Cas11 enables genome engineering in human cells with compact CRISPR-Cas3 systems

TAK1-mediated phosphorylation of PLCE1 represses PIP2 hydrolysis to impede esophageal squamous cancer metastasis

TAK1 is a serine/threonine protein kinase that is a key regulator in a wide variety of cellular processes. However, the functions and mechanisms involved in cancer metastasis are still not well understood. Here, we found that TAK1 knockdown promoted esophageal squamous cancer carcinoma (ESCC) migration and invasion, whereas TAK1 overexpression resulted in the opposite outcome. These in vitro findings were recapitulated in vivo in a xenograft metastatic mouse model. Mechanistically, co-immunoprecipitation and mass spectrometry demonstrated that TAK1 interacted with phospholipase C epsilon 1 (PLCE1) and phosphorylated PLCE1 at serine 1060 (S1060). Functional studies revealed that phosphorylation at S1060 in PLCE1 resulted in decreased enzyme activity, leading to the repression of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis. As a result, the degradation products of PIP2 including diacylglycerol (DAG) and inositol IP3 were reduced, which thereby suppressed signal transduction in the axis of PKC/GSK-3β/β-Catenin. Consequently, expression of cancer metastasis-related genes was impeded by TAK1. Overall, our data indicate that TAK1 plays a negative role in ESCC metastasis, which depends on the TAK1-induced phosphorylation of PLCE1 at S1060.

pH-Induced Conformational Change of the Chromophore of the Large Stokes Shift Fluorescent Protein tKeima

Fluorescent proteins (FPs) are widely used as optical probes in molecular and cell biology. tKeima is a tetrameric, large Stokes shift red fluorescent protein and the ancestral protein of mt-Keima, which is widely applied as a pH-sensitive fluorescent probe. While the pH sensitivity of mt-Keima is well characterized, the pH-dependent properties of the ancestral tKeima have not been comprehensively elucidated. To obtain a better understanding of the effects of pH on tKeima, its fluorescent emission intensity at various pH levels was measured, and its crystal structure at pH 4.0 was determined at a resolution of 2.2 Å. The fluorescence emission intensity of tKeima at pH 4.0 decreased by approximately 65% compared with its peak emission at pH 10.0. The crystal structure of tKeima at pH 4.0 revealed both cis and trans conformations of the chromophore, in contrast to previously determined structures at pH 8.0, which showed only the cis conformation. This indicates that pH induces a conformational change of the chromophore in tKeima. Both the cis and trans conformations in tKeima were stabilized by hydrogen bonds with neighboring residues. A comparison of tKeima at pH 4.0 with tKeima at basic pH, as well as with mKeima, highlights its unique structural properties. These results provide a deeper understanding of the structural basis for the pH-induced fluorescence emission changes in the Keima family.

Developing a robust genome editing tool based on an endogenous type I-B CRISPR-Cas system in Saccharopolyspora spinosa

Saccharopolyspora spinosa is an industrial rare actinomycete capable of producing important environmental-friendly biopesticides, spinosyns. However, exploitation of S. spinosa has been limited due to its genetic inaccessibility and lack of effective genome engineering tools. In this work, we characterized the activity of an endogenous type I-B CRISPR-Cas system as well as its recognized protospacer adjacent motifs (PAMs) based on bioinformatics analysis combined with a plasmid interference assay in S. spinosa. By delivering editing plasmids containing a designed miniCRISPR array (repeat+self-targeting spacer+repeat) and repair templates, we achieved 100% editing efficiency for gene deletion. Using this tool, the genetic barrier composed of the restriction-modification (RM) systems was systematically disarmed. We showed that by disarming one type I RM system (encoded by A8926_1903/1904/1905) and two type II RM systems (encoded by A8926_1725/1726 and A8926_2652/2653) simultaneously, the transformation efficiency of the replicative and integrative plasmids (pSP01 and pSI01) was increased by approximately 3.9-fold and 4.2-fold, respectively. Using the engineered strain with simultaneous knock-out of these three RM genes as the starting strain, we achieved the deletion of 75-kb spinosyns biosynthetic gene cluster (BGC) as well as gene insertion at high efficiency. Collectively, we developed a reliable and highly efficient genome editing tool based on the endogenous type I CRISPR-Cas system combined with the disarmament of the RM systems in S. spinosa. This is the first time to establish an endogenous CRISPR-Cas-based genome editing tool in the non-model industrial actinomycetes.

Internal in-frame translation generates Cas11b, which is important for effective interference in an archaeal CRISPR-Cas system

CRISPR-Cas is a sophisticated defence system used by bacteria and archaea to fend off invaders. CRISPR-Cas systems vary in their Cas protein composition and have therefore been divided into different classes and types. Type I systems of bacteria have been shown to contain the small protein Cas11 as part of the interference complex known as Cascade. Here we show for the first time that an archaeal CRISPR-Cas type I-B system also contains a homolog of Cas11. The Cas11b protein, encoded by the cas8b gene in Haloferax volcanii, represents the first known case of an internal in-frame translation of an archaeal protein. Translation initiation at an internal methionine of the cas8b open reading frame results in synthesis of Cas11b. Cas11b is required for an effective CRISPR-Cas interference reaction, and in its absence fewer Cascade complexes are formed. Comparison of transcriptomes from wild type and a Cas11b-less strain shows that the depletion of Cas11b also results in differential transcript abundance of many genes, presumably affecting their regulation. Taken together, Cas11b is important for the defence reaction of the type I-B CRISPR-Cas system and seems to play an additional cellular role.

Ablation of CD44 Attenuates Adipogenesis in White Adipocytes via the Tryptophan 5-Hydroxylase 2/5-Hydroxytryptamine Axis to Protect Mice from High-Fat Diet-Induced Obesity

CD44 is a transmembrane protein that plays an essential role in transducing extracellular stimuli into intracellular signaling cascades, especially in cancer cells. Recent studies have shown that CD44 contributes to metabolic regulation. However, the effect of CD44 on adipogenesis in white adipose tissue (WAT) remains unclear. Herein, the expression of CD44 was largely increased in the inguinal and epididymal WAT of obese mice. Ablation or neutralization of CD44 inhibited adipogenesis in cultured adipocytes. CD44-deficient mice were resistant to high-fat diet-induced obesity and metabolic dysfunction. RNA-sequencing, together with functional studies, revealed that reduced expression of tryptophan 5-hydroxylase 2 (Tph2) in WAT was responsible for the repressed adipogenesis in the absence of CD44. The application of 5-hydroxytryptamine, a product of TPH2, rescued the repressed adipogenesis induced by CD44 neutralization. Moreover, the inhibition of TPH2 by p-chlorophenylalanine recapitulated the beneficial phenotypes observed in CD44-deficient mice. Taken together, these data indicate that CD44 plays a pivotal role in adipogenesis in WAT. In this regard, CD44 and its downstream target TPH2 may hold great therapeutic potential for treating excessive adiposity-related metabolic disorders, such as obesity, insulin resistance, and type 2 diabetes.

