Improving therapeutic protein secretion in the probiotic yeast Saccharomyces boulardii using a multifactorial engineering approach

Strategic approaches for designing yeast strains as protein secretion and display platforms

Yeast has been established as a versatile platform for expressing functional molecules, owing to its well-characterized biology and extensive genetic modification tools. Compared to prokaryotic systems, yeast possesses advanced cellular mechanisms that ensure accurate protein folding and post-translational modifications. These capabilities are particularly advantageous for the expression of human-derived functional proteins. However, designing yeast strains as an expression platform for proteins requires the integration of molecular and cellular functions. By delving into the complexities of yeast-based expression systems, this review aims to empower researchers with the knowledge to fully exploit yeast as a functional platform to produce a diverse range of proteins. This review includes an exploration of the host strains, gene cassette structures, as well as considerations for maximizing the efficiency of the expression system. Through this in-depth analysis, the review anticipates stimulating further innovation in the field of yeast biotechnology and protein engineering.

Bioengineered Probiotics for Clostridioides difficile Infection: An Overview of the Challenges and Potential for This New Treatment Approach

The rapid increase in microbial antibiotic resistance in Clostridioides difficile (C. difficile) strains and the formation of hypervirulent strains have been associated with a global increase in the incidence of C. difficile infection (CDI) and subsequently, an increase in the rate of recurrence. These consequences have led to an urgent need to develop new and promising alternative strategies to control this pathogen. Engineered probiotics are exciting new bacterial strains produced by editing the genome of the original probiotics. Recently, engineered probiotics have been used to develop delivery vehicles for vaccines, diagnostics, and therapeutics. Recent studies have demonstrated engineered probiotics may potentially be an effective approach to control or treat CDI. This review provides a brief overview of the considerations for engineered probiotics for medicinal use, with a focus on recent preclinical research using engineered probiotics to prevent or treat CDI. We also address the challenges faced in the production of engineered strains and how they may be overcome in the application of these agents to meet patient needs in the future.

Engineering Saccharomyces boulardii for enhanced surface display capacity

Saccharomyces boulardii (Sb) has gained significant attention for its potential therapeutic application as a probiotic yeast strain. Current approaches often leverage its secretion and display capabilities to deliver therapeutic agents aimed at alleviating intestinal disorders. However, relatively few studies have focused on optimizing its display efficiency. In this study, we evaluated two surface display systems, Aga2- and Sed1-based, for use in Sb by systematically modifying display cassette components and the host strain. Initially, both systems were tested in Saccharomyces cerevisiae (Sc) and Sb to validate their design. Sc consistently outperformed Sb in both display expression and efficiency, highlighting the need for further optimization in Sb. To enhance the display efficiency in Sb, we investigated specific modifications to the display cassette, including the use of linker sequences for Aga2 and variations in anchor length for Sed1. These experiments identified key factors influencing display performance. Subsequently, we engineered a modified Sb strain, LIP02, by overexpressing AGA1 and deleting cell wall-related genes (CCW12, CCW14, and FYV5). These modifications were expected to expand the available docking sites for the protein of interest (POI) and improve overall protein secretion and display efficiency. As a result, the modified strain exhibited a significant enhancement in display capacity compared to the wild-type Sb strain. Furthermore, genome integration of the display cassette in LIP02 enhanced both stability and expression compared to plasmid-based systems. Importantly, the functionality of β-glucosidase displayed on LIP02 was preserved, as demonstrated by improved enzymatic activity and robust growth on cellobiose as the sole carbon source. These findings establish LIP02 as a superior host for surface display applications in Sb, offering a more stable and efficient platform for the expression of therapeutic proteins and other functional biomolecules.

