Guide RNA Engineering Enables Dual Purpose CRISPR-Cpf1 for Simultaneous Gene Editing and Gene Regulation in Yarrowia lipolytica

Rewiring Metabolic Flux in Corynebacterium glutamicum Using a CRISPR/dCpf1-Based Bifunctional Regulation System

Corynebacterium glutamicum, a microorganism classified as generally recognized as safe for use in the industrial production of food raw materials and additives, has encountered challenges in achieving widespread adoption and popularization as microbial cell factories. These obstacles arise from the intricate nature of manipulating metabolic flux through conventional methods, such as gene knockout and enzyme overexpression. To address this challenge, we developed a CRISPR/dCpf1-based bifunctional regulation system to bidirectionally regulate the expression of multiple genes in C. glutamicum. Specifically, through fusing various transcription factors to the C-terminus of dCpf1, the resulting dCpf1-SoxS exhibited both CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) capabilities in C. glutamicum by altering the binding sites of crRNAs. The bifunctional regulation system was used to fine-tune metabolic flux from shikimic acid (SA) and l-serine biosynthesis, resulting in 27-fold and 10-fold increases in SA and l-serine production, respectively, compared to the original strain. These findings highlight the potential of the CRISPR/dCpf1-based bifunctional regulation system in effectively enhancing the yield of target products in C. glutamicum.

Recent application of CRISPR-Cas12 and OMEGA system for genome editing

In 2012, it was discovered that precise gene editing could be induced in target DNA using the reprogrammable characteristics of the CRISPR system. Since then, several studies have investigated the potential of the CRISPR system to edit various biological organisms. For the typical CRISPR system obtained from bacteria and archaea, many application studies have been conducted and have spread to various fields. To date, orthologs with various characteristics other than CRISPR-Cas9 have been discovered and are being intensively studied in the field of gene editing. CRISPR-Cas12 and its varied orthologs are representative examples of genome editing tools and have superior properties in terms of in vivo target gene editing compared with Cas9. Recently, TnpB and Fanzor of the OMEGA (obligate mobile element guided activity) system were identified to be the ancestor of CRISPR-Cas12 on the basis of phylogenetic analysis. Notably, the compact sizes of Cas12 and OMEGA endonucleases allow adeno-associated virus (AAV) delivery; hence, they are set to challenge Cas9 for in vivo gene therapy. This review is focused on these RNA-guided reprogrammable endonucleases: their structure, biochemistry, off-target effects, and applications in therapeutic gene editing.

Genome Editing, Transcriptional Regulation, and Forward Genetic Screening Using CRISPR-Cas12a Systems in Yarrowia lipolytica

Class II Type V endonucleases have increasingly been adapted to develop sophisticated and easily accessible synthetic biology tools for genome editing, transcriptional regulation, and functional genomic screening in a wide range of organisms. One such endonuclease, Cas12a, presents itself as an attractive alternative to Cas9-based systems. The ability to mature its own guide RNAs (gRNAs) from a single transcript has been leveraged for easy multiplexing, and its lack of requirement of a tracrRNA element, also allows for short gRNA expression cassettes. To extend these functionalities into the industrially relevant oleaginous yeast Yarrowia lipolytica, we developed a set of CRISPR-Cas12a vectors for easy multiplexed gene knockout, repression, and activation. We further extended the utility of this CRISPR-Cas12a system to functional genomic screening by constructing a genome-wide guide library targeting every gene with an eightfold coverage. Pooled CRISPR screens conducted with this library were used to profile Cas12a guide activities and develop a machine learning algorithm that could accurately predict highly efficient Cas12a gRNA. In this protocols chapter, we first present a method by which protein coding genes may be functionally disrupted via indel formation with CRISPR-Cas12a systems. Further, we describe how Cas12a fused to a transcriptional regulator can be used in conjunction with shortened gRNA to achieve transcriptional repression or activation. Finally, we describe the design, cloning, and validation of a genome-wide library as well as a protocol for the execution of a pooled CRISPR screen, to determine guide activity profiles in a genome-wide context in Y. lipolytica. The tools and strategies discussed here expand the list of available synthetic biology tools for facile genome engineering in this industrially important host.

