Multiplexed CRISPR/Cas9 Targeting of Genes Implicated in Retinal Regeneration and Degeneration

Unleashing the potential of CRISPR multiplexing: Harnessing Cas12 and Cas13 for precise gene modulation in eye diseases

Gene therapy is a flourishing field with the potential to revolutionize the treatment of genetic diseases. The emergence of CRISPR-Cas9 has significantly advanced targeted and efficient genome editing. Although CRISPR-Cas9 has demonstrated promising potential applications in various genetic disorders, it faces limitations in simultaneously targeting multiple genes. Novel CRISPR systems, such as Cas12 and Cas13, have been developed to overcome these challenges, enabling multiplexing and providing unique advantages. Cas13, in particular, targets mRNA instead of genomic DNA, permitting precise gene expression control and mitigating off-target effects. This review investigates the potential of Cas12 and Cas13 in ocular gene therapy applications, such as suppression of inflammation and cell death. In addition, the capabilities of Cas12 and Cas13 are explored in addressing potential targets related with disease mechanisms such as aberrant isoforms, mitochondrial genes, cis-regulatory sequences, modifier genes, and long non-coding RNAs. Anatomical accessibility and relative immune privilege of the eye provide an ideal organ system for evaluating these novel techniques' efficacy and safety. By targeting multiple genes concurrently, CRISPR-Cas12 and Cas13 systems hold promise for treating a range of ocular disorders, including glaucoma, retinal dystrophies, and age-related macular degeneration. Nonetheless, additional refinement is required to ascertain the safety and efficacy of these approaches in ocular disease treatments. Thus, the development of Cas12 and Cas13 systems marks a significant advancement in gene therapy, offering the potential to devise effective treatments for ocular disorders.

A CRISPR-Cas9-mediated F0 screen to identify pro-regenerative genes in the zebrafish retinal pigment epithelium

Ocular diseases resulting in death of the retinal pigment epithelium (RPE) lead to vision loss and blindness. There are currently no FDA-approved strategies to restore damaged RPE cells. Stimulating intrinsic regenerative responses within damaged tissues has gained traction as a possible mechanism for tissue repair. Zebrafish possess remarkable regenerative abilities, including within the RPE; however, our understanding of the underlying mechanisms remains limited. Here, we conducted an F0 in vivo CRISPR-Cas9-mediated screen of 27 candidate RPE regeneration genes. The screen involved injection of a ribonucleoprotein complex containing three highly mutagenic guide RNAs per target gene followed by PCR-based genotyping to identify large intragenic deletions and MATLAB-based automated quantification of RPE regeneration. Through this F0 screening pipeline, eight positive and seven negative regulators of RPE regeneration were identified. Further characterization of one candidate, cldn7b, revealed novel roles in regulating macrophage/microglia infiltration after RPE injury and in clearing RPE/pigment debris during late-phase RPE regeneration. Taken together, these data support the utility of targeted F0 screens for validating pro-regenerative factors and reveal novel factors that could regulate regenerative responses within the zebrafish RPE.

A robust pipeline for efficient knock-in of point mutations and epitope tags in zebrafish using fluorescent PCR based screening

BACKGROUND

Genome editing using CRISPR/Cas9 has become a powerful tool in zebrafish to generate targeted gene knockouts models. However, its use for targeted knock-in remains challenging due to inefficient homology directed repair (HDR) pathway in zebrafish, highlighting the need for efficient and cost-effective screening methods.  RESULTS: Here, we present our fluorescent PCR and capillary electrophoresis based screening approach for knock-in using a single-stranded oligodeoxynucleotide donor (ssODN) as a repair template for the targeted insertion of epitope tags, or single nucleotide changes to recapitulate pathogenic human alleles. For the insertion of epitope tags, we took advantage of the expected change in size of the PCR product. For point mutations, we combined fluorescent PCR with restriction fragment length polymorphism (RFLP) analysis to distinguish the fish with the knock-in allele. As a proof-of-principle, we present our data on the generation of fish lines with insertion of a FLAG tag at the tcnba locus, an HA tag at the gata2b locus, and a point mutation observed in Gaucher disease patients in the gba gene. Despite the low number of germline transmitting founders (1-5%), combining our screening methods with prioritization of founder fish by fin biopsies allowed us to establish stable knock-in lines by screening 12 or less fish per gene.

CONCLUSIONS

We have established a robust pipeline for the generation of zebrafish models with precise integration of small DNA sequences and point mutations at the desired sites in the genome. Our screening method is very efficient and easy to implement as it is PCR-based and only requires access to a capillary sequencer.