Expanding the frontiers of genome engineering: A comprehensive review of CRISPR-associated transposons

Genome engineering is extensively utilized in diverse scientific disciplines, advancing human welfare and addressing various challenges. Numerous genome engineering tools have been developed to modify genomic sequences. Among these, the CRISPR-Cas system has transformed the field and remains the most commonly employed genome-editing tool. However, the CRISPR-Cas system relies on induced double-strand breaks, with editing efficiency often limited by factors such as cell type and homologous recombination, impeding further progress. CRISPR-associated transposons (CASTs) represent programmable mobile genetic elements. CASTs identified as active were developed as CAST systems, which can perform RNA-guided DNA integration and are featured by high precision, programmability, and kilobase-level payload capacity. Moreover, CAST system allows for precise genome modifications independent of host DNA repair mechanisms, addressing the constraints of conventional CRISPR-Cas systems. It expands the genome engineering toolkit and is poised to become a representative of next-generation genome editing tools. This review thoroughly examines the research progress on CASTs, highlighting the current challenges faced in genome engineering based on CASTs, and offering insights into the ongoing development of this transformative technology.

Research Progress on the Mechanism and Application of the Type I CRISPR-Cas System

The CRISPR-Cas system functions as an adaptive immune mechanism in archaea and bacteria, providing defense against the invasion of foreign nucleic acids. Most CRISPR-Cas systems are classified into class 1 or class 2, with further subdivision into several subtypes. The primary distinction between class 1 and class 2 systems lies in the assembly of their effector modules. In class 1 systems, the effector complex consists of multiple proteins with distinct functions, whereas in class 2 systems, the effector is associated with a single protein. Class 1 systems account for approximately 90% of the CRISPR-Cas repertoire and are categorized into three types (type I, type IV, and type III) and 12 subtypes. To date, various CRISPR-Cas systems have been widely employed in the field of genetic engineering as essential tools and techniques for genome editing. Type I CRISPR-Cas systems remain a valuable resource for developing sophisticated application tools. This review provides a comprehensive review of the characteristics, mechanisms of action, and applications of class 1 type I CRISPR-Cas systems, as well as transposon-associated systems, offering effective approaches and insights for future research on the mechanisms of action, as well as the subsequent development and application of type I CRISPR-Cas systems.

Versatile and efficient mammalian genome editing with Type I-C CRISPR System of Desulfovibrio vulgaris

CRISPR-Cas tools for mammalian genome editing typically rely on single Cas9 or Cas12a proteins. While type I CRISPR systems in Class I may offer greater specificity and versatility, they are not well-developed for genome editing. Here, we present an alternative type I-C CRISPR system from Desulfovibrio vulgaris (Dvu) for efficient and precise genome editing in mammalian cells and animals. We optimized the Dvu type I-C editing complex to generate precise deletions at multiple loci in various cell lines and pig primary fibroblast cells using a paired PAM-in crRNA strategy. These edited pig cells can serve as donors for generating transgenic cloned piglets. The Dvu type I-C editor also enabled precise large fragment replacements with homology-directed repair. Additionally, we adapted the Dvu-Cascade effector for cytosine and adenine base editing, developing Dvu-CBE and Dvu-ABE systems. These systems efficiently induced C-to-T and A-to-G substitutions in human genes without double-strand breaks. Off-target analysis confirmed the high specificity of the Dvu type I-C editor. Our findings demonstrate the Dvu type I-C editor's potential for diverse mammalian genome editing applications, including deletions, fragment replacement, and base editing, with high efficiency and specificity for biomedicine and agriculture.

Strategies for improving the genome-editing efficiency of class 2 CRISPR/Cas system

Since its advent, gene-editing technology has been widely used in microorganisms, animals, plants, and other species. This technology shows remarkable application prospects, giving rise to a new biotechnological industry. In particular, third-generation gene editing technology, represented by the CRISPR/Cas9 system, has become the mainstream gene editing technology owing to its advantages of high efficiency, simple operation, and low cost. These systems can be widely used because they have been modified and optimized, leading to notable improvements in the efficiency of gene editing. This review introduces the characteristics of popular CRISPR/Cas systems and optimization methods aimed at improving the editing efficiency of class 2 CRISPR/Cas systems, providing a reference for the development of superior gene editing systems. Additionally, the review discusses the development and optimization of base editors, primer editors, gene activation and repression tools, as well as the advancement and refinement of compact systems such as IscB, TnpB, Fanzor, and Cas12f.

Structural basis of Cas3 activation in type I-C CRISPR-Cas system

CRISPR-Cas systems function as adaptive immune mechanisms in bacteria and archaea and offer protection against phages and other mobile genetic elements. Among many types of CRISPR-Cas systems, Type I CRISPR-Cas systems are most abundant, with target interference depending on a multi-subunit, RNA-guided complex known as Cascade that recruits a transacting helicase nuclease, Cas3, to degrade the target. While structural studies on several other types of Cas3 have been conducted long ago, it was only recently that the structural study of Type I-C Cas3 in complex with Cascade was revealed, shedding light on how Cas3 achieve its activity in the Cascade complex. In the present study, we elucidated the first structure of standalone Type I-C Cas3 from Neisseria lactamica (NlaCas3). Structural analysis revealed that the histidine-aspartate (HD) nuclease active site of NlaCas3 was bound to two Fe2+ ions that inhibited its activity. Moreover, NlaCas3 could cleave both single-stranded and double-stranded DNA in the presence of Ni2+ or Co2+, showing the highest activity in the presence of both Ni2+ and Mg2+ ions. By comparing the structural studies of various Cas3 proteins, we determined that our NlaCas3 stays in an inactive conformation, allowing us to understand the structural changes associated with its activation and their implication.

Engineered minimal type I CRISPR-Cas system for transcriptional activation and base editing in human cells

Type I CRISPR-Cas systems are widespread and have exhibited high versatility and efficiency in genome editing and gene regulation in prokaryotes. However, due to the multi-subunit composition and large size, their application in eukaryotes has not been thoroughly investigated. Here, we demonstrate that the type I-F2 Cascade, the most compact among type I systems, with a total gene size smaller than that of SpCas9, can be developed for transcriptional activation in human cells. The efficiency of the engineered I-F2 tool can match or surpass that of dCas9. Additionally, we create a base editor using the I-F2 Cascade, which induces a considerably wide editing window (~30 nt) with a bimodal distribution. It can expand targetable sites, which is useful for disrupting functional sequences and genetic screening. This research underscores the application of compact type I systems in eukaryotes, particularly in the development of a base editor with a wide editing window.