High-Efficiency Multiplexed Cytosine Base Editors for Natural Product Synthesis in Yarrowia lipolytica

Yarrowia lipolytica is an industrial host with a high fatty acid flux. Even though CRISPR-based tools have accelerated its metabolic engineering, there remains a need to develop tools for rapid multiplexed strain engineering to accelerate the design-build-test-learn cycle. Base editors have the potential to perform high-efficiency multiplexed gene editing because they do not depend upon double-stranded DNA breaks. Here, we identified that base editors are less toxic than CRISPR-Cas9 for multiplexed gene editing. We increased the editing efficiency by removing the extra nucleotides between tRNA and gRNA and increasing the base editor and gRNA copy number in a Ku70 deficient strain. We achieved five multiplexed gene editing in the ΔKu70 strain at 42% efficiency. Initially, we were unsuccessful at performing multiplexed base editing in NHEJ competent strain; however, we increased the editing efficiency by using a co-selection approach to enrich base editing events. Base editor-mediated canavanine gene (CAN1) knockout provided resistance to the import of canavanine, which enriched the base editing in other unrelated genetic loci. We performed multiplexed editing of up to three genes at 40% efficiency in the Po1f strain through the CAN1 co-selection approach. Finally, we demonstrated the application of multiplexed cytosine base editor for rapid multigene knockout to increase naringenin production by 2-fold from glucose or glycerol as a carbon source.

acCRISPR: an activity-correction method for improving the accuracy of CRISPR screens

High throughput CRISPR screens are revolutionizing the way scientists unravel the genetic underpinnings of engineered and evolved phenotypes. One of the critical challenges in accurately assessing screening outcomes is accounting for the variability in sgRNA cutting efficiency. Poorly active guides targeting genes essential to screening conditions obscure the growth defects that are expected from disrupting them. Here, we develop acCRISPR, an end-to-end pipeline that identifies essential genes in pooled CRISPR screens using sgRNA read counts obtained from next-generation sequencing. acCRISPR uses experimentally determined cutting efficiencies for each guide in the library to provide an activity correction to the screening outcomes via calculation of an optimization metric, thus determining the fitness effect of disrupted genes. CRISPR-Cas9 and -Cas12a screens were carried out in the non-conventional oleaginous yeast Yarrowia lipolytica and acCRISPR was used to determine a high-confidence set of essential genes for growth under glucose, a common carbon source used for the industrial production of oleochemicals. acCRISPR was also used in screens quantifying relative cellular fitness under high salt conditions to identify genes that were related to salt tolerance. Collectively, this work presents an experimental-computational framework for CRISPR-based functional genomics studies that may be expanded to other non-conventional organisms of interest.

SGRN: A Cas12a-driven Synthetic Gene Regulatory Network System

Gene regulatory networks, which control gene expression patterns in development and in response to stimuli, use regulatory logic modules to coordinate inputs and outputs. One example of a regulatory logic module is the gene regulatory cascade (GRC), where a series of transcription factor genes turn on in order. Synthetic biologists have derived artificial systems that encode regulatory rules, including GRCs. Furthermore, the development of single-cell approaches has enabled the discovery of gene regulatory modules in a variety of experimental settings. However, the tools available for validating these observations remain limited. Based on a synthetic GRC using DNA cutting-defective Cas9 (dCas9), we designed and implemented an alternative synthetic GRC utilizing DNA cutting-defective Cas12a (dCas12a). Comparing the ability of these two systems to express a fluorescent reporter, the dCas9 system was initially more active, while the dCas12a system was more streamlined. Investigating the influence of individual components of the systems identified nuclear localization as a major driver of differences in activity. Improving nuclear localization for the dCas12a system resulted in 1.5-fold more reporter-positive cells and a 15-fold increase in reporter intensity relative to the dCas9 system. We call this optimized system the "Synthetic Gene Regulatory Network" (SGRN, pronounced "sojourn").

Metabolic Engineering of Yarrowia lipolytica for Terpenoid Production: Tools and Strategies

Terpenoids are a diverse group of compounds with isoprene units as basic building blocks. They are widely used in the food, feed, pharmaceutical, and cosmetic industries due to their diverse biological functions such as antioxidant, anticancer, and immune enhancement. With an increase in understanding the biosynthetic pathways of terpenoids and advances in synthetic biology techniques, microbial cell factories have been built for the heterologous production of terpenoids, with the oleaginous yeast Yarrowia lipolytica emerging as an outstanding chassis. In this paper, recent progress in the development of Y. lipolytica cell factories for terpenoid production with a focus on the advances in novel synbio tools and metabolic engineering strategies toward enhanced terpenoid biosynthesis is reviewed.

