Optimized protocols for the characterization of Cas12a activities

Key thermodynamic characteristics of Cas9 and Cas12a endonucleases' cleavage of a DNA substrate containing a nucleotide mismatch in the region complementary to RNA

CRISPR-Cas9 and CRISPR-Cas12a are endonuclease systems widely used for genome editing, but their mechanisms of DNA cleavage, particularly in the presence of nucleotide mismatches, remain incompletely understood. This study deals with thermodynamic parameters governing the cleavage of DNA substrates-containing a mismatch in the region complementary to RNA-by Cas9 and Cas12a. Using a series of 55 bp DNA substrates with various mismatches, we investigated the cleavage efficiency, reaction kinetics, and thermodynamic stability of the Cas12a-crRNA complex and compared it with Cas9-sgRNA on the same substrates. Cas12a manifested strict specificity, with a mismatch leading to a significant reduction in cleavage efficiency or to nonspecific trans-cleavage, whereas Cas9 showed higher tolerance to each mismatch, especially in internal and distal regions. Thermodynamic calculations indicated that Cas12a-crRNA complexes are generally stabler with fully complementary DNA but are more destabilized by a mismatch than Cas9-sgRNA complexes are. Molecular dynamics simulations revealed that a mismatch causes greater structural destabilization in Cas12a than in Cas9, correlating with reduced cleavage efficiency. These findings highlight distinct mechanisms of mismatch recognition by Cas9 and Cas12a, provide insights into their enzymatic behavior, and inform the design of more precise genome-editing tools.

Enhanced production of functional CRISPR-AsCas12a protein in Escherichia coli

The CRISPR-Cas12a system is a groundbreaking tool widely used for genome editing and diagnostics in biotechnology and biomedicine research laboratories. Despite its growing application, studies optimizing Cas12a protein production at the laboratory scale using straightforward protocols remains scarce. This study aimed to enhance the lab-scale recombinant production of Acidaminococcus sp Cas12a protein (AsCas12a) in E. coli. Through targeted adjustments of simple parameters, AsCas12a production was significantly increased. The optimized conditions included the use of E. coli BL21(DE3), TB medium supplemented with 1 % glucose, induction with 0.3 mM IPTG for at least 6-9 h, and incubation at 30 °C. Notably, these conditions differ from conventional protocols typically used for Cas12a and related proteins, such as Streptococcus pyogenes Cas9. Upon combining all optimized parameters, AsCas12a production increased approximately 3-fold, from 0.95 mg/mL of bacterial lysate under non-optimized conditions to 3.73 mg/mL under optimized ones. After chromatographic purification, the final protein yield rose approximately 4.5-fold, from 5.2 to 23.4 mg/L of culture volume, without compromising functional activity. The purified AsCas12a retained full activity for programmable in vitro DNA cis-cleavage and collateral trans-cleavage, which was successfully applied to detect the N gene of SARS-CoV-2. This optimized method provide an efficient and high-yield approach for producing functional AsCas12a protein using accessible materials and conditions available to many research laboratories worldwide.

A novel method of species-specific molecular target mining and accurate discrimination of Bacillus cereus sensu lato

Bacillus cereus, a member of the Bacillus cereus sensu lato (B. cereus s.l.), is widely distributed in nature and can contaminate a variety of foods, leading to foodborne illnesses and substantial losses in the food industry. Although culture-based methods remain the gold standard for identifying B. cereus due to their high sensitivity under specific conditions, they are often complex and labor-intensive to implement. Furthermore, the high genetic similarity among certain members of the B. cereus s.l. makes it challenging to identify species-specific molecular targets, hindering the rapid and accurate differentiation of these bacteria. In this study, we introduce a novel method, comparative analysis based on whole genome slices (CAWGS), combined with the Basic Local Alignment Search Tool (BLAST) for efficient molecular target mining. Using CAWGS-BLAST and pan-genome analysis, we successfully identified new molecular targets for B. cereus, Bacillus thuringiensis, emetic B. cereus, Bacillus anthracis, Bacillus mycoides, Bacillus weihenstephanensis, and Bacillus megaterium. Based on these newly discovered targets, we developed a PCR-CRISPR/Cas12a method for detecting B. cereus s.l. and related species. Our research not only provides a rapid and accurate approach for discriminating B. cereus s.l. and related species, but also offers a universal and valuable reference for detecting foodborne pathogens, especially those with highly similar phenotypic and genetic characteristics.