Mechanisms for HNH-mediated target DNA cleavage in type I CRISPR-Cas systems

The metagenome-derived type I-E and type I-F variant CRISPR-associated complex for antiviral defense (Cascade) complexes, fused with HNH domains, precisely cleave target DNA, representing recently identified genome editing tools. However, the underlying working mechanisms remain unknown. Here, structures of type I-FHNH and I-EHNH Cascade complexes at different states are reported. In type I-FHNH Cascade, Cas8fHNH loosely attaches to Cascade head and is adjacent to the 5' end of the target single-stranded DNA (ssDNA). Formation of the full R-loop drives the Cascade head to move outward, allowing Cas8fHNH to detach and rotate ∼150° to accommodate target ssDNA for cleavage. In type I-EHNH Cascade, Cas5eHNH domain is adjacent to the 5' end of the target ssDNA. Full crRNA-target pairing drives the lift of the Cascade head, widening the substrate channel for target ssDNA entrance. Altogether, these analyses into both complexes revealed that crRNA-guided positioning of target DNA and target DNA-induced HNH unlocking are two key factors for their site-specific cleavage of target DNA.

SIRT1 improves lactate homeostasis in the brain to alleviate parkinsonism via deacetylation and inhibition of PKM2

Sirtuin 1 (SIRT1) is a histone deacetylase and plays diverse functions in various physiological events, from development to lifespan regulation. Here, in Parkinson's disease (PD) model mice, we demonstrated that SIRT1 ameliorates parkinsonism, while SIRT1 knockdown further aggravates PD phenotypes. Mechanistically, SIRT1 interacts with and deacetylates pyruvate kinase M2 (PKM2) at K135 and K206, thus leading to reduced PKM2 enzyme activity and lactate production, which eventually results in decreased glial activation in the brain. Administration of lactate in the brain recapitulates PD-like phenotypes. Furthermore, increased expression of PKM2 worsens PD symptoms, and, on the contrary, inhibition of PKM2 by shikonin or PKM2-IN-1 alleviates parkinsonism in mice. Collectively, our data indicate that excessive lactate in the brain might be involved in the progression of PD. By improving lactate homeostasis, SIRT1, together with PKM2, are likely drug targets for developing agents for the treatment of neurodegeneration in PD.

Genome editing using type I-E CRISPR-Cas3 in mice and rat zygotes

The type I CRISPR system has recently emerged as a promising tool, especially for large-scale genomic modification, but its application to generate model animals by editing zygotes had not been established. In this study, we demonstrate genome editing in zygotes using the type I-E CRISPR-Cas3 system, which efficiently generates deletions of several thousand base pairs at targeted loci in mice with 40%-70% editing efficiency without off-target mutations. To overcome the difficulties associated with detecting the variable deletions, we used a newly long-read sequencing-based multiplex genotyping approach. Demonstrating remarkable versatility, our Cas3-based technique was successfully extended to rats as well as mice, even by zygote electroporation methods. Knockin for SNP exchange and genomic replacement with a donor plasmid were also achieved in mice. This pioneering work with the type I CRISPR zygote editing system offers increased flexibility and broader applications in genetic engineering across different species.

Plasmid targeting and destruction by the DdmDE bacterial defence system

Although eukaryotic Argonautes have a pivotal role in post-transcriptional gene regulation through nucleic acid cleavage, some short prokaryotic Argonaute variants (pAgos) rely on auxiliary nuclease factors for efficient foreign DNA degradation1. Here we reveal the activation pathway of the DNA defence module DdmDE system, which rapidly eliminates small, multicopy plasmids from the Vibrio cholerae seventh pandemic strain (7PET)2. Through a combination of cryo-electron microscopy, biochemistry and in vivo plasmid clearance assays, we demonstrate that DdmE is a catalytically inactive, DNA-guided, DNA-targeting pAgo with a distinctive insertion domain. We observe that the helicase-nuclease DdmD transitions from an autoinhibited, dimeric complex to a monomeric state upon loading of single-stranded DNA targets. Furthermore, the complete structure of the DdmDE-guide-target handover complex provides a comprehensive view into how DNA recognition triggers processive plasmid destruction. Our work establishes a mechanistic foundation for how pAgos utilize ancillary factors to achieve plasmid clearance, and provides insights into anti-plasmid immunity in bacteria.

Structural Analysis of the Large Stokes Shift Red Fluorescent Protein tKeima

The Keima family comprises large Stokes shift fluorescent proteins that are useful for dual-color fluorescence cross-correlation spectroscopy and multicolor imaging. The tKeima is a tetrameric large Stokes shift fluorescent protein and serves as the ancestor fluorescent protein for both dKeima and mKeima. The spectroscopic properties of tKeima have been previously reported; however, its structural basis and molecular properties have not yet been elucidated. In this study, we present the crystallographic results of the large Stokes shift fluorescent protein tKeima. The purified tKeima protein spontaneously crystallized after purification without further crystallization. The crystal structure of tKeima was determined at 3.0 Å resolution, revealing a β-barrel fold containing the Gln-Tyr-Gly chromophores mainly with cis-conformation. The tetrameric interfaces of tKeima were stabilized by numerous hydrogen bonds and salt-bridge interactions. These key residues distinguish the substituted residues in dKeima and mKeima. The key structure-based residues involved in the tetramer formation of tKeima provide insights into the generation of a new type of monomeric mKeima. This structural analysis expands our knowledge of the Keima family and provides insights into its protein engineering.

Structure and genome editing of type I-B CRISPR-Cas

Type I CRISPR-Cas systems employ multi-subunit effector Cascade and helicase-nuclease Cas3 to target and degrade foreign nucleic acids, representing the most abundant RNA-guided adaptive immune systems in prokaryotes. Their ability to cause long fragment deletions have led to increasing interests in eukaryotic genome editing. While the Cascade structures of all other six type I systems have been determined, the structure of the most evolutionarily conserved type I-B Cascade is still missing. Here, we present two cryo-EM structures of the Synechocystis sp. PCC 6714 (Syn) type I-B Cascade, revealing the molecular mechanisms that underlie RNA-directed Cascade assembly, target DNA recognition, and local conformational changes of the effector complex upon R-loop formation. Remarkably, a loop of Cas5 directly intercalated into the major groove of the PAM and facilitated PAM recognition. We further characterized the genome editing profiles of this I-B Cascade-Cas3 in human CD3+ T cells using mRNA-mediated delivery, which led to unidirectional 4.5 kb deletion in TRAC locus and achieved an editing efficiency up to 41.2%. Our study provides the structural basis for understanding target DNA recognition by type I-B Cascade and lays foundation for harnessing this system for long range genome editing in human T cells.