An update on CRISPR-Cas12 as a versatile tool in genome editing

Gene editing techniques, which help in modification of any DNA sequence at ease, have revolutionized the world of Genetic engineering. Although there are other gene-editing techniques, CRISPR has emerged as the chief and most preferred tool due to its simplicity and capacity to execute effective gene editing in a wide range of organisms. Although Cas9 has widely been employed for genetic modification over the years, Cas12 systems have lately emerged as a viable option. This review primarily focuses on assessing Cas12-mediated mutagenesis and elucidating the editing efficacy of both Cpf1 (Cas12a) and C2c1 (Cas12b) systems in microbes, plants, and other species. Also, we reviewed several genetic alterations that have been performed with these Cas12 systems to improve editing efficiency. Furthermore, the experimental benefits and applications of Cas12 systems are highlighted in this study.

Phosphate limitation as crucial factor to enhance yeast lipid production from short-chain fatty acids

Microbial lipids for chemical synthesis are commonly obtained from sugar-based substrates which in most cases is not economically viable. As a low-cost carbon source, short-chain fatty acids (SCFAs) that can be obtained from food wastes offer an interesting alternative for achieving an affordable lipid production process. In this study, SCFAs were employed to accumulate lipids using Yarrowia lipolytica ACA DC 50109. For this purpose, different amounts of SCFAs, sulfate, phosphate and carbon: phosphate ratios were used in both synthetic and real SCFAs-rich media. Although sulfate limitation did not increase lipid accumulation, phosphate limitation was proved to be an optimal strategy for increasing lipid content and lipid yields in both synthetic and real media, reaching a lipid productivity up to 8.95 g/L h. Remarkably, the highest lipid yield (0.30 g/g) was achieved under phosphate absence condition (0 g/L). This fact demonstrated the suitability of using low-phosphate concentrations to boost lipid production from SCFAs.

Genome-wide CRISPR-Cas9 screen reveals a persistent null-hyphal phenotype that maintains high carotenoid production in Yarrowia lipolytica

Yarrowia lipolytica is a metabolic engineering host of growing industrial interest due to its ability to metabolize hydrocarbons, fatty acids, glycerol, and other renewable carbon sources. This dimorphic yeast undergoes a stress-induced transition to a multicellular hyphal state, which can negatively impact biosynthetic activity, reduce oxygen and nutrient mass transfer in cell cultures, and increase culture viscosity. Identifying mutations that prevent the formation of hyphae would help alleviate the bioprocess challenges that they create. To this end, we conducted a genome-wide CRISPR screen to identify genetic knockouts that prevent the transition to hyphal morphology. The screen identified five mutants with a null-hyphal phenotype-ΔRAS2, ΔRHO5, ΔSFL1, ΔSNF2, and ΔPAXIP1. Of these hits, only ΔRAS2 suppressed hyphal formation in an engineered lycopene production strain over a multiday culture. The RAS2 knockout was also the only genetic disruption characterized that did not affect lycopene production, producing more than 5 mg L^-1 OD^-1 from a heterologous pathway with enhanced carbon flux through the mevalonate pathway. These data suggest that a ΔRAS2 mutant of Y. lipolytica could prove useful in engineering a metabolic engineering host of the production of carotenoids and other biochemicals.

