A Cas12a-based CRISPR interference system for multigene regulation in mycobacteria

CRISPR/Cas12a-mediated gene silencing across diverse functional genes demonstrates single gene-specific spacer efficacy in Mycobacterium smegmatis

BACKGROUND

Tuberculosis, a persistent global health threat, necessitates a comprehensive understanding of the genes and pathways crucial for the survival and virulence of the causative pathogen, Mycobacterium tuberculosis. Working with M. tuberculosis (M.tb) presents significant challenges; therefore, the use of M. smegmatis as a surrogate system for conducting genetic studies of M.tb has proven to be highly valuable. Development of novel genetic tools to probe cellular processes accelerates the progress in the field of drug development and also helps in understanding the basic physiology of the bacterium.

RESULTS

This study reports the successful implementation and evaluation of the CRISPR-Cas12a system for gene repression in Mycobacterium smegmatis, a surrogate for M. tuberculosis. We engineered a Cas12a-based CRISPR interference (CRISPRi) system and assessed its functionality. Targeting 45 genes with a single sgRNA per gene, we achieved efficient gene repression, leading to marked phenotypic changes. Each knockdown strain was evaluated individually for growth phenotypes, and a comparison of the results with the reported essential gene library probed with dCas9 demonstrated congruous results across diverse gene categories. The study shows that CRISPR/Cas12a system can be effectively utilised with a single gene specific target for efficient silencing of the gene and highlights the importance of subsequent growth assays required to evaluate the vulnerability of targeted gene silencing.

CONCLUSION

Our findings reveal the robustness and versatility of the dCas12a-CRISPRi system in M. smegmatis, providing a valuable tool for functional genomics research. This work showcases the potential of the dCas12a-CRISPRi system in investigating essential genes, enabling a deeper understanding of the biology and potential therapeutic targets in mycobacterium species.

CRISPR applications in microbial World: Assessing the opportunities and challenges

Genome editing has emerged during the past few decades in the scientific research area to manipulate genetic composition, obtain desired traits, and deal with biological challenges by exploring genetic traits and their sequences at a level of precision. The discovery of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) as a genome editing tool has offered a much better understanding of cellular and molecular mechanisms. This technology emerges as one of the most promising candidates for genome editing, offering several advantages over other techniques such as high accuracy and specificity. In the microbial world, CRISPR/Cas technology enables researchers to manipulate the genetic makeup of micro-organisms, allowing them to achieve almost impossible tasks. This technology initially discovered as a bacterial defense mechanism, is now being used for gene cutting and editing to explore more of its dimensions. CRISPR/Cas 9 systems are highly efficient and flexible, leading to its widespread uses in microbial research areas. Although this technology is widely used in the scientific community, many challenges, including off-target activity, low efficiency of Homology Directed Repair (HDR), and ethical considerations, still need to be overcome before it can be widely used. As CRISPR/Cas technology has revolutionized the field of microbiology, this review article aimed to present a comprehensive overview highlighting a brief history, basic mechanisms, and its application in the microbial world along with accessing the opportunities and challenges.

The rise and future of CRISPR-based approaches for high-throughput genomics

Clustered regularly interspaced short palindromic repeats (CRISPR) has revolutionized the field of genome editing. To circumvent the permanent modifications made by traditional CRISPR techniques and facilitate the study of both essential and nonessential genes, CRISPR interference (CRISPRi) was developed. This gene-silencing technique employs a deactivated Cas effector protein and a guide RNA to block transcription initiation or elongation. Continuous improvements and a better understanding of the mechanism of CRISPRi have expanded its scope, facilitating genome-wide high-throughput screens to investigate the genetic basis of phenotypes. Additionally, emerging CRISPR-based alternatives have further expanded the possibilities for genetic screening. This review delves into the mechanism of CRISPRi, compares it with other high-throughput gene-perturbation techniques, and highlights its superior capacities for studying complex microbial traits. We also explore the evolution of CRISPRi, emphasizing enhancements that have increased its capabilities, including multiplexing, inducibility, titratability, predictable knockdown efficacy, and adaptability to nonmodel microorganisms. Beyond CRISPRi, we discuss CRISPR activation, RNA-targeting CRISPR systems, and single-nucleotide resolution perturbation techniques for their potential in genome-wide high-throughput screens in microorganisms. Collectively, this review gives a comprehensive overview of the general workflow of a genome-wide CRISPRi screen, with an extensive discussion of strengths and weaknesses, future directions, and potential alternatives.

Application of functional genomics for domestication of novel non-model microbes

With the expansion of domesticated microbes producing biomaterials and chemicals to support a growing circular bioeconomy, the variety of waste and sustainable substrates that can support microbial growth and production will also continue to expand. The diversity of these microbes also requires a range of compatible genetic tools to engineer improved robustness and economic viability. As we still do not fully understand the function of many genes in even highly studied model microbes, engineering improved microbial performance requires introducing genome-scale genetic modifications followed by screening or selecting mutants that enhance growth under prohibitive conditions encountered during production. These approaches include adaptive laboratory evolution, random or directed mutagenesis, transposon-mediated gene disruption, or CRISPR interference (CRISPRi). Although any of these approaches may be applicable for identifying engineering targets, here we focus on using CRISPRi to reduce the time required to engineer more robust microbes for industrial applications.

ONE-SENTENCE SUMMARY

The development of genome scale CRISPR-based libraries in new microbes enables discovery of genetic factors linked to desired traits for engineering more robust microbial systems.