Cartilage stem/progenitor cells-derived exosomes facilitate knee cartilage repair in a subacute osteoarthritis rat model

Cartilage defects in the knee are often associated with the progression of degenerative osteoarthritis (OA), and cartilage repair is a useful strategy for managing this disease. However, cartilage repair is challenging because of the unique environment within the tissue. Recently, stem cell-based therapies have shed new light on this issue. In this study, we prepared exosomes (EXOs) from cartilage stem/progenitor cells (CSPCs) and found that treatment with EXOs increased the viability, migration, and proliferation of cultured primary chondrocytes. In a subacute OA rat model, the application of EXOs facilitated cartilage regeneration as evidenced by histological staining. Exosomal protein analysis together with bioinformatics suggested that cyclin-dependent kinase 9 (CDK9) is a key factor for chondrocyte growth and migration. Functional studies confirmed this prediction, that is, inhibiting CDK9 reduced the beneficial effects induced by EXOs in primary chondrocytes; while overexpression of CDK9 recapitulated the EXOs-induced phenotypes. RNA-Seq data showed that a set of genes involved in cell growth and migration were up-regulated by EXOs in chondrocytes. These changes could be partially reproduced by CDK9 overexpression. Overall, our data suggest that EXOs derived from primary CSPCs hold great therapeutic potential for treating cartilage defect-associated disorders such as degenerative OA, and that CDK9 is a key factor in this process.

Precise large-fragment deletions in mammalian cells and mice generated by dCas9-controlled CRISPR/Cas3

Currently, the Cas9 and Cas12a systems are widely used for genome editing, but their ability to precisely generate large chromosome fragment deletions is limited. Type I-E CRISPR mediates broad and unidirectional DNA degradation, but controlling the size of Cas3-mediated DNA deletions has proven elusive thus far. Here, we demonstrate that the endonuclease deactivation of Cas9 (dCas9) can precisely control Cas3-mediated large-fragment deletions in mammalian cells. In addition, we report the elimination of the Y chromosome and precise retention of the Sry gene in mice using CRISPR/Cas3 and dCas9-controlled CRISPR/Cas3, respectively. In conclusion, dCas9-controlled CRISPR/Cas3-mediated precise large-fragment deletion provides an approach for establishing animal models by chromosome elimination. This method also holds promise as a potential therapeutic strategy for treating fragment mutations or human aneuploidy diseases that involve additional chromosomes.

Immune-cell-mediated tissue engineering strategies for peripheral nerve injury and regeneration

During the process of peripheral nerve repair, there are many complex pathological and physiological changes, including multi-cellular responses and various signaling molecules, and all these events establish a dynamic microenvironment for axon repair, regeneration, and target tissue/organ reinnervation. The immune system plays an indispensable role in the process of nerve repair and function recovery. An effective immune response not only involves innate-immune and adaptive-immune cells but also consists of chemokines and cytokines released by these immune cells. The elucidation of the orchestrated interplay of immune cells with nerve regeneration and functional restoration is meaningful for the exploration of therapeutic strategies. This review mainly enumerates the general immune cell response to peripheral nerve injury and focuses on their contributions to functional recovery. The tissue engineering-mediated strategies to regulate macrophages and T cells through physical and biochemical factors combined with scaffolds are discussed. The dynamic immune responses during peripheral nerve repair and immune-cell-mediated tissue engineering methods are presented, which provide a new insight and inspiration for immunomodulatory therapies in peripheral nerve regeneration.

Development and implementation of a Type I-C CRISPR-based programmable repression system for Neisseria gonorrhoeae

Clustered regularly interspaced short palindromic repeats (CRISPR) are prokaryotic adaptive immune systems regularly utilized as DNA-editing tools. While Neisseria gonorrhoeae does not have an endogenous CRISPR, the commensal species Neisseria lactamica encodes a functional Type I-C CRISPR-Cas system. We have established an isopropyl β-d-1-thiogalactopyranoside added (IPTG)-inducible, CRISPR interference (CRISPRi) platform based on the N. lactamica Type I-C CRISPR missing the Cas3 nuclease to allow locus-specific transcriptional repression. As proof of principle, we targeted a non-phase-variable version of the opaD gene. We show that CRISPRi can downregulate opaD gene and protein expression, resulting in bacterial inability to stimulate neutrophil oxidative responses and to bind to an N-terminal fragment of CEACAM1. Importantly, we used CRISPRi to effectively knockdown all the transcripts of all 11 opa genes using a five-spacer CRISPR array, allowing control of the entire phase-variable opa family in strain FA1090. We also report that repression is reversible following IPTG removal. Finally, we showed that the Type I-C CRISPRi system can conditionally reduce the expression of two essential genes. This CRISPRi system will allow the interrogation of every Gc gene, essential and non-essential, to study physiology and pathogenesis and aid in antimicrobial development.IMPORTANCEClustered regularly interspaced short palindromic repeats (CRISPR)-Cas systems have proven instrumental in genetically manipulating many eukaryotic and prokaryotic organisms. Despite its usefulness, a CRISPR system had yet to be developed for use in Neisseria gonorrhoeae (Gc), a bacterium that is the main etiological agent of gonorrhea infection. Here, we developed a programmable and IPTG-inducible Type I-C CRISPR interference (CRISPRi) system derived from the commensal species Neisseria lactamica as a gene repression system in Gc. As opposed to generating genetic knockouts, the Type I-C CRISPRi system allows us to block transcription of specific genes without generating deletions in the DNA. We explored the properties of this system and found that a minimal spacer array is sufficient for gene repression while also facilitating efficient spacer reprogramming. Importantly, we also show that we can use CRISPRi to knockdown genes that are essential to Gc that cannot normally be knocked out under laboratory settings. Gc encodes ~800 essential genes, many of which have no predicted function. We predict that this Type I-C CRISPRi system can be used to help categorize gene functions and perhaps contribute to the development of novel therapeutics for gonorrhea.

Exploiting activation and inactivation mechanisms in type I-C CRISPR-Cas3 for genome-editing applications

Type I CRISPR-Cas systems utilize the RNA-guided Cascade complex to identify matching DNA targets and the nuclease-helicase Cas3 to degrade them. Among the seven subtypes, type I-C is compact in size and highly active in creating large-sized genome deletions in human cells. Here, we use four cryoelectron microscopy snapshots to define its RNA-guided DNA binding and cleavage mechanisms in high resolution. The non-target DNA strand (NTS) is accommodated by I-C Cascade in a continuous binding groove along the juxtaposed Cas11 subunits. Binding of Cas3 further traps a flexible bulge in NTS, enabling NTS nicking. We identified two anti-CRISPR proteins AcrIC8 and AcrIC9 that strongly inhibit Neisseria lactamica I-C function. Structural analysis showed that AcrIC8 inhibits PAM recognition through allosteric inhibition, whereas AcrIC9 achieves so through direct competition. Both Acrs potently inhibit I-C-mediated genome editing and transcriptional modulation in human cells, providing the first off-switches for type I CRISPR eukaryotic genome engineering.