Purification and in vivo, cell-free, and in vitro characterization of CRISPR-Cas12a2

The CRISPR-associated (Cas) nuclease Cas12a2 from Sulfuricurvum sp. PC08-66 (SuCas12a2) binds RNA targets with a complementary guide (g)RNA. Target RNA binding causes a major conformational rearrangement in Cas12a2 that activates a RuvC nuclease domain to collaterally cleave RNA, ssDNA and dsDNA, arresting growth and providing population-level immunity. Here, we report in vivo, cell-free, and in vitro methods to characterize the collateral cleavage activity of SuCas12a2 as well as a procedure for gRNA design. As part of the in vivo methods, we describe how to capture growth arrest through plasmid interference and induction of an SOS DNA damage response in the bacterium Escherichia coli. We further apply cell-free transcription-translation to affirm collateral cleavage activity triggered by an expressed RNA target. Finally, as part of the in vitro methods, we describe how to purify active nuclease and subsequently conduct biochemical cleavage assays. In total, the outlined methods should accelerate the exploration of SuCas12a2 and other related Cas nucleases, revealing new features of CRISPR biology and helping develop new CRISPR technologies for molecular diagnostics and other applications.

Advancements in genome editing tools for genetic studies and crop improvement

The rapid increase in global population poses a significant challenge to food security, compounded by the adverse effects of climate change, which limit crop productivity through both biotic and abiotic stressors. Despite decades of progress in plant breeding and genetic engineering, the development of new crop varieties with desirable agronomic traits remains a time-consuming process. Traditional breeding methods often fall short of addressing the urgent need for improved crop varieties. Genome editing technologies, which enable precise modifications at specific genomic loci, have emerged as powerful tools for enhancing crop traits. These technologies, including RNA interference, Meganucleases, ZFNs, TALENs, and CRISPR/Cas systems, allow for the targeted insertion, deletion, or alteration of DNA fragments, facilitating improvements in traits such as herbicide and insect resistance, nutritional quality, and stress tolerance. Among these, CRISPR/Cas9 stands out for its simplicity, efficiency, and ability to reduce off-target effects, making it a valuable tool in both agricultural biotechnology and plant functional genomics. This review examines the functional mechanisms and applications of various genome editing technologies for crop improvement, highlighting their advantages and limitations. It also explores the ethical considerations associated with genome editing in agriculture and discusses the potential of these technologies to contribute to sustainable food production in the face of growing global challenges.

Helical transition of the bridge helix of Cas12a is an allosteric regulator of R-loop formation and RuvC activation

CRISPR-Cas12a is widely used for genome editing and biomarker detection since it can create targeted double-stranded DNA breaks and promote non-specific DNA cleavage after identifying specific DNA. To mitigate the off-target DNA cleavage of Cas12a, we previously developed a Francisella novicida Cas12a variant (FnoCas12a KD2P ) by introducing double proline substitutions (K969P/D970P) in a conserved helix called the bridge helix (BH). In this work, we used cryogenic electron microscopy (cryoEM) to understand the molecular mechanisms of BH-mediated activation of Cas12a. We captured five structures of FnoCas12a KD2P at different states of conformational activation. Comparison with wild-type (FnoCas12a WT ) structures unravels a mechanism where BH acts as a trigger that allosterically activates REC lobe movements by tracking the number of base pairs in the growing RNA-DNA hybrid to undergo a loop-to-helical transition and bending to latch onto the hybrid. The transition of the BH is coupled to the previously reported loop-to-helix transition of the "lid", essential for opening RuvC endonuclease, through direct interactions of residues of the BH and the lid. We also observe structural details of cooperativity of BH and "helix-1" of RuvC for activation, a previously proposed interaction. Overall, our study enables development of high-fidelity Cas12a and Cas9 variants by BH-modifications.

CRISPR-Cas12a exhibits metal-dependent specificity switching

Cas12a is the immune effector of type V-A CRISPR-Cas systems and has been co-opted for genome editing and other biotechnology tools. The specificity of Cas12a has been the subject of extensive investigation both in vitro and in genome editing experiments. However, in vitro studies have often been performed at high magnesium ion concentrations that are inconsistent with the free Mg2+ concentrations that would be present in cells. By profiling the specificity of Cas12a orthologs at a range of Mg2+ concentrations, we find that Cas12a switches its specificity depending on metal ion concentration. Lowering Mg2+ concentration decreases cleavage defects caused by seed mismatches, while increasing the defects caused by PAM-distal mismatches. We show that Cas12a can bind seed mutant targets more rapidly at low Mg2+ concentrations, resulting in faster cleavage. In contrast, PAM-distal mismatches cause substantial defects in cleavage following formation of the Cas12a-target complex at low Mg2+ concentrations. We observe differences in Cas12a specificity switching between three orthologs that results in variations in the routes of phage escape from Cas12a-mediated immunity. Overall, our results reveal the importance of physiological metal ion conditions on the specificity of Cas effectors that are used in different cellular environments.