Genome-wide functional screens enable the prediction of high activity CRISPR-Cas9 and -Cas12a guides in Yarrowia lipolytica

Genome-wide functional genetic screens have been successful in discovering genotype-phenotype relationships and in engineering new phenotypes. While broadly applied in mammalian cell lines and in E. coli, use in non-conventional microorganisms has been limited, in part, due to the inability to accurately design high activity CRISPR guides in such species. Here, we develop an experimental-computational approach to sgRNA design that is specific to an organism of choice, in this case the oleaginous yeast Yarrowia lipolytica. A negative selection screen in the absence of non-homologous end-joining, the dominant DNA repair mechanism, was used to generate single guide RNA (sgRNA) activity profiles for both SpCas9 and LbCas12a. This genome-wide data served as input to a deep learning algorithm, DeepGuide, that is able to accurately predict guide activity. DeepGuide uses unsupervised learning to obtain a compressed representation of the genome, followed by supervised learning to map sgRNA sequence, genomic context, and epigenetic features with guide activity. Experimental validation, both genome-wide and with a subset of selected genes, confirms DeepGuide's ability to accurately predict high activity sgRNAs. DeepGuide provides an organism specific predictor of CRISPR guide activity that with retraining could be applied to other fungal species, prokaryotes, and other non-conventional organisms.

Potential of food waste hydrolysate as an alternative carbon source for microbial oil synthesis

Volatile fatty acids (VFAs) have great potential as cheap raw materials in microbial oil synthesis and reducing the cost of substrates is essential for the development of microbial oil biosynthesis. In this study, the food waste hydrolysate and synthetic VFAs media were both used as substrate to synthesis microbial oil. The optimal short-chain VFAs ratio for microbial oil synthesis is 20:5:5 and increasing the proportion of propionic acid is the key to obtaining odd fatty acids. The hydrolysate obtained from food waste under the total solid condition of 2:1 and pH 5 is the most suitable medium for microbial oil synthesis. The biological products obtained from food waste hydrolysate were comparable to synthetic VFAs media, obtaining a 34.02% of lipid content. Results prove that food waste hydrolysate has great potential as the available feedstock for microbial oil synthesis and a promising application value in food waste recycling.

A versatile toolbox for CRISPR-based genome engineering in Pichia pastoris

Pichia pastoris has gained much attention as a popular microbial cell factory for the production of recombinant proteins and high-value chemicals from laboratory to industrial scale. However, the lack of convenient and efficient genome engineering tools has impeded further applications of Pichia pastoris towards metabolic engineering and synthetic biology. Here, we report a CRISPR-based toolbox for gene editing and transcriptional regulation in P. pastoris. Based on the previous attempts in P. pastoris, we constructed a CRISPR/Cas9 system for gene editing using the RNA Pol-III-driven expression of sgRNA. The system was used to rapidly recycle the selectable marker with an eliminable episomal plasmid and achieved up to 100% knockout efficiency. Via dCas9 fused with transcriptional repressor (Mix1/RD1152) or activator (VPR), a flexible toolbox for regulation of gene expression was developed. The reporter gene eGFP driven by yeast pGAP or pCYC1 promoter showed strong inhibition (above 70%) and up to ~ 3.5-fold activation. To implement the combinatorial genetic engineering strategy, the CRISPR system contained a single Cas9-VPR protein, and engineered gRNA was introduced in P. pastoris for simultaneous gene activation, repression, and editing (CRISPR-ARE). We demonstrated that CRISPR-ARE was highly efficient for eGFP activation, mCherry repression, and ADE2 disruption, individually or in a combinatorial manner with a stable expression of multiplex sgRNAs. The simple and multifunctional toolkit demonstrated in this study will accelerate the application of P. pastoris in metabolic engineering and synthetic biology. KEY POINTS: • An eliminable CRISPR/Cas9 system yielded a highly efficient knockout of genes. • Simplified CRISPR/dCas9-based tools enabled transcriptional regulation of targeted genes. • CRISPR-ARE system achieved simultaneous gene activation, repression, and editing in P. pastoris.

Advances and Opportunities of CRISPR/Cas Technology in Bioengineering Non-conventional Yeasts

Non-conventional yeasts have attracted a growing interest on account of their excellent characteristics. In recent years, the emerging of CRISPR/Cas technology has improved the efficiency and accuracy of genome editing. Utilizing the advantages of CRISPR/Cas in bioengineering of non-conventional yeasts, quite a few advancements have been made. Due to the diversity in their genetic background, the ways for building a functional CRISPR/Cas system of various species non-conventional yeasts were also species-specific. Herein, we have summarized the different strategies for optimizing CRISPR/Cas systems in different non-conventional yeasts and their biotechnological applications in the construction of cell factories. In addition, we have proposed some potential directions for broadening and improving the application of CRISPR/Cas technology in non-conventional yeasts.