Mechanical Factors Regulate Annulus Fibrosus (AF) Injury Repair and Remodeling: A Review

Low back pain is a common chronic disease that can severely affect the patient's work and daily life. The breakdown of spinal mechanical homeostasis caused by intervertebral disc (IVD) degeneration is a leading cause of low back pain. Annulus fibrosus (AF), as the outer layer structure of the IVD, is often the first affected part. AF injury caused by consistent stress overload will further accelerate IVD degeneration. Therefore, regulating AF injury repair and remodeling should be the primary goal of the IVD repair strategy. Mechanical stimulation has been shown to promote AF regeneration and repair, but most studies only focus on the effect of single stress on AF, and lack realistic models and methods that can mimic the actual mechanical environment of AF. In this article, we review the effects of different types of stress stimulation on AF injury repair and remodeling, suggest possible beneficial load combinations, and explore the underlying molecular mechanisms. It will provide the theoretical basis for designing better tissue engineering therapy using mechanical factors to regulate AF injury repair and remodeling in the future.

Type I CRISPR-Cas-mediated microbial gene editing and regulation

There are six major types of CRISPR-Cas systems that provide adaptive immunity in bacteria and archaea against invasive genetic elements. The discovery of CRISPR-Cas systems has revolutionized the field of genetics in many organisms. In the past few years, exploitations of the most abundant class 1 type I CRISPR-Cas systems have revealed their great potential and distinct advantages to achieve gene editing and regulation in diverse microorganisms in spite of their complicated structures. The widespread and diversified type I CRISPR-Cas systems are becoming increasingly attractive for the development of new biotechnological tools, especially in genetically recalcitrant microbial strains. In this review article, we comprehensively summarize recent advancements in microbial gene editing and regulation by utilizing type I CRISPR-Cas systems. Importantly, to expand the microbial host range of type I CRISPR-Cas-based applications, these structurally complicated systems have been improved as transferable gene-editing tools with efficient delivery methods for stable expression of CRISPR-Cas elements, as well as convenient gene-regulation tools with the prevention of DNA cleavage by obviating deletion or mutation of the Cas3 nuclease. We envision that type I CRISPR-Cas systems will largely expand the biotechnological toolbox for microbes with medical, environmental and industrial importance.

Bionic peptide scaffold in situ polarization and recruitment of M2 macrophages to promote peripheral nerve regeneration

Tissue regeneration requires exogenous and endogenous signals, and there is increasing evidence that the exogenous microenvironment may play an even more dominant role in the complex process of coordinated multiple cells. The short-distance peripheral nerve showed a spontaneous regenerative phenomenon, which was initiated by the guiding role of macrophages. However, it cannot sufficiently restore long-distance nerve injury by itself. Based on this principle, we firstly constructed a proinflammatory model to prove that abnormal M2 expression reduce the guidance and repair effect of long-distance nerves. Furthermore, a bionic peptide hydrogel scaffold based on self-assembly was developed to envelop M2-derived regenerative cytokines and extracellular vesicles (EVs). The cytokines and EVs were quantified to mimic the guidance and regenerative microenvironment in a direct and mild manner. The bionic scaffold promoted M2 transformation in situ and led to proliferation and migration of Schwann cells, neuron growth and motor function recovery. Meanwhile, the peptide scaffold combined with CX3CL1 recruited more blood-derived M2 macrophages to promote long-distance nerve reconstruction. Overall, we systematically confirmed the important role of M2 in regulating and restoring the injury peripheral nerve. This bionic peptide hydrogel scaffold mimicked and remodeled the local environment for M2 transformation and recruitment, favoring long-distance peripheral nerve regeneration. It can help to explicate regulative effect of M2 may be a cause not just a consequence in nerve repair and tissue integration, which facilitating the development of pro-regenerative biomaterials.

In-silico whole-genome sequence analysis of a halotolerant filamentous mangrove cyanobacterium revealed CRISPR-Cas systems with unique properties

Novel CRISPR systems capable of cleaving both DNA and RNA are progressively emerging as attractive tools for genome manipulation of prokaryotic and eukaryotic organisms. We report specific characteristics of CRISPR systems present in Oxynema aestuarii AP17, a halotolerant, filamentous cyanobacterium and the second known member of the Oxynema genus. In-silico analyses of its whole-genome sequence revealed the presence of multiple Type I and Type III CRISPR loci with one Type I-G system previously unreported in cyanobacteria. We further identified the leader sequences at the 5' end of multiple CRISPR loci, many of which were distinct from previously reported cyanobacterial CRISPR leaders. Phylogenetic analyses of the O. aestuarii AP17 Cas1 proteins revealed two protein sequences that were unique and distantly related to other cyanobacterial Cas1 protein sequences. Our findings are significant because novel Class 1 CRISPR systems possess multi-subunit effectors and are highly flexible for repurposing by protein domain fusions made to the effector complex. Additionally, Type III CRISPRs are particularly useful for genome editing in certain extremophiles for which mesophilic Type II CRISPRs are ineffective.

Genome and transcriptome engineering by compact and versatile CRISPR-Cas systems

Comparative genomics has enabled the discovery of tiny clustered regularly interspaced short palindromic repeat (CRISPR) bacterial immune system effectors with enormous potential for manipulating eukaryotic genomes. Recently, smaller Cas proteins, including miniature Cas9, Cas12, and Cas13 proteins, have been identified and validated as efficient genome editing and base editing tools in human cells. The compact size of these novel CRISPR effectors is highly desirable for generating CRISPR-based therapeutic approaches, mainly to overcome in vivo delivery constraints, providing a promising opportunity for editing pathogenic mutations of clinical relevance and knocking down RNAs in human cells without inducing chromosomal insertions or genome alterations. Thus, these tiny CRISPR-Cas systems represent new and highly programmable, specific, and efficient platforms, which expand the CRISPR toolkit for potential therapeutic opportunities.

Molecular breeding of livestock for disease resistance

Animal infectious diseases pose a significant threat to the global agriculture and biomedicine industries, leading to significant economic losses and public health risks. The emergence and spread of viral infections such as African swine fever virus (ASFV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine epidemic diarrhea virus (PEDV), and avian influenza virus (AIV) have highlighted the need for innovative approaches to develop resilient and disease-resistant animal populations. Gene editing technologies, such as CRISPR/Cas9, offer a promising avenue for generating animals with enhanced disease resistance. This review summarizes recent advances in molecular breeding strategies for generating disease-resistant animals, focusing on the development of disease-resistant livestock. We also highlight the potential applications of genome-wide CRISPR/Cas9 library screening and base editors in producing precise gene modified livestock for disease resistance in the future. Overall, gene editing technologies have the potential to revolutionize animal breeding and improve animal health and welfare.

