of-principle for initiation of a clinical trial using AAV-mediated gene augmentation for the treatment of children with CLN3-associated retinal degeneration.

Multilineage transduction of resident lung cells in vivo by AAV2/8 for α1-antitrypsin gene therapy

In vivo gene delivery has long represented an appealing potential treatment approach for monogenic diseases such as α1-antitrypsin deficiency (AATD) but has proven challenging to achieve in practice. Alternate pseudotyping of recombinant adeno-associated virus (AAV) vectors is producing vectors with increasingly heterogeneous tropic specificity, giving researchers the ability to target numerous end-organs affected by disease. Herein, we describe sustained pulmonary transgene expression for at least 52 weeks after a single intratracheal instillation of AAV2/8 and characterize the multiple cell types transduced within the lung utilizing this approach. We demonstrate that lung-directed AAV2/8 is able to achieve therapeutic α-1 antitrypsin (AAT) protein levels within the lung epithelial lining fluid and that AAT gene delivery ameliorates the severity of experimental emphysema in mice. We find that AAV2/8 efficiently transduces hepatocytes in vivo after intratracheal administration, a finding that may have significance for AAV-based human gene therapy studies. These results support direct transgene delivery to the lung as a potential alternative approach to achieve the goal of developing a gene therapy for AATD.

In vitro and in vivo rescue of aberrant splicing in CEP290-associated LCA by antisense oligonucleotide delivery

Leber congenital amaurosis (LCA) is a severe disorder resulting in visual impairment usually starting in the first year of life. The most frequent genetic cause of LCA is an intronic mutation in CEP290 (c.2991 + 1655A > G) that creates a cryptic splice donor site resulting in the insertion of a pseudoexon (exon X) into CEP290 mRNA. Previously, we showed that naked antisense oligonucleotides (AONs) effectively restored normal CEP290 splicing in patient-derived lymphoblastoid cells. We here explore the therapeutic potential of naked and adeno-associated virus (AAV)-packaged AONs in vitro and in vivo In both cases, AON delivery fully restored CEP290 pre-mRNA splicing, significantly increased CEP290 protein levels and rescued a ciliary phenotype present in patient-derived fibroblast cells. Moreover, administration of naked and AAV-packaged AONs to the retina of a humanized mutant Cep290 mouse model, carrying the intronic mutation, showed a statistically significant reduction of exon X-containing Cep290 transcripts, without compromising the retinal structure. Together, our data highlight the tremendous therapeutic prospective of AONs for the treatment of not only CEP290-associated LCA but potentially many other subtypes of retinal dystrophy caused by splicing mutations.

Distinct Expression Patterns of AAV8 Vectors with Broadly Active Promoters from Subretinal Injections of Neonatal Mouse Eyes at Two Different Ages

The retinal expression patterns were analyzed following the injection of serotype 8 adeno-associated virus (AAV8) vectors that utilize two broadly active and commonly used sets of transcription regulatory sequences. These include the human cytomegalovirus (CMV) immediate early (IE) enhancer/promoter and the hybrid CAG element (also known as CAGGS or CBA) composed of a partial human CMV IE enhancer and the chicken β-actin promoter and intron. Subretinal delivery to postnatal day 0 (P0) or 6 (P6) mouse eyes resulted in efficient labeling of retinal cells, but with very distinct patterns. With P0 delivery, AAV8-CMV-GFP selectively labelled photoreceptors, while AAV8-CAG-GFP efficiently labeled both outer and inner retinal neurons, including photoreceptors, horizontal cells, amacrine cells and retinal ganglion cells. With P6 delivery, both vectors led to efficient labeling of photoreceptors and Müller glia cells, but not of inner retinal neurons. Our results suggest that the cell types that express the genes encoded by subretinally delivered AAV8 vectors are determined by both the timing of the injection and the regulatory sequences.

In vivo cellular imaging of various stress/response pathways using AAV following axonal injury in mice

Glaucoma, a leading cause of blindness worldwide, is instigated by various factors, including axonal injury, which eventually leads to a progressive loss of retinal ganglion cells (RGCs). To study various pathways reportedly involved in the pathogenesis of RGC death caused by axonal injury, seven pathways were investigated. Pathway-specific fluorescent protein-coded reporters were each packaged into an adeno-associated virus (AAV). After producing axonal injury in the eye, injected with AAV to induce RGC death, the temporal activity of each stress-related pathway was monitored in vivo through the detection of fluorescent RGCs using confocal ophthalmoscopy. We identified the activation of ATF6 and MCP-1 pathways involved in endoplasmic reticulum stress and macrophage recruitment, respectively, as early markers of RGC stress that precede neuronal death. Conversely, inflammatory responses probed by NF-κB and cell-death-related pathway p53 were most prominent in the later phases, when RGC death was already ongoing. AAV-mediated delivery of stress/response reporters followed by in vivo cellular imaging is a powerful strategy to characterize the temporal aspects of complex molecular pathways involved in retinal diseases. The identification of promoter elements that are activated before the death of RGCs enables the development of pre-emptive gene therapy, exclusively targeting the early phases of diseased cells.