A robust method for protein depletion based on gene editing

The technology of clustered regularly interspaced short palindromic repeats (CRISPR)-associated nuclease Cas9 (CRISPR-Cas9) is a powerful system for protein depletion resulting from insertions and deletions following Cas9 cleavage of genome at specific site in vitro and in vivo. We herein present a relatively standard protocol for protein depletion in a step-by-step procedure, including guide RNA designation and vector construction, lentivirus production, cell selection, and experimentally validate the function of targeted protein. We exemplified this approach by editing PDGFRβ in human epithelial cells, and expected that this simplified and detailed protocol will be more broadly applied on specific genes to aid understanding gene functions.

A Cascade Signal Amplification Based on Dynamic DNA Nanodevices and CRISPR/Cas12a Trans-cleavage for Highly Sensitive MicroRNA Sensing

The variations of microRNA (miRNA) expression can be valuable biomarkers in disease diagnosis and prognosis. However, current miRNA detection techniques mainly rely on reverse transcription and template replication, which suffer from slowness, contamination risk, and sample loss. To address these limitations, here we introduce a cascade toehold-mediated strand displacement reaction (CTSDR) and CRISPR/Cas12a trans-cleavage for highly sensitive fluorescent miRNA sensing, namely CTSDR-Cas12a. In this work, the target miRNA hybridizes with the terminal toehold site of a rationally designed probe and subsequently initiates dynamic CTSDR, leading to enzyme-free target recycling and the production of multiple programmable DNA duplexes. The obtained DNA duplex acts as an activator to trigger Cas12a trans-cleavage, generating significantly amplified fluorescence readout for highly sensitive detection of the miRNA target. Under the optimal conditions, the developed sensing method can detect target miRNA down to 70.28 fM with a wide linear range from 100 fM to 100 pM. In particular, by designing a set of probes and crRNAs, we demonstrate its broad applicability for the detection of six kinds of miRNAs with high sequence specificity. Furthermore, the method can be satisfactorily applied to monitor miR-21 in total RNA extracted from cells and clinical serum samples. Considering the high sensitivity, specificity, universality, and ease of handling, this strategy provides a great potential platform for the detection of miRNA biomarkers in molecular diagnostic practice.

Systems-level approaches for understanding and engineering of the oleaginous cell factory Yarrowia lipolytica

Concerns about climate change and the search for renewable energy sources together with the goal of attaining sustainable product manufacturing have boosted the use of microbial platforms to produce fuels and high-value chemicals. In this regard, Yarrowia lipolytica has been known as a promising yeast with potentials in diverse array of biotechnological applications such as being a host for different oleochemicals, organic acid, and recombinant protein production. Having a rapidly increasing number of molecular and genetic tools available, Y. lipolytica has been well studied amongst oleaginous yeasts and metabolic engineering has been used to explore its potentials. More recently, with the advancement in systems biotechnology and the implementation of mathematical modeling and high throughput omics data-driven approaches, in-depth understanding of cellular mechanisms of cell factories have been made possible resulting in enhanced rational strain design. In case of Y. lipolytica, these systems-level studies and the related cutting-edge technologies have recently been initiated which is expected to result in enabling the biotechnology sector to rationally engineer Y. lipolytica-based cell factories with favorable production metrics. In this regard, here, we highlight the current status of systems metabolic engineering research and assess the potential of this yeast for future cell factory design development.

Microbial lipids from organic wastes: Outlook and challenges

Microbial lipids have recently drawn a lot of attention as renewable sources for biochemicals production. Strong research efforts have been addressed to efficiently use organic wastes as carbon source for microbial lipids, which would definitively increase the profitability of the production process and boost a bio-based economy. This review compiles interesting traits of oleaginous microorganisms and highlights current trends on microbial- and process-oriented approaches to maximize microbial oil production from inexpensive substrates like lignocellulosic sugars, volatile fatty acids and glycerol. Furthermore, downstream processes such as cell harvesting or lipid extraction, that are decisive for the cost-effectiveness of the process, are discussed. To underpin microbial oils within the so demanded circular economy, associated challenges, recent advances and possible industrial applications that are also identified in this review.