Pre- and postsynaptic alterations in the visual cortex of the P23H-1 retinal degeneration rat model

P23H rats express a variant of rhodopsin with a mutation that leads to loss of visual function with similar properties as human autosomal dominant retinitis pigmentosa (RP). The advances made in different therapeutic strategies to recover visual system functionality reveal the need to know whether progressive retina degeneration affects the visual cortex structure. Here we are interested in detecting cortical alterations in young rats with moderate retinal degeneration, and in adulthood when degeneration is severer. For this purpose, we studied the synaptic architecture of the primary visual cortex (V1) by analyzing a series of pre- and postsynaptic elements related to excitatory glutamatergic transmission. Visual cortices from control Sprague Dawley (SD) and P23H rats at postnatal days 30 (P30) and P230 were used to evaluate the distribution of vesicular glutamate transporters VGLUT1 and VGLUT2 by immunofluorescence, and to analyze the expression of postsynaptic density protein-95 (PSD-95) by Western blot. The amount and dendritic spine distribution along the apical shafts of the layer V pyramidal neurons, stained by the Golgi-Cox method, were also studied. We observed that at P30, RP does not significantly affect any of the studied markers and structures, which suggests in young P23H rats that visual cortex connectivity seems preserved. However, in adult rats, although VGLUT1 immunoreactivity and PSD-95 expression were similar between both groups, a narrower and stronger VGLUT2-immunoreactive band in layer IV was observed in the P23H rats. Furthermore, RP significantly decreased the density of dendritic spines and altered their distribution along the apical shafts of pyramidal neurons, which remained in a more immature state compared to the P230 SD rats. Our results indicate that the most notable changes in the visual cortex structure take place after a prolonged retinal degeneration period that affected the presynaptic thalamocortical VGLUT2-immunoreactive terminals and postsynaptic dendritic spines from layer V pyramidal cells. Although plasticity is more limited at these ages, future studies will determine how reversible these changes are and to what extent they can affect the visual system’s functionality.

Disruption of mstn Gene by CRISPR/Cas9 in Large Yellow Croaker (Larimichthys crocea)

The large yellow croaker (Larimichthys crocea) plays an economically vital role in the marine aquaculture in China. Suffering from infection of bacteria and protozoon, effect of extreme weather and stress from high-density farming, genome editing is thought to be an important tool applied to L. croea for enhancing commercial traits such as growth rate, disease resistance, and nutrition component. In this study, we identified two mstn genes in L. croea and investigated the different phylogenetic clades, gene structures, and conserved syntenic relationships. To obtain fast-growing large yellow croaker, we specially selected two validated targets for mstnb knockout, which was homologous to mammalian myostatin gene (MSTN) and downregulated skeletal muscle growth and development. Five significant mutation types were generated in two mosaic mutants by transferring specific CRISPR/Cas9 RNPs (ribonucleoprotein) into the one-cell fertilized embryos based on CRISPR/Cas9 technology. Subsequently, we also elucidated the obstacles and possible measures to improve the success rate of inducing modified large yellow croaker. Our results would provide valuable method and reference for facilitating genome editing programs of the large yellow croaker in the future.

A Comprehensive Review of Indel Detection Methods for Identification of Zebrafish Knockout Mutants Generated by Genome-Editing Nucleases

The use of zebrafish in functional genomics and disease modeling has become popular due to the ease of targeted mutagenesis with genome editing nucleases, i.e., zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and clustered regularly interspaced short palindromic repeats/Cas9 (CRISPR/Cas9). These nucleases, specifically CRISPR/Cas9, are routinely used to generate gene knockout mutants by causing a double stranded break at the desired site in the target gene and selecting for frameshift insertions or deletions (indels) caused by the errors during the repair process. Thus, a variety of methods have been developed to identify fish with indels during the process of mutant generation and phenotypic analysis. These methods range from PCR and gel-based low-throughput methods to high-throughput methods requiring specific reagents and/or equipment. Here, we provide a comprehensive review of currently used indel detection methods in zebrafish. By discussing the molecular basis for each method as well as their pros and cons, we hope that this review will serve as a comprehensive resource for zebrafish researchers, allowing them to choose the most appropriate method depending upon their budget, access to required equipment and the throughput needs of the projects.

NTR 2.0: a rationally engineered prodrug-converting enzyme with substantially enhanced efficacy for targeted cell ablation

Transgenic expression of bacterial nitroreductase (NTR) enzymes sensitizes eukaryotic cells to prodrugs such as metronidazole (MTZ), enabling selective cell-ablation paradigms that have expanded studies of cell function and regeneration in vertebrates. However, first-generation NTRs required confoundingly toxic prodrug treatments to achieve effective cell ablation, and some cell types have proven resistant. Here we used rational engineering and cross-species screening to develop an NTR variant, NTR 2.0, which exhibits ~100-fold improvement in MTZ-mediated cell-specific ablation efficacy, eliminating the need for near-toxic prodrug treatment regimens. NTR 2.0 therefore enables sustained cell-loss paradigms and ablation of previously resistant cell types. These properties permit enhanced interrogations of cell function, extended challenges to the regenerative capacities of discrete stem cell niches, and novel modeling of chronic degenerative diseases. Accordingly, we have created a series of bipartite transgenic reporter/effector resources to facilitate dissemination of NTR 2.0 to the research community.

The application of genome editing technology in fish

The advent and development of genome editing technology has opened up the possibility of directly targeting and modifying genomic sequences in the field of life sciences with rapid developments occurring in the last decade. As a powerful tool to decipher genome data at the molecular biology level, genome editing technology has made important contributions to elucidating many biological problems. Currently, the three most widely used genome editing technologies include: zinc finger nucleases (ZFN), transcription activator like effector nucleases (TALEN), and clustered regularly interspaced short palindromic repeats (CRISPR). Researchers are still striving to create simpler, more efficient, and accurate techniques, such as engineered base editors and new CRISPR/Cas systems, to improve editing efficiency and reduce off-target rate, as well as a near-PAMless SpCas9 variants to expand the scope of genome editing. As one of the important animal protein sources, fish has significant economic value in aquaculture. In addition, fish is indispensable for research as it serves as the evolutionary link between invertebrates and higher vertebrates. Consequently, genome editing technologies were applied extensively in various fish species for basic functional studies as well as applied research in aquaculture. In this review, we focus on the application of genome editing technologies in fish species detailing growth, gender, and pigmentation traits. In addition, we have focused on the construction of a zebrafish (Danio rerio) disease model and high-throughput screening of functional genes. Finally, we provide some of the future perspectives of this technology.

Zebrafish Model in Ophthalmology to Study Disease Mechanism and Drug Discovery

Visual impairment and blindness are common and seriously affect people’s work and quality of life in the world. Therefore, the effective therapies for eye diseases are of high priority. Zebrafish (Danio rerio) is an alternative vertebrate model as a useful tool for the mechanism elucidation and drug discovery of various eye disorders, such as cataracts, glaucoma, diabetic retinopathy, age-related macular degeneration, photoreceptor degeneration, etc. The genetic and embryonic accessibility of zebrafish in combination with a behavioral assessment of visual function has made it a very popular model in ophthalmology. Zebrafish has also been widely used in ocular drug discovery, such as the screening of new anti-angiogenic compounds or neuroprotective drugs, and the oculotoxicity test. In this review, we summarized the applications of zebrafish as the models of eye disorders to study disease mechanism and investigate novel drug treatments.

Large-scale phenotypic drug screen identifies neuroprotectants in zebrafish and mouse models of retinitis pigmentosa

Retinitis pigmentosa (RP) and associated inherited retinal diseases (IRDs) are caused by rod photoreceptor degeneration, necessitating therapeutics promoting rod photoreceptor survival. To address this, we tested compounds for neuroprotective effects in multiple zebrafish and mouse RP models, reasoning drugs effective across species and/or independent of disease mutation may translate better clinically. We first performed a large-scale phenotypic drug screen for compounds promoting rod cell survival in a larval zebrafish model of inducible RP. We tested 2934 compounds, mostly human-approved drugs, across six concentrations, resulting in 113 compounds being identified as hits. Secondary tests of 42 high-priority hits confirmed eleven lead candidates. Leads were then evaluated in a series of mouse RP models in an effort to identify compounds effective across species and RP models, that is, potential pan-disease therapeutics. Nine of 11 leads exhibited neuroprotective effects in mouse primary photoreceptor cultures, and three promoted photoreceptor survival in mouse rd1 retinal explants. Both shared and complementary mechanisms of action were implicated across leads. Shared target tests implicated parp1-dependent cell death in our zebrafish RP model. Complementation tests revealed enhanced and additive/synergistic neuroprotective effects of paired drug combinations in mouse photoreceptor cultures and zebrafish, respectively. These results highlight the value of cross-species/multi-model phenotypic drug discovery and suggest combinatorial drug therapies may provide enhanced therapeutic benefits for RP patients.

Zebrafish Models of Photoreceptor Dysfunction and Degeneration

Zebrafish are an instrumental system for the generation of photoreceptor degeneration models, which can be utilized to determine underlying causes of photoreceptor dysfunction and death, and for the analysis of potential therapeutic compounds, as well as the characterization of regenerative responses. We review the wealth of information from existing zebrafish models of photoreceptor disease, specifically as they relate to currently accepted taxonomic classes of human rod and cone disease. We also highlight that rich, detailed information can be derived from studying photoreceptor development, structure, and function, including behavioural assessments and in vivo imaging of zebrafish. Zebrafish models are available for a diversity of photoreceptor diseases, including cone dystrophies, which are challenging to recapitulate in nocturnal mammalian systems. Newly discovered models of photoreceptor disease and drusenoid deposit formation may not only provide important insights into pathogenesis of disease, but also potential therapeutic approaches. Zebrafish have already shown their use in providing pre-clinical data prior to testing genetic therapies in clinical trials, such as antisense oligonucleotide therapy for Usher syndrome.

BE4max and AncBE4max Are Efficient in Germline Conversion of C:G to T:A Base Pairs in Zebrafish

The ease of use and robustness of genome editing by CRISPR/Cas9 has led to successful use of gene knockout zebrafish for disease modeling. However, it still remains a challenge to precisely edit the zebrafish genome to create single-nucleotide substitutions, which account for ~60% of human disease-causing mutations. Recently developed base editing nucleases provide an excellent alternate to CRISPR/Cas9-mediated homology dependent repair for generation of zebrafish with point mutations. A new set of cytosine base editors, termed BE4max and AncBE4max, demonstrated improved base editing efficiency in mammalian cells but have not been evaluated in zebrafish. Therefore, we undertook this study to evaluate their efficiency in converting C:G to T:A base pairs in zebrafish by somatic and germline analysis using highly active sgRNAs to twist and ntl genes. Our data demonstrated that these improved BE4max set of plasmids provide desired base substitutions at similar efficiency and without any indels compared to the previously reported BE3 and Target-AID plasmids in zebrafish. Our data also showed that AncBE4max produces fewer incorrect and bystander edits, suggesting that it can be further improved by codon optimization of its components for use in zebrafish.

Application of CRISPR Tools for Variant Interpretation and Disease Modeling in Inherited Retinal Dystrophies

Inherited retinal dystrophies are an assorted group of rare diseases that collectively account for the major cause of visual impairment of genetic origin worldwide. Besides clinically, these vision loss disorders present a high genetic and allelic heterogeneity. To date, over 250 genes have been associated to retinal dystrophies with reported causative variants of every nature (nonsense, missense, frameshift, splice-site, large rearrangements, and so forth). Except for a fistful of mutations, most of them are private and affect one or few families, making it a challenge to ratify the newly identified candidate genes or the pathogenicity of dubious variants in disease-associated loci. A recurrent option involves altering the gene in in vitro or in vivo systems to contrast the resulting phenotype and molecular imprint. To validate specific mutations, the process must rely on simulating the precise genetic change, which, until recently, proved to be a difficult endeavor. The rise of the CRISPR/Cas9 technology and its adaptation for genetic engineering now offers a resourceful suite of tools to alleviate the process of functional studies. Here we review the implementation of these RNA-programmable Cas9 nucleases in culture-based and animal models to elucidate the role of novel genes and variants in retinal dystrophies.

Model systems for regeneration: zebrafish

Tissue damage can resolve completely through healing and regeneration, or can produce permanent scarring and loss of function. The response to tissue damage varies across tissues and between species. Determining the natural mechanisms behind regeneration in model organisms that regenerate well can help us develop strategies for tissue recovery in species with poor regenerative capacity (such as humans). The zebrafish (Danio rerio) is one of the most accessible vertebrate models to study regeneration. In this Primer, we highlight the tools available to study regeneration in the zebrafish, provide an overview of the mechanisms underlying regeneration in this system and discuss future perspectives for the field.

Expanding the CRISPR Toolbox in Zebrafish for Studying Development and Disease

The study of model organisms has revolutionized our understanding of the mechanisms underlying normal development, adult homeostasis, and human disease. Much of what we know about gene function in model organisms (and its application to humans) has come from gene knockouts: the ability to show analogous phenotypes upon gene inactivation in animal models. The zebrafish (Danio rerio) has become a popular model organism for many reasons, including the fact that it is amenable to various forms of genetic manipulation. The RNA-guided CRISPR/Cas9-mediated targeted mutagenesis approaches have provided powerful tools to manipulate the genome toward developing new disease models and understanding the pathophysiology of human diseases. CRISPR-based approaches are being used for the generation of both knockout and knock-in alleles, and also for applications including transcriptional modulation, epigenome editing, live imaging of the genome, and lineage tracing. Currently, substantial effort is being made to improve the specificity of Cas9, and to expand the target coverage of the Cas9 enzymes. Novel types of naturally occurring CRISPR systems [Cas12a (Cpf1); engineered variants of Cas9, such as xCas9 and SpCas9-NG], are being studied and applied to genome editing. Since the majority of pathogenic mutations are single point mutations, development of base editors to convert C:G to T:A or A:T to G:C has further strengthened the CRISPR toolbox. In this review, we provide an overview of the increasing number of novel CRISPR-based tools and approaches, including lineage tracing and base editing.