Enhancing intestinal absorption of a macromolecule through engineered probiotic yeast in the murine gastrointestinal tract

Oral administration of therapeutic peptides is limited by poor intestinal absorption. Use of engineered microorganisms as drug delivery vehicles can overcome the challenges faced by conventional delivery methods. The potential of engineered microorganisms to act synergistically with the therapeutics they deliver opens new horizons for noninvasive treatment modalities. This study engineered a probiotic yeast, Saccharomyces boulardii, to produce cell-penetrating peptides (CPPs) in situ for enhanced intestinal permeability. Four CPPs were integrated into the yeast chromosome: RRL helix, Shuffle, Penetramax, and PN159. In vitro tests on a Caco-2 cell model showed that three CPP-producing strains increased permeability without causing permanent damage. In vivo experiments on mice revealed that Sb PN159 administration over 10 days significantly increased FITC-dextran translocation into the bloodstream without causing inflammation. This study demonstrates, for the first time, the ability of an engineered microorganism to modulate host permeability for improved intestinal absorption of a macromolecule.

A yeast-based oral therapeutic delivers immune checkpoint inhibitors to reduce intestinal tumor burden

Engineered probiotics are an emerging platform for in situ delivery of therapeutics to the gut. Herein, we developed an orally administered, yeast-based therapeutic delivery system to deliver next-generation immune checkpoint inhibitor (ICI) proteins directly to gastrointestinal tumors. We engineered Saccharomyces cerevisiae var. boulardii (Sb), a probiotic yeast with high genetic tractability and innate anticancer activity, to secrete "miniature" antibody variants that target programmed death ligand 1 (Sb_haPD-1). When tested in an ICI-refractory colorectal cancer (CRC) mouse model, Sb_haPD-1 significantly reduced intestinal tumor burden and resulted in significant shifts to the immune cell profile and microbiome composition. This oral therapeutic platform is modular and highly customizable, opening new avenues of targeted drug delivery that can be applied to treat a myriad of gastrointestinal malignancies.

Omics-driven onboarding of the carotenoid producing red yeast Xanthophyllomyces dendrorhous CBS 6938

Transcriptomics is a powerful approach for functional genomics and systems biology, yet it can also be used for genetic part discovery. Here, we derive constitutive and light-regulated promoters directly from transcriptomics data of the basidiomycete red yeast Xanthophyllomyces dendrorhous CBS 6938 (anamorph Phaffia rhodozyma) and use these promoters with other genetic elements to create a modular synthetic biology parts collection for this organism. X. dendrorhous is currently the sole biotechnologically relevant yeast in the Tremellomycete class-it produces large amounts of astaxanthin, especially under oxidative stress and exposure to light. Thus, we performed transcriptomics on X. dendrorhous under different wavelengths of light (red, green, blue, and ultraviolet) and oxidative stress. Differential gene expression analysis (DGE) revealed that terpenoid biosynthesis was primarily upregulated by light through crtI, while oxidative stress upregulated several genes in the pathway. Further gene ontology (GO) analysis revealed a complex survival response to ultraviolet (UV) where X. dendrorhous upregulates aromatic amino acid and tetraterpenoid biosynthesis and downregulates central carbon metabolism and respiration. The DGE data was also used to identify 26 constitutive and regulated genes, and then, putative promoters for each of the 26 genes were derived from the genome. Simultaneously, a modular cloning system for X. dendrorhous was developed, including integration sites, terminators, selection markers, and reporters. Each of the 26 putative promoters were integrated into the genome and characterized by luciferase assay in the dark and under UV light. The putative constitutive promoters were constitutive in the synthetic genetic context, but so were many of the putative regulated promoters. Notably, one putative promoter, derived from a hypothetical gene, showed ninefold activation upon UV exposure. Thus, this study reveals metabolic pathway regulation and develops a genetic parts collection for X. dendrorhous from transcriptomic data. Therefore, this study demonstrates that combining systems biology and synthetic biology into an omics-to-parts workflow can simultaneously provide useful biological insight and genetic tools for nonconventional microbes, particularly those without a related model organism. This approach can enhance current efforts to engineer diverse microbes. KEY POINTS: • Transcriptomics revealed further insights into the photobiology of X. dendrorhous, specifically metabolic nodes that are transcriptionally regulated by light. • A modular genetic part collection was developed, including 26 constitutive and regulated promoters derived from the transcriptomics of X. dendrorhous. • Omics-to-parts can be applied to nonconventional microbes for rapid "onboarding".