The Versatile Type V CRISPR Effectors and Their Application Prospects

The class II clustered regularly interspaced short palindromic repeats (CRISPR)–Cas systems, characterized by a single effector protein, can be further subdivided into types II, V, and VI. The application of the type II CRISPR effector protein Cas9 as a sequence-specific nuclease in gene editing has revolutionized this field. Similarly, Cas13 as the effector protein of type VI provides a convenient tool for RNA manipulation. Additionally, the type V CRISPR–Cas system is another valuable resource with many subtypes and diverse functions. In this review, we summarize all the subtypes of the type V family that have been identified so far. According to the functions currently displayed by the type V family, we attempt to introduce the functional principle, current application status, and development prospects in biotechnology for all major members.

Guide RNA Design for Genome-Wide CRISPR Screens in Yarrowia lipolytica

Genome-wide functional genomic screens are essential to determining the genetic underpinning of a biological process. Novel and powerful tools for perturbing gene function, with the help of genetic and epigenetic information, have made it possible to systematically investigate the contribution of every gene to evolved and engineered phenotypes. Functional genomics and screening for enhanced phenotypes become ever more important when dealing with nonconventional hosts. Non-model organisms are valuable to metabolic engineering as they present a range of desirable phenotypes and can help in avoiding complex and intensive engineering of less suitable hosts that do not possess the desired phenotype(s). Domestication of such hosts however requires a suite of synthetic biology tools that allow for targeted genome engineering, regulation of gene expression, and genome-wide mutational screens. The widespread adoption of CRISPR-Cas9 and CRISPR-Cpf1 based systems has allowed for such screens in many organisms. Key considerations in any genome-wide CRISPR screen are the design of a set of unique guide RNAs targeting the required set of genes in the genome and the design of nontargeting guide RNAs that function as appropriate negative controls for the experiment. In this methods chapter, we present protocols for the design of guides for a CRISPR screen, targeting every gene in the genome of the industrially relevant oleaginous yeast Yarrowia lipolytica. The first set of protocols describes the algorithm for the design of genome targeting and nontargeting guides for a genome-wide CRISPR-Cpf1 screen. The second set of protocols describes modifications to the first for the design of guides for a CRISPR-Cas9 screen. The strategies described here should serve as an efficient guide to design a library of gRNAs for most genome-wide CRISPR screens.

Genome editing systems across yeast species

Yeasts are used to produce a myriad of value-added compounds. Engineering yeasts into cost-efficient cell factories is greatly facilitated by the availability of genome editing tools. While traditional engineering techniques such as homologous recombination-based gene knockout and pathway integration continue to be widely used, novel genome editing systems including multiplexed approaches, bacteriophage integrases, CRISPR-Cas systems, and base editors are emerging as more powerful toolsets to accomplish rapid genome scale engineering and phenotype screening. In this review, we summarized the techniques which have been successfully implemented in model yeast Saccharomyces cerevisiae as well as non-conventional yeast species. The mechanisms and applications of various genome engineering systems are discussed and general guidelines to expand genome editing systems from S. cerevisiae to other yeast species are also highlighted.

A Single Cas9-VPR Nuclease for Simultaneous Gene Activation, Repression, and Editing in Saccharomyces cerevisiae

Combinatorial metabolic engineering has been widely established for the development of efficient microbial cell factories to produce the products of interest by precisely regulating the expression levels of multiple genes simultaneously. Here, we report a novel multifunctional CRISPR system that enables simultaneous gene activation, repression, and editing (CRISPR-ARE) with a single Cas9-VPR protein for combinatorial metabolic engineering applications in Saccharomyces cerevisiae. Via gRNA engineering, we achieved orthogonal transcriptional regulations and genome editing using the nuclease active Cas9-VPR fusion protein, individually or in a combinatorial manner. After establishing a system for stable expression of multiple gRNAs on the same plasmid, we first demonstrated CRISPR-ARE for simultaneous mCherry activation, mVenus repression, and ADE2 disruption in a fluorescence reporter strain. Subsequently, we adopted CRISPR-ARE for simple and fast combinatorial metabolic engineering, which improved the production of α-santalene for 2.66-fold in a single step. Because of its simplicity and modularity, the developed CRISPR-ARE system could be applied for facile multifunctional metabolic engineering of microbial cell factories, particularly for which only a few CRISPR proteins have been characterized.