Molecular mechanism for Tn7-like transposon recruitment by a type I-B CRISPR effector

Tn7-like transposons have co-opted CRISPR-Cas systems to facilitate the movement of their own DNA. These CRISPR-associated transposons (CASTs) are promising tools for programmable gene knockin. A key feature of CASTs is their ability to recruit Tn7-like transposons to nuclease-deficient CRISPR effectors. However, how Tn7-like transposons are recruited by diverse CRISPR effectors remains poorly understood. Here, we present the cryo-EM structure of a recruitment complex comprising the Cascade complex, TniQ, TnsC, and the target DNA in the type I-B CAST from Peltigera membranacea cyanobiont 210A. Target DNA recognition by Cascade induces conformational changes in Cas6 and primes TniQ recruitment through its C-terminal domain. The N-terminal domain of TniQ is bound to the seam region of the TnsC spiral heptamer. Our findings provide insights into the diverse mechanisms for the recruitment of Tn7-like transposons to CRISPR effectors and will aid in the development of CASTs as gene knockin tools.

Intervertebral disc degeneration and inflammatory microenvironment: expression, pathology, and therapeutic strategies

BACKGROUND

Intervertebral disc degeneration (IDD) is a leading cause of low back pain (LBP), posing a significant socioeconomic burden. Recent studies highlight the crucial role of inflammatory microenvironment in IDD progression.

METHOD

A keyword-based search was performed using the PubMed database for published articles.

RESULTS AND CONCLUSIONS

Dysregulated expression of inflammatory cytokines disrupts intervertebral disc (IVD) homeostasis, causing atrophy, fibrosis, and phenotypic changes in nucleus pulposus cells. Modulating the inflammatory microenvironment and restoring cytokine balance hold promise for IVD repair and regeneration. This comprehensive review systematically examines the expression regulation, pathological effects, therapeutic strategies, and future challenges associated with the inflammatory microenvironment and relevant cytokines in IDD. Key inflammatory cytokines, including interleukins (IL), tumor necrosis factor-alpha (TNF-α), and chemokines, exhibit significant pathological effects in IDD. Furthermore, major therapeutic modalities such as chemical antagonists, biologics, plant extracts, and gene transcription therapies are introduced to control and ameliorate the inflammatory microenvironment. These approaches provide valuable insights for identifying potential targets in future anti-inflammatory treatments for IDD.

Exploiting Activation and Inactivation Mechanisms in Type I-C CRISPR-Cas3 for Genome Editing Applications

Type I CRISPR-Cas systems utilize the RNA-guided Cascade complex to identify matching DNA targets, and the nuclease-helicase Cas3 to degrade them. Among seven subtypes, Type I-C is compact in size and highly active in creating large-sized genome deletions in human cells. Here we use four cryo-electron microscopy snapshots to define its RNA-guided DNA binding and cleavage mechanisms in high resolution. The non-target DNA strand (NTS) is accommodated by I-C Cascade in a continuous binding groove along the juxtaposed Cas11 subunits. Binding of Cas3 further traps a flexible bulge in NTS, enabling efficient NTS nicking. We identified two anti-CRISPR proteins AcrIC8 and AcrIC9, that strongly inhibit N. lactamica I-C function. Structural analysis showed that AcrIC8 inhibits PAM recognition through direct competition, whereas AcrIC9 achieves so through allosteric inhibition. Both Acrs potently inhibit I-C-mediated genome editing and transcriptional modulation in human cells, providing the first off-switches for controllable Type I CRISPR genome engineering.

Repurposing the atypical type I-G CRISPR system for bacterial genome engineering

The CRISPR-Cas system functions as a prokaryotic immune system and is highly diverse, with six major types and numerous sub-types. The most abundant are type I CRISPR systems, which utilize a multi-subunit effector, Cascade, and a CRISPR RNA (crRNA) to detect invading DNA species. Detection leads to DNA loading of the Cas3 helicase-nuclease, leading to long-range deletions in the targeted DNA, thus providing immunity against mobile genetic elements (MGE). Here, we focus on the type I-G system, a streamlined, 4-subunit complex with an atypical Cas3 enzyme. We demonstrate that Cas3 helicase activity is not essential for immunity against MGE in vivo and explore applications of the Thioalkalivibrio sulfidiphilus Cascade effector for genome engineering in Escherichia coli. Long-range, bidirectional deletions were observed when the lacZ gene was targeted. Deactivation of the Cas3 helicase activity dramatically altered the types of deletions observed, with small deletions flanked by direct repeats that are suggestive of microhomology mediated end joining. When donor DNA templates were present, both the wild-type and helicase-deficient systems promoted homology-directed repair (HDR), with the latter system providing improvements in editing efficiency, suggesting that a single nick in the target site may promote HDR in E. coli using the type I-G system. These findings open the way for further application of the type I-G CRISPR systems in genome engineering.

Type IV-A CRISPR-Csf complex: Assembly, dsDNA targeting, and CasDinG recruitment

Type IV CRISPR-Cas systems, which are primarily found on plasmids and exhibit a strong plasmid-targeting preference, are the only one of the six known CRISPR-Cas types for which the mechanistic details of their function remain unknown. Here, we provide high-resolution functional snapshots of type IV-A Csf complexes before and after target dsDNA binding, either in the absence or presence of CasDinG, revealing the mechanisms underlying Csf^crRNA complex assembly, "DWN" PAM-dependent dsDNA targeting, R-loop formation, and CasDinG recruitment. Furthermore, we establish that CasDinG, a signature DinG family helicase, harbors ssDNA-stimulated ATPase activity and ATP-dependent 5'-3' DNA helicase activity. In addition, we show that CasDinG unwinds the non-target strand (NTS) and target strand (TS) of target dsDNA from the Csf^crRNA complex. These molecular details advance our mechanistic understanding of type IV-A CRISPR-Csf function and should enable Csf complexes to be harnessed as genome-engineering tools for biotechnological applications.

CRISPR-Based Genome Editing Tools: An Accelerator in Crop Breeding for a Changing Future

Genome editing is an important strategy to maintain global food security and achieve sustainable agricultural development. Among all genome editing tools, CRISPR-Cas is currently the most prevalent and offers the most promise. In this review, we summarize the development of CRISPR-Cas systems, outline their classification and distinctive features, delineate their natural mechanisms in plant genome editing and exemplify the applications in plant research. Both classical and recently discovered CRISPR-Cas systems are included, detailing the class, type, structures and functions of each. We conclude by highlighting the challenges that come with CRISPR-Cas and offer suggestions on how to tackle them. We believe the gene editing toolbox will be greatly enriched, providing new avenues for a more efficient and precise breeding of climate-resilient crops.

Mechanistic insights into DNA binding and cleavage by a compact type I-F CRISPR-Cas system in bacteriophage

CRISPR-Cas systems are widespread adaptive antiviral systems used in prokaryotes. Some phages, in turn, although have small genomes can economize the use of genetic space to encode compact or incomplete CRISPR-Cas systems to inhibit the host and establish infection. Phage ICP1, infecting Vibrio cholerae, encodes a compact type I-F CRISPR-Cas system to suppress the antiphage mobile genetic element in the host genome. However, the mechanism by which this compact system recognizes the target DNA and executes interference remains elusive. Here, we present the electron cryo-microscopy (cryo-EM) structures of both apo- and DNA-bound ICP1 surveillance complexes (Aka Csy complex). Unlike most other type I surveillance complexes, the ICP1 Csy complex lacks the Cas11 subunit or a structurally homologous domain, which is crucial for dsDNA binding and Cas3 activation in other type I CRISPR-Cas systems. Structural and functional analyses revealed that the compact ICP1 Csy complex alone is inefficient in binding to dsDNA targets, presumably stalled at a partial R-loop conformation. The presence of Cas2/3 facilitates dsDNA binding and allows effective dsDNA target cleavage. Additionally, we found that Pseudomonas aeruginosa Cas2/3 efficiently cleaved the dsDNA target presented by the ICP1 Csy complex, but not vice versa. These findings suggest a unique mechanism for target dsDNA binding and cleavage by the compact phage-derived CRISPR-Cas system.

The biology and type I/III hybrid nature of type I-D CRISPR-Cas systems

Prokaryotes have adaptive defence mechanisms that protect them from mobile genetic elements and viral infection. One defence mechanism is called CRISPR-Cas (clustered regularly interspaced short palindromic repeats and CRISPR-associated proteins). There are six different types of CRISPR-Cas systems and multiple subtypes that vary in composition and mode of action. Type I and III CRISPR-Cas systems utilise multi-protein complexes, which differ in structure, nucleic acid binding and cleaving preference. The type I-D system is a chimera of type I and III systems. Recently, there has been a burst of research on the type I-D CRISPR-Cas system. Here, we review the mechanism, evolution and biotechnological applications of the type I-D CRISPR-Cas system.

Functional PAM sequence for DNA interference by CRISPR-Cas I-B system of Leptospira interrogans and the role of LinCas11b encoded within lincas8b

Pathogenic species of Leptospira are recalcitrant for genetic manipulation using conventional tools, and therefore there is a need to explore techniques of higher efficiency. Application of endogenous CRISPR-Cas tool is emerging and efficient; nevertheless, it is limited by a poor understanding of interference machinery in the bacterial genome and its associated protospacer adjacent motif (PAM). In this study, interference machinery of CRISPR-Cas subtype I-B (Lin_I-B) from L. interrogans was experimentally validated in E. coli using the various identified PAM (TGA, ATG, ATA). The overexpression of the Lin_I-B interference machinery in E. coli demonstrated that LinCas5, LinCas6, LinCas7, and LinCas8b can self-assemble on cognate CRISPR RNA to form an interference complex (LinCascade). Moreover, a robust interference of target plasmids containing a protospacer with a PAM suggested a functional LinCascade. We also recognized a small open reading frame within lincas8b that independently co-translates LinCas11b. A mutant variant of LinCascade^-Cas11b that lacks LinCas11b co-expression erred to mount target plasmid interference. At the same time, LinCas11b complementation in LinCascade^-Cas11b rescued target plasmid interference. Thus, the present study establishes Leptospira subtype I-B interference machinery to be functional and, soon, may pave the way for scientists to harness it as a programmable endogenous genetic manipulation tool.

Recent advances in CRISPR-based genome editing technology and its applications in cardiovascular research

The rapid development of genome editing technology has brought major breakthroughs in the fields of life science and medicine. In recent years, the clustered regularly interspaced short palindromic repeats (CRISPR)-based genome editing toolbox has been greatly expanded, not only with emerging CRISPR-associated protein (Cas) nucleases, but also novel applications through combination with diverse effectors. Recently, transposon-associated programmable RNA-guided genome editing systems have been uncovered, adding myriads of potential new tools to the genome editing toolbox. CRISPR-based genome editing technology has also revolutionized cardiovascular research. Here we first summarize the advances involving newly identified Cas orthologs, engineered variants and novel genome editing systems, and then discuss the applications of the CRISPR-Cas systems in precise genome editing, such as base editing and prime editing. We also highlight recent progress in cardiovascular research using CRISPR-based genome editing technologies, including the generation of genetically modified in vitro and animal models of cardiovascular diseases (CVD) as well as the applications in treating different types of CVD. Finally, the current limitations and future prospects of genome editing technologies are discussed.

Structural snapshots of R-loop formation by a type I-C CRISPR Cascade

Type I CRISPR-Cas systems employ multi-subunit Cascade effector complexes to target foreign nucleic acids for destruction. Here, we present structures of D. vulgaris type I-C Cascade at various stages of double-stranded (ds)DNA target capture, revealing mechanisms that underpin PAM recognition and Cascade allosteric activation. We uncover an interesting mechanism of non-target strand (NTS) DNA stabilization via stacking interactions with the "belly" subunits, securing the NTS in place. This "molecular seatbelt" mechanism facilitates efficient R-loop formation and prevents dsDNA reannealing. Additionally, we provide structural insights into how two anti-CRISPR (Acr) proteins utilize distinct strategies to achieve a shared mechanism of type I-C Cascade inhibition by blocking PAM scanning. These observations form a structural basis for directional R-loop formation and reveal how different Acr proteins have converged upon common molecular mechanisms to efficiently shut down CRISPR immunity.

Type I-D CRISPR System-Mediated Genome Editing in Plants

Genome editing has revolutionized plant research and plant breeding by enabling precise genome manipulation. In particular, the application of type II CRISPR-Cas9 systems to genome editing has proved an important milestone, accelerating genetic engineering and the analysis of gene function. On the other hand, the potential of other types of CRISPR-Cas systems, especially many of the most abundant type I CRISPR-Cas systems, remains unexplored. We recently developed a novel genome editing tool, TiD, based on the type I-D CRISPR-Cas system. In this chapter, we describe a protocol for genome editing of plant cells using TiD. This protocol allows the application of TiD to induce short insertion and deletions (indels) or long-range deletions at target sites with high specificity in tomato cells.

Visualizing the Nucleome Using the CRISPR–Cas9 System: From in vitro to in vivo

One of the latest methods in modern molecular biology is labeling genomic loci in living cells using fluorescently labeled Cas protein. The NIH Foundation has made the mapping of the 4D nucleome (the three-dimensional nucleome on a timescale) a priority in the studies aimed to improve our understanding of chromatin organization. Fluorescent methods based on CRISPR-Cas are a significant step forward in visualization of genomic loci in living cells. This approach can be used for studying epigenetics, cell cycle, cellular response to external stimuli, rearrangements during malignant cell transformation, such as chromosomal translocations or damage, as well as for genome editing. In this review, we focused on the application of CRISPR-Cas fluorescence technologies as components of multimodal imaging methods for in vivo mapping of chromosomal loci, in particular, attribution of fluorescence signal to morphological and anatomical structures in a living organism. The review discusses the approaches to the highly sensitive, high-precision labeling of CRISPR-Cas components, delivery of genetically engineered constructs into cells and tissues, and promising methods for molecular imaging.

