Improved production of 2′-fucosyllactose in engineered Escherichia coli by expressing putative α-1,2-fucosyltransferase, WcfB from Bacteroides fragilis

Structural insight of cell surface sugars in viral infection and human milk glycans as natural antiviral substance

Viral infections are caused by the adhesion of viruses to host cell receptors, including sialylated glycans, glycosaminoglycans, and human blood group antigens (HBGAs). Atomic-level structural information on the interactions between viral particles or proteins with glycans can be determined to provide precise targets for designing antiviral drugs. Milk glycans, existing as free oligosaccharides or glycoconjugates, have attracted increasing attention; milk glycans protect infants against infectious diseases, particularly poorly manageable viral infections. Furthermore, several glycans containing structurally distinct sialic acid/fucose/sulfate modifications in human milk acting as a "receptor decoy" and serving as the natural antiviral library, could interrupt virus-receptor interaction in the first line of defense for viral infection. This review highlights the basis of virus-glycan interactions, presents specific glycan receptor binding by gastroenterovirus viruses, including norovirus, enteroviruses, and the breakthroughs in the studies on the antiviral properties of human milk glycans, and also elucidates the role of glycans in respiratory viruses infection. In addition, recent advances in methods for performing virus/viral protein-glycan interactions were reported. Finally, we discuss the prospects and challenges of the studies on the clinical application of human milk glycan for viral interventions.

Enhancing the soluble expression of α-1,2-fucosyltransferase in E. coli using high-throughput flow cytometry screening coupled with a split-GFP

2'-Fucosyllactose (2'-FL), one of the major human milk oligosaccharides, was produced in several engineered microorganisms. However, the low solubility of α-1,2-fucosyltransferase (α1,2-FucT) often becomes a bottleneck to produce maximum amount of 2'-FL in the microorganisms. To overcome this solubility issue, the following studies were conducted to improve the soluble expression of α1,2-FucT. Initially, hydrophobic amino acids in the hydrophilic region of the 6 α-helices were mutated, adhering to the α-helix rule. Subsequently, gfp11 was fused to the C-terminal of futC gene encoding α1,2-FucT (FutC), enabling selection of high-fluorescence mutants through split-GFP. Each mutant library was screened via fluorescence activated cell sorting (FACS) to separate soluble mutants for high-throughput screening. As a result, L80C single mutant and A121D/P124A/L125R triple mutant were found, and a combined quadruple mutant was created. Furthermore, we combined mutations of conserved sequences (Q150H/C151R/Q239S) of FutC, which showed positive effects in the previous studies from our lab, with the above quadruple mutants (L80C/A121D/P124A/L125R). The resulting strain produced approximately 3.4-fold higher 2'-FL titer than that of the wild-type, suggesting that the conserved sequence mutations are an independent subset of the mutations that further improve the solubility of the target protein acquired by random mutagenesis using split-GFP.

De novo biosynthesis of 2'-fucosyllactose by bioengineered Corynebacterium glutamicum

2'-Fucosyllactose (2'-FL) which is well-known human milk oligosaccharide was biotechnologically synthesized using engineered Corynebacterium glutamicum, a GRAS microbial workhorse. By construction of the complete de novo pathway for GDP-L-fucose supply and heterologous expression of Escherichia coli lactose permease and Helicobacter pylori α-1,2-fucosyltransferase, bioengineered C. glutamicum BCGW_TL successfully biosynthesized 0.25 g L-1 2'-FL from glucose. The additional genetic perturbations including the expression of a putative 2'-FL exporter and disruption of the chromosomal pfkA gene allowed C. glutamicum BCGW_cTTLEΔP to produce 2.5 g L-1 2'-FL batchwise. Finally, optimized fed-batch cultivation of the BCGW_cTTLEΔP using glucose, fructose, and lactose resulted in 21.5 g L-1 2'-FL production with a productivity of 0.12 g L-1 •h, which were more than 3.3 times higher value relative to the batch culture of the BCGW_TL. Conclusively, it would be a groundwork to adopt C. glutamicum for biotechnological production of other food additives including human milk oligosaccharides.

Human Milk Oligosaccharides: A Critical Review on Structure, Preparation, Their Potential as a Food Bioactive Component, and Future Perspectives

Human milk is the gold standard for infant feeding. Human milk oligosaccharides (HMOs) are a unique group of oligosaccharides in human milk. Great interest in HMOs has grown in recent years due to their positive effects on various aspects of infant health. HMOs provide various physiologic functions, including establishing a balanced infant's gut microbiota, strengthening the gastrointestinal barrier, preventing infections, and potential support to the immune system. However, the clinical application of HMOs is challenging due to their specificity to human milk and the difficulties and high costs associated with their isolation and synthesis. Here, the differences in oligosaccharides in human and other mammalian milk are compared, