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Oxytocin, vasopressin and social behavior in the age of genome editing: A comparative perspective.
Behavioral neuroendocrinology has a rich history of using diverse model organisms to elucidate general principles and evolution of hormone-brain-behavior relationships. The oxytocin and vasopressin systems have been studied in many species, revealing their role in regulating social behaviors. Oxytocin and vasopressin receptors show remarkable species and individual differences in distribution in the brain that have been linked to diversity in social behaviors. New technologies allow for unprecedented interrogation of the genes and neural circuitry regulating behaviors, but these approaches often require transgenic models and are most often used in mice. Here we discuss seminal findings relating the oxytocin and vasopressin systems to social behavior with a focus on non-traditional animal models. We then evaluate the potential of using CRISPR/Cas9 genome editing to examine the roles of genes and enable circuit dissection, manipulation and activity monitoring of the oxytocin and vasopressin systems. We believe that it is essential to incorporate these genetic and circuit level techniques in comparative behavioral neuroendocrinology research to ensure that our field remains innovative and attractive for the next generation of investigators and funding agencies.
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Guide RNAs: A Glimpse at the Sequences that Drive CRISPR-Cas Systems.
CRISPR-Cas systems provide adaptive immunity in bacteria and archaea. Although there are two main classes of CRISPR-Cas systems defined by gene content, interfering RNA biogenesis, and effector proteins, Type II systems have recently been exploited on a broad scale to develop next-generation genetic engineering and genome-editing tools. Conveniently, Type II systems are streamlined and rely on a single protein, Cas9, and a guide RNA molecule, comprised of a CRISPR RNA (crRNA) and trans-acting CRISPR RNA (tracrRNA), to achieve effective and programmable nucleic acid targeting and cleavage. Currently, most commercially available Cas9-based genome-editing tools use the CRISPR-Cas system from Streptococcus pyogenes (SpyCas9), although many orthogonal Type II systems are available for diverse and multiplexable genome engineering applications. Here, we discuss the biological significance of Type II CRISPR-Cas elements, including the tracrRNA, crRNA, Cas9, and protospacer-adjacent motif (PAM), and look at the native function of these elements to understand how they can be engineered, enhanced, and optimized for genome editing applications. Additionally, we discuss the basis for orthogonal Cas9 and guide RNA systems that would allow researchers to concurrently use multiple Cas9-based systems for different purposes. Understanding the native function of endogenous Type II CRISPR-Cas systems can lead to new Cas9 tool development to expand the genetic manipulation toolbox.
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Delivery of biomacromolecules for therapeutic genome editing.
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Single-strand annealing: Molecular mechanisms and potential applications in CRISPR-Cas-based precision genome editing.
BACKGROUND: Spontaneous double-stranded DNA breaks (DSBs) frequently occur within the genome of all living organisms and must be well repaired for survival. Recently, more important roles of the DSB repair pathways that were previously thought to be minor pathways, such as single-strand annealing (SSA), have been shown. Nevertheless, the biochemical mechanisms and applications of the SSA pathway in genome editing have not been updated. PURPOSE AND SCOPE: Understanding the molecular mechanism of SSA is important to design potential applications in gene editing. This review provides insights into the recent progress of SSA studies and establishes a model for their potential applications in precision genome editing. SUMMARY AND CONCLUSION: The SSA mechanism involved in DNA DSB repair appears to be activated by a complex signaling cascade starting with broken end sensing and 5'-3' resection to reveal homologous repeats on the 3' ssDNA overhangs that flank the DSB. Annealing the repeats would help to amend the discontinuous ends and restore the intact genome, resulting in the missing of one repeat and the intervening sequence between the repeats. We proposed a model for CRISPR-Cas-based precision insertion or replacement of DNA fragments to take advantage of the characteristics. The proposed model can add a tool to extend the choice for precision gene editing. Nevertheless, the model needs to be experimentally validated and optimized with SSA-favorable conditions for practical applications.
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Biomedical applications of gene editing.
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Repair-seq: Seeking and Perturbing DNA Repair.
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Naïve Primary Mouse CD8+ T Cells Retain In Vivo Immune Responsiveness After Electroporation-Based CRISPR/Cas9 Genetic Engineering.
CRISPR/Cas9 technology has revolutionized genetic engineering of primary cells. Although its use is gaining momentum in studies on CD8+ T cell biology, it remains elusive to what extent CRISPR/Cas9 affects in vivo function of CD8+ T cells. Here, we optimized nucleofection-based CRISPR/Cas9 genetic engineering of naïve and in vitro-activated primary mouse CD8+ T cells and tested their in vivo immune responses. Nucleofection of naïve CD8+ T cells preserved their in vivo antiviral immune responsiveness to an extent that is indistinguishable from non-nucleofected cells, whereas nucleofection of in vitro-activated CD8+ T cells led to slightly impaired expansion/survival at early time point after adoptive transfer and more pronounced contraction. Of note, different target proteins displayed distinct decay rates after gene editing. This is in stark contrast to a comparable period of time required to complete gene inactivation. Thus, for optimal experimental design, it is crucial to determine the kinetics of the loss of target gene product to adapt incubation period after gene editing. In sum, nucleofection-based CRISPR/Cas9 genome editing achieves efficient and rapid generation of mutant CD8+ T cells without imposing detrimental constraints on their in vivo functions.
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Cloning-Free Genome Editing by CRISPR/T7RNAP/Cas9 in Trypanosoma cruzi.
The genetic manipulation of the human parasite Trypanosoma cruzi has been significantly improved since the implementation of the CRISPR/Cas9 technology for genome editing in this organism. Initially, the system was successfully used for gene knockout and endogenous C-terminal tagging in T. cruzi. Recently, an updated version of this technology has been used for gene complementation, site-directed mutagenesis, and N-terminal tagging in trypanosomatids. This cloning-free strategy, called CRISPR/T7RNAP/Cas9, is extremely useful for identifying essential genes when null mutants are not viable. Mutant cell lines obtained by this new system have been used for the functional characterization of proteins in different developmental stages of this parasite's life cycle, including infective trypomastigotes and intracellular amastigotes. In this chapter, we describe the methodology to achieve genome editing by CRISPR/T7RNAP/Cas9 in T. cruzi. Our method involves the generation of T. cruzi epimastigotes that constitutively express the T7 RNA polymerase (T7RNAP) and SpCas9, and their co-transfection with an sgRNA template and donor DNA(s) as polymerase chain reaction (PCR) products. Using this strategy, we have generated genetically modified parasites in 2-3 weeks without the need for gene cloning, cell sorting, or having to perform several transfection attempts to verify the sgRNA efficiency for targeting the gene of interest. The methodology has been organized according to three main genetic purposes: gene knockout, gene complementation of knockout cell lines, and endogenous (N- or C-terminal) tagging in T. cruzi.
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CRISPR-Cas Gene Perturbation and Editing in Human Induced Pluripotent Stem Cells.
Directing the fates of human pluripotent stem cells (hPSC) to generate a multitude of differentiated cell types allows the study of the genetic regulation of human development and disease. The translational potential of hPSC is maximized by exploiting CRISPR to silence or activate genes with spatial and temporal precision permanently or reversibly. Here, we summarize the increasingly refined and diverse CRISPR toolkit for the latter forms of gene perturbation in hPSC and their downstream applications. We discuss newer methods to install edits efficiently with single nucleotide resolution and describe pooled CRISPR screens as a powerful means of unbiased discovery of genes associated with a phenotype of interest. Last, we discuss the potential of these combined technologies in the treatment of hitherto intractable human diseases and the challenges to their implementation in the clinic.
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Therapeutic genome editing with engineered nucleases.
Targeted genome editing with designer nucleases, such as zinc finger nucleases, TALE nucleases, and CRISPR-Cas nucleases, has heralded a new era in gene therapy. Genetic disorders, which have not been amenable to conventional gene-addition-type gene therapy approaches, such as disorders with dominant inheritance or diseases caused by mutations in tightly regulated genes, can now be treated by precise genome surgery. Moreover, engineered nucleases enable novel genetic interventions to fight infectious diseases or to improve cancer immunotherapies. Here, we review the development of the different classes of programmable nucleases, discuss the challenges and improvements in translating gene editing into clinical use, and give an outlook on what applications can expect to enter the clinic in the near future.
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