Electrochemical biosensing for E.coli detection based on triple helix DNA inhibition of CRISPR/Cas12a cleavage activity

BACKGROUND

Escherichia coli (E.coli) is both a commensal and a foodborne pathogenic bacterium in the human gastrointestinal tract, posing significant potential risks to human health and food safety. However, one of the major challenges in E.coli detection lies in the preparation and storage of antibodies. In traditional detection methods, antibodies are indispensable, but their instability often leads to experimental complexity and increased false positives. This underscores the need for new technologies and novel sensors. Therefore, the development of a simple and sensitive method for analyzing E.coli would make significant contributions to human health and food safety.

RESULTS

We constructed an electrochemical biosensor based on triple-helical DNA and entropy-driven amplification reaction (EDC) to inhibit the cleavage activity of Cas12a, enabling high-specificity detection of E.coli. Replacing antibodies with nucleic acid aptamers (Apt) as recognition elements, we utilized the triple-helical DNA generated by the binding of DNA2 and DNA5/DNA6 double-helical DNA through the entropy-driven amplification reaction to inhibit the collateral cleavage activity of clustered regularly interspaced short palindromic repeats gene editing system (CRISPR) and its associated proteins (Cas). By converting E.coli into electrical signals and recording signal changes in the form of square wave voltammetry (SWV), rapid detection of E.coli was achieved. Optimization of experimental conditions and data detection under the optimal conditions provided high sensitivity, low detection limits, and high specificity.

SIGNIFICANCE

With a minimal detection limit of 5.02 CFU/mL and a linear range of 1 × 102 - 1 × 107 CFU/mL, the suggested approach was successfully verified to analyze E.coli at various concentrations. Additionally, after examining E.coli samples from pure water and pure milk, the recoveries ranged between 95.76 and 101.20%, demonstrating the method's applicability. Additionally, it provides a feasible research direction for the detection of pathogenic bacteria causing other diseases using the CRISPR/Cas gene editing system.

Enhanced detection of Listeria monocytogenes using tetraethylenepentamine-functionalized magnetic nanoparticles and LAMP-CRISPR/Cas12a-based biosensor

BACKGROUND

Listeria monocytogenes is a pathogenic bacterium that can lead to severe illnesses, especially among vulnerable populations. Therefore, the development of rapid and sensitive detection methods is vital to prevent and manage foodborne diseases. In this study, we used tetraethylenepentamine (TEPA)-functionalized magnetic nanoparticles (MNPs) and a loop-mediated isothermal amplification (LAMP)-based CRISPR/Cas12a-based biosensor to concentrate and detect, respectively, L. monocytogenes. LAMP enables DNA amplification at a constant temperature, providing a highly suitable approach for point-of-care testing (POCT). The ability of CRISPR/Cas12a to cleave ssDNA reporter, coupled with TEPA-functionalized MNPs effective attachment to negatively charged bacteria, forms a promising biosensor.

RESULTS

The LAMP assay was meticulously developed by selecting specific primers and designing crRNA sequences targeting a specific region within the hly gene of L. monocytogenes. We selected primer and refined the amplification conditions by systematically exploring a temperature range from 59 °C to 69 °C, ensuring the attainment of optimal performance. This process was complemented by systematic optimization of LAMP-CRISPR/Cas12a system parameters. In particular, we successfully established the optimal ssDNA reporter concentrations (0-1.2 μM) and Cas12a-mediated trans-cleavage times (0-20 min), crucial components that underpin the effectiveness of the LAMP-CRISPR/Cas12a-based biosensor. For optimizing parameters in capturing L. monocytogenes using TEPA-functionalized MNPs, capture efficiency was significantly enhanced through adjustments in TEPA-functionalized MNPs concentration, incubation times, and magnetic separation duration. Large-volume (20 mL) magnetic separation exhibited a 10-fold sensitivity improvement over conventional methods. Utilizing TEPA-functionalized MNPs, the LAMP-CRISPR/Cas12a-based biosensor achieved detection limits of 100 CFU mL-1 in pure cultures and 100 CFU g-1 in enoki mushrooms.

SIGNIFICANCE

The integration of this novel technique with the LAMP-CRISPR/Cas12a-based biosensor enhances the accuracy and sensitivity of L. monocytogenes detection in foods, and it can be a promising biosensor for POCT. The 10-fold increase in sensitivity compared to conventional methods makes this approach a groundbreaking advancement in pathogenic bacteria detection for food safety and public health.