Systematic and scalable genome-wide essentiality mapping to identify nonessential genes in phages

Phages are one of the key ecological drivers of microbial community dynamics, function, and evolution. Despite their importance in bacterial ecology and evolutionary processes, phage genes are poorly characterized, hampering their usage in a variety of biotechnological applications. Methods to characterize such genes, even those critical to the phage life cycle, are labor intensive and are generally phage specific. Here, we develop a systematic gene essentiality mapping method scalable to new phage-host combinations that facilitate the identification of nonessential genes. As a proof of concept, we use an arrayed genome-wide CRISPR interference (CRISPRi) assay to map gene essentiality landscape in the canonical coliphages λ and P1. Results from a single panel of CRISPRi probes largely recapitulate the essential gene roster determined from decades of genetic analysis for lambda and provide new insights into essential and nonessential loci in P1. We present evidence of how CRISPRi polarity can lead to false positive gene essentiality assignments and recommend caution towards interpreting CRISPRi data on gene essentiality when applied to less studied phages. Finally, we show that we can engineer phages by inserting DNA barcodes into newly identified inessential regions, which will empower processes of identification, quantification, and tracking of phages in diverse applications.

Genome-scale CRISPRi screening: A powerful tool in engineering microbiology

Deciphering gene function is fundamental to engineering of microbiology. The clustered regularly interspaced short palindromic repeats (CRISPR) system has been adapted for gene repression across a range of hosts, creating a versatile tool called CRISPR interference (CRISPRi) that enables genome-scale analysis of gene function. This approach has yielded significant advances in the design of genome-scale CRISPRi libraries, as well as in applications of CRISPRi screening in medical and industrial microbiology. This review provides an overview of the recent progress made in pooled and arrayed CRISPRi screening in microorganisms and highlights representative studies that have employed this method. Additionally, the challenges associated with CRISPRi screening are discussed, and potential solutions for optimizing this strategy are proposed.

Mycobacterial Genetic Technologies for Probing the Host-Pathogen Microenvironment

Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, is one of the oldest and most successful pathogens in the world. Diverse selective pressures encountered within host cells have directed the evolution of unique phenotypic traits, resulting in the remarkable evolutionary success of this largely obligate pathogen. Despite centuries of study, the genetic repertoire utilized by Mtb to drive virulence and host immune evasion remains to be fully understood. Various genetic approaches have been and continue to be developed to tackle the challenges of functional gene annotation and validation in an intractable organism such as Mtb. In vitro and ex vivo systems remain the primary approaches to generate and confirm hypotheses that drive a general understanding of mycobacteria biology. However, it remains of great importance to characterize genetic requirements for successful infection within a host system as in vitro and ex vivo studies fail to fully replicate the complex microenvironment experienced by Mtb. In this review, we evaluate the employment of the mycobacterial genetic toolkit to probe the host-pathogen interface by surveying the current state of mycobacterial genetic studies within host systems, with a major focus on the murine model. Specifically, we discuss the different ways that these tools have been utilized to examine various aspects of infection, including bacterial survival/virulence, bacterial evasion of host immunity, and development of novel antibacterial/vaccine strategies.

Implementation of a mycobacterial CRISPRi platform in Mycobacterium abscessus and demonstration of the essentiality of ftsZMab

Mycobacterium abscessus (Mab) is a highly drug-resistant non-tuberculous mycobacterial species that causes debilitating TB-like pulmonary infections. The lack of genetic tools has hampered characterization of its extensive repertoire of virulence factors, antimicrobial resistance mechanisms, and drug targets. In this study, we evaluated the performance of a mycobacterial single plasmid CRISPRi-dCas9 system optimized for M. tuberculosis and M. smegmatis for inducible gene silencing in Mab. The efficacy of CRISPRi-mediated repression of two antibiotic resistance genes (blaMab, whiB7Mab) and two putative essential genes (ftsZMab,topAMab) was determined by measuring mRNA transcript levels and phenotypic outcomes. While our results support the utility of this mycobacterial CRISPRi dCas9Sth1 single-plasmid platform for inducible silencing of specific target genes in Mab, they also highlighted several caveats and nuances that may warrant species-specific optimization for Mab. We observed overall lower levels of gene repression in Mab including variable silencing of different target genes despite use of PAMs of similar predicted strength. In addition, leaky gene repression in the absence of inducer was noted for some genes but not others. Nonetheless, using CRISPRi we demonstrated the silencing of multiple target genes and validated ftsZMab as an essential gene and promising drug target for the first time.

CRISPR-Based Approaches for Gene Regulation in Non-Model Bacteria

CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) have become ubiquitous approaches to control gene expression in bacteria due to their simple design and effectiveness. By regulating transcription of a target gene(s), CRISPRi/a can dynamically engineer cellular metabolism, implement transcriptional regulation circuitry, or elucidate genotype-phenotype relationships from smaller targeted libraries up to whole genome-wide libraries. While CRISPRi/a has been primarily established in the model bacteria Escherichia coli and Bacillus subtilis, a growing numbering of studies have demonstrated the extension of these tools to other species of bacteria (here broadly referred to as non-model bacteria). In this mini-review, we discuss the challenges that contribute to the slower creation of CRISPRi/a tools in diverse, non-model bacteria and summarize the current state of these approaches across bacterial phyla. We find that despite the potential difficulties in establishing novel CRISPRi/a in non-model microbes, over 190 recent examples across eight bacterial phyla have been reported in the literature. Most studies have focused on tool development or used these CRISPRi/a approaches to interrogate gene function, with fewer examples applying CRISPRi/a gene regulation for metabolic engineering or high-throughput screens and selections. To date, most CRISPRi/a reports have been developed for common strains of non-model bacterial species, suggesting barriers remain to establish these genetic tools in undomesticated bacteria. More efficient and generalizable methods will help realize the immense potential of programmable CRISPR-based transcriptional control in diverse bacteria.