A role of Heat Shock Protein 70 in Photoreceptor Cell Death: Potential as a Novel Therapeutic Target in Retinal Degeneration

Retinal degenerative diseases (RDs) such as retinitis pigmentosa (RP) are a genetically heterogeneous group of disorders characterized by night blindness and peripheral vision loss, which caused by the dysfunction and death of photoreceptor cells. Although many causative gene mutations have been reported, the final common end stage is photoreceptor cell death. Unfortunately, no effective treatments or therapeutic agents have been discovered. Heat shock protein 70 (HSP70) is highly conserved and has antiapoptotic activities. A few reports have shown that HSP70 plays a role in RDs. Thus, we focused on the role of HSP70 in photoreceptor cell death. Using the N-methyl-N-nitrosourea (MNU)-induced photoreceptor cell death model in mice, we could examine two stages of the novel cell death mechanism; the early stage, including HSP70 cleavage through protein carbonylation by production of reactive oxygen species, lipid peroxidation and Ca(2+) influx/calpain activation, and the late stage of cathepsin and/or caspase activation. The upregulation of intact HSP70 expression by its inducer is likely to protect photoreceptor cells. In this review, we focus on the role of HSP70 and the novel cell death signaling process in RDs. We also describe candidate therapeutic agents for RDs.

Efficient gene delivery to photoreceptors using AAV2/rh10 and rescue of the Rho(-/-) mouse

As gene therapies for various forms of retinal degeneration progress toward human clinical trial, it will be essential to have a repertoire of safe and efficient vectors for gene delivery to the target cells. Recombinant adeno-associated virus (AAV) serotype 2/2 has been shown to be well tolerated in the human retina and has provided efficacy in human patients for some inherited retinal degenerations. In this study, the AAV2/8 and AAV2/rh10 serotypes have been compared as a means of gene delivery to mammalian photoreceptor cells using a photoreceptor specific promoter for transgene expression. Both AAV2/8 and AAV2/rh10 provided rescue of the retinal degeneration present in the rhodopsin knockout mouse, with similar levels of benefit as evaluated by molecular, histological, and functional readouts. Transgene expression levels were significantly higher (fivefold) 1 week postsubretinal injection when employing AAV2/8 for rhodopsin gene delivery compared to AAV2/rh10, and were indistinguishable by 6 weeks postadministration of vector. This study reports the use of the AAV2/rh10 serotype to provide rescue in a degenerating retina and provides a comparative evaluation of AAV2/rh10 with respect to AAV2/8, a serotype regarded as providing efficient delivery to photoreceptors.

NRF2 promotes neuronal survival in neurodegeneration and acute nerve damage

Oxidative stress contributes to the loss of neurons in many disease conditions as well as during normal aging; however, small-molecule agents that reduce oxidation have not been successful in preventing neurodegeneration. Moreover, even if an efficacious systemic reduction of reactive oxygen and/or nitrogen species (ROS/NOS) could be achieved, detrimental side effects are likely, as these molecules regulate normal physiological processes. A more effective and targeted approach might be to augment the endogenous antioxidant defense mechanism only in the cells that suffer from oxidation. Here, we created several adeno-associated virus (AAV) vectors to deliver genes that combat oxidation. These vectors encode the transcription factors NRF2 and/or PGC1a, which regulate hundreds of genes that combat oxidation and other forms of stress, or enzymes such as superoxide dismutase 2 (SOD2) and catalase, which directly detoxify ROS. We tested the effectiveness of this approach in 3 models of photoreceptor degeneration and in a nerve crush model. AAV-mediated delivery of NRF2 was more effective than SOD2 and catalase, while expression of PGC1a accelerated photoreceptor death. Since the NRF2-mediated neuroprotective effects extended to photoreceptors and retinal ganglion cells, which are 2 very different types of neurons, these results suggest that this targeted approach may be broadly applicable to many diseases in which cells suffer from oxidative damage.

Mechanisms of blindness: animal models provide insight into distinct CRX-associated retinopathies

BACKGROUND

The homeodomain transcription factor CRX is a crucial regulator of mammalian photoreceptor gene expression. Mutations in the human CRX gene are associated with dominant inherited retinopathies Retinitis Pigmentosa (RP), Cone-Rod Dystrophy (CoRD), and Leber Congenital Amaurosis (LCA), of varying severity. In vitro and in vivo assessment of mutant CRX proteins have revealed pathogenic mechanisms for several mutations, but no comprehensive mutation-disease correlation has yet been reported.

RESULTS

Here we describe four different classes of disease-causing CRX mutations, characterized by mutation type, pathogenetic mechanism, and the molecular activity of the mutant protein: (1) hypomorphic missense mutations with reduced DNA binding, (2) antimorphic missense mutations with variable DNA binding, (3) antimorphic frameshift/nonsense mutations with intact DNA binding, and (4) antimorphic frameshift mutations with reduced DNA binding. Mammalian models representing three of these classes have been characterized.

CONCLUSIONS

Models carrying Class I mutations display a mild dominant retinal phenotype and recessive LCA, while models carrying Class III and IV mutations display characteristically distinct dominant LCA phenotypes. These animal models also reveal unexpected pathogenic mechanisms underlying CRX-associated retinopathies. The complexity of genotype-phenotype correlation for CRX-associated diseases highlights the value of developing comprehensive "true-to-disease" animal models for understanding pathologic mechanisms and testing novel therapeutic approaches.

The retinal phenotype of Usher syndrome: pathophysiological insights from animal models

The Usher syndrome (USH) is the most prevalent cause of inherited deaf-blindness. Three clinical subtypes, USH1-3, have been defined, and ten USH genes identified. The hearing impairment due to USH gene defects has been shown to result from improper organisation of the hair bundle, the sound receptive structure of sensory hair cells. In contrast, the cellular basis of the visual defect is less well understood as this phenotype is absent in almost all the USH mouse models that faithfully mimic the human hearing impairment. Structural and molecular interspecies discrepancies regarding photoreceptor calyceal processes and the association with the distribution of USH1 proteins have recently been unravelled, and have led to the conclusion that a defect in the USH1 protein complex-mediated connection between the photoreceptor outer segment and the surrounding calyceal processes (in both rods and cones), and the inner segment (in rods only), probably causes the USH1 retinal dystrophy in humans.

Dre - Cre sequential recombination provides new tools for retinal ganglion cell labeling and manipulation in mice

BACKGROUND

Genetic targeting methods have greatly advanced our understanding of many of the 20 Retinal Ganglion Cell (RGC) types conveying visual information from the eyes to the brain. However, the complexity and partial overlap of gene expression patterns in RGCs call for genetic intersectional or sparse labeling strategies. Loci carrying the Cre recombinase in conjunction with conditional knock-out, reporter or other genetic tools can be used for targeted cell type ablation and functional manipulation of specific cell populations. The three members of the Pou4f family of transcription factors, Brn3a, Brn3b and Brn3c, expressed early during RGC development and in combinatorial pattern amongst RGC types are excellent candidates for such gene manipulations.

METHODS AND FINDINGS

We generated conditional Cre knock-in alleles at the Brn3a and Brn3b loci, Brn3a(CKOCre) and Brn3b(CKOCre). When crossed to mice expressing the Dre recombinase, the endogenous Brn3 gene expressed by Brn3a(CKOCre) or Brn3b(CKOCre) is removed and replaced with a Cre recombinase, generating Brn3a(Cre) and Brn3b(Cre) knock-in alleles. Surprisingly both Brn3a(Cre) and Brn3b(Cre) knock-in alleles induce early ubiquitous recombination, consistent with germline expression. However in later stages of development, their expression is limited to the expected endogenous pattern of the Brn3a and Brn3b genes. We use the Brn3a(Cre) and Brn3b(Cre) alleles to target a Cre dependent Adeno Associated Virus (AAV) reporter to RGCs and demonstrate its use in morphological characterization, early postnatal gene delivery and tracing the expression of Brn3 genes in RGCs.

CONCLUSIONS

Dre recombinase effectively recombines the Brn3a(CKOCre) and Brn3b(CKOCre) alleles containing its roxP target sites. Sequential Dre to Cre recombination reveals Brn3a and Brn3b expression in early mouse development. The generated Brn3a(Cre) and Brn3b(Cre) alleles are useful tools that can target exogenously delivered Cre dependent reagents to RGCs in early postnatal development, opening up a large range of potential applications.

A nanobody-based system using fluorescent proteins as scaffolds for cell-specific gene manipulation

Fluorescent proteins are commonly used to label cells across organisms, but the unmodified forms cannot control biological activities. Using GFP-binding proteins derived from Camelid antibodies, we co-opted GFP as a scaffold for inducing formation of biologically active complexes, developing a library of hybrid transcription factors that control gene expression only in the presence of GFP or its derivatives. The modular design allows for variation in key properties such as DNA specificity, transcriptional potency, and drug dependency. Production of GFP controlled cell-specific gene expression and facilitated functional perturbations in the mouse retina and brain. Further, retrofitting existing transgenic GFP mouse and zebrafish lines for GFP-dependent transcription enabled applications such as optogenetic probing of neural circuits. This work establishes GFP as a multifunctional scaffold and opens the door to selective manipulation of diverse GFP-labeled cells across transgenic lines. This approach may also be extended to exploit other intracellular products as cell-specific scaffolds in multicellular organisms.