Thermo-amplifier circuit in probiotic E. coli for stringently temperature-controlled release of a novel antibiotic

Peptide drugs have seen rapid advancement in biopharmaceutical development, with over 80 candidates approved globally. Despite their therapeutic potential, the clinical translation of peptide drugs is hampered by challenges in production yields and stability. Engineered bacterial therapeutics is a unique approach being explored to overcome these issues by using bacteria to produce and deliver therapeutic compounds at the body site of use. A key advantage of this technology is the possibility to control drug delivery within the body in real time using genetic switches. However, the performance of such genetic switches suffers when used to control drugs that require post-translational modifications or are toxic to the host. In this study, these challenges were experienced when attempting to establish a thermal switch for the production of a ribosomally synthesized and post-translationally modified peptide antibiotic, darobactin, in probiotic E. coli. These challenges were overcome by developing a thermo-amplifier circuit that combined the thermal switch with a T7 RNA Polymerase. Due to the orthogonality of the Polymerase, this strategy overcame limitations imposed by the host transcriptional machinery. This circuit enabled production of pathogen-inhibitory levels of darobactin at 40 °C while maintaining leakiness below the detection limit at 37 °C. Furthermore, the thermo-amplifier circuit sustained gene expression beyond the thermal induction duration such that with only 2 h of induction, the bacteria were able to produce pathogen-inhibitory levels of darobactin. This performance was maintained even in physiologically relevant simulated conditions of the intestines that include bile salts and low nutrient levels.

A Saccharomyces boulardii-derived antioxidant protein, thioredoxin, ameliorates intestinal inflammation through transactivating epidermal growth factor receptor

Saccharomyces boulardii (Sb) is a probiotic yeast for the treatment of gastrointestinal disorders, including inflammatory bowel disease (IBD). Little is known about the modulatory capacity of the Sb in IBD. Here, we found that oral gavage of Sb supernatant (SbS) alleviated gut inflammation, protected the intestinal barrier, and reversed DSS-induced down-regulated activation of epidermal growth factor receptor (EGFR) in colitis. Mass spectrum analysis showed that thioredoxin (Trx) is one of the critical secreted soluble proteins participating in EGFR activation detected in SbS. Trx exerted an array of significant effects on anti-inflammatory activity, including alleviating inflammation, protecting gut barrier, suppressing apoptosis, as well as reducing oxidative stress. Mechanistically, Trx promoted EGFR ligand gene expression and transactivated EGFR in a concentration-dependent manner. EGFR kinase inhibitor could block Trx-mediated preventive effects of intestinal epithelial injury. Our data suggested that Sb-derived soluble protein Trx could serve as a potential prophylactic, as a novel postbiotic against colitis, which provides a new strategy for the precision prevention and treatment of IBD.

Programming Probiotics: Diet-Responsive Gene Expression and Colonization Control in Engineered S. boulardii

Saccharomyces boulardii (Sb) is an emerging probiotic chassis for delivering biomolecules to the mammalian gut, offering unique advantages as the only eukaryotic probiotic. However, precise control over gene expression and gut residence time in Sb have remained challenging. To address this, we developed five ligand-responsive gene expression systems and repaired galactose metabolism in Sb, enabling inducible gene expression in this strain. Engineering these systems allowed us to construct AND logic gates, control the surface display of proteins, and turn on protein production in the mouse gut in response to dietary sugar. Additionally, repairing galactose metabolism expanded Sb's habitat within the intestines and resulted in galactose-responsive control over gut residence time. This work opens new avenues for precise dosing of therapeutics by Sb via control over its in vivo gene expression levels and localization within the gastrointestinal tract.

Next generation probiotics: Engineering live biotherapeutics

The population dynamics of the human microbiome have been associated with inflammatory bowel disease, cancer, obesity, autoimmune diseases, and many other human disease states. An emerging paradigm in treatment is the administration of live engineered organisms, also called next-generation probiotics. However, the efficacy of these microbial therapies can be limited by the organism's overall performance in the harsh and nutrient-limited environment of the gut. In this review, we summarize the current state of the art use of bacterial and yeast strains as probiotics, highlight the recent development of genetic tools for engineering new therapeutic functions in these organisms, and report on the latest therapeutic applications of engineered probiotics, including recent clinical trials. We also discuss the supplementation of prebiotics as a method of manipulating the microbiome and improving the overall performance of engineered live biotherapeutics.