Constructing next-generation CRISPR-Cas tools from structural blueprints

Clustered regularly interspaced short palindromic repeats - CRISPR-associated protein (CRISPR-Cas) systems are a critical component of the bacterial adaptive immune response. Since the discovery that they can be reengineered as programmable RNA-guided nucleases, there has been significant interest in using these systems to perform diverse and precise genetic manipulations. Here, we outline recent advances in the mechanistic understanding of CRISPR-Cas9, how these findings have been leveraged in the rational redesign of Cas9 variants with altered activities, and how these novel tools can be exploited for biotechnology and therapeutics. We also discuss the potential of the ubiquitous, yet often-overlooked, multisubunit CRISPR effector complexes for large-scale genomic deletions. Furthermore, we highlight how future structural studies will bolster these technologies.

Structure and mechanism of the type I-G CRISPR effector

Type I CRISPR systems are the most common CRISPR type found in bacteria. They use a multisubunit effector, guided by crRNA, to detect and bind dsDNA targets, forming an R-loop and recruiting the Cas3 enzyme to facilitate target DNA destruction, thus providing immunity against mobile genetic elements. Subtypes have been classified into families A-G, with type I-G being the least well understood. Here, we report the composition, structure and function of the type I-G Cascade CRISPR effector from Thioalkalivibrio sulfidiphilus, revealing key new molecular details. The unique Csb2 subunit processes pre-crRNA, remaining bound to the 3' end of the mature crRNA, and seven Cas7 subunits form the backbone of the effector. Cas3 associates stably with the effector complex via the Cas8g subunit and is important for target DNA recognition. Structural analysis by cryo-Electron Microscopy reveals a strikingly curved backbone conformation with Cas8g spanning the belly of the structure. These biochemical and structural insights shed new light on the diversity of type I systems and open the way to applications in genome engineering.

Disarming of type I-F CRISPR-Cas surveillance complex by anti-CRISPR proteins AcrIF6 and AcrIF9

CRISPR-Cas systems are prokaryotic adaptive immune systems that protect against phages and other invading nucleic acids. The evolutionary arms race between prokaryotes and phages gave rise to phage anti-CRISPR (Acr) proteins that act as a counter defence against CRISPR-Cas systems by inhibiting the effector complex. Here, we used a combination of bulk biochemical experiments, X-ray crystallography and single-molecule techniques to explore the inhibitory activity of AcrIF6 and AcrIF9 proteins against the type I-F CRISPR-Cas system from Aggregatibacter actinomycetemcomitans (Aa). We showed that AcrIF6 and AcrIF9 proteins hinder Aa-Cascade complex binding to target DNA. We solved a crystal structure of Aa1-AcrIF9 protein, which differ from other known AcrIF9 proteins by an additional structurally important loop presumably involved in the interaction with Cascade. We revealed that AcrIF9 association with Aa-Cascade promotes its binding to off-target DNA sites, which facilitates inhibition of CRISPR-Cas protection.

Allosteric control of type I-A CRISPR-Cas3 complexes and establishment as effective nucleic acid detection and human genome editing tools

Type I CRISPR-Cas systems typically rely on a two-step process to degrade DNA. First, an RNA-guided complex named Cascade identifies the complementary DNA target. The helicase-nuclease fusion enzyme Cas3 is then recruited in trans for processive DNA degradation. Contrary to this model, here, we show that type I-A Cascade and Cas3 function as an integral effector complex. We provide four cryoelectron microscopy (cryo-EM) snapshots of the Pyrococcus furiosus (Pfu) type I-A effector complex in different stages of DNA recognition and degradation. The HD nuclease of Cas3 is autoinhibited inside the effector complex. It is only allosterically activated upon full R-loop formation, when the entire targeted region has been validated by the RNA guide. The mechanistic insights inspired us to convert Pfu Cascade-Cas3 into a high-sensitivity, low-background, and temperature-activated nucleic acid detection tool. Moreover, Pfu CRISPR-Cas3 shows robust bi-directional deletion-editing activity in human cells, which could find usage in allele-specific inactivation of disease-causing mutations.

Introducing Large Genomic Deletions in Human Pluripotent Stem Cells Using CRISPR-Cas3

CRISPR-Cas systems provide researchers with eukaryotic genome editing tools and therapeutic platforms that make it possible to target disease mutations in somatic organs. Most of these tools employ Type II (e.g., Cas9) or Type V (e.g., Cas12a) CRISPR enzymes to create RNA-guided precise double-strand breaks in the genome. However, such technologies are limited in their capacity to make targeted large deletions. Recently, the Type I CRISPR system, which is prevalent in microbes and displays unique enzymatic features, has been harnessed to effectively create large chromosomal deletions in human cells. Type I CRISPR first uses a multisubunit ribonucleoprotein (RNP) complex called Cascade to find its guide-complementary target site, and then recruits a helicase-nuclease enzyme, Cas3, to travel along and shred the target DNA over a long distance with high processivity. When introduced into human cells as purified RNPs, the CRISPR-Cas3 complex can efficiently induce large genomic deletions of varying lengths (1-100 kb) from the CRISPR-targeted site. Because of this unique editing outcome, CRISPR-Cas3 holds great promise for tasks such as the removal of integrated viral genomes and the interrogation of structural variants affecting gene function and human disease. Here, we provide detailed protocols for introducing large deletions using CRISPR-Cas3. We describe step-by-step procedures for purifying the Type I-E CRISPR proteins Cascade and Cas3 from Thermobifida fusca, electroporating RNPs into human cells, and characterizing DNA deletions using PCR and sequencing. We focus here on human pluripotent stem cells due to their clinical potential, but these protocols will be broadly useful for other cell lines and model organisms for applications including large genomic deletion, full-gene or -chromosome removal, and CRISPR screening for noncoding elements, among others. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Expression and purification of Tfu Cascade RNP Support Protocol 1: Expression and purification of TfuCas3 protein Support Protocol 2: Culture of human pluripotent stem cells Basic Protocol 2: Introduction of Tfu Cascade RNP and Cas3 protein into hPSCs via electroporation Basic Protocol 3: Characterization of genomic DNA lesions using long-range PCR, TOPO cloning, and Sanger sequencing Alternate Protocol: Comprehensive analysis of genomic lesions by Tn5-based next-generation sequencing Support Protocol 3: Single-cell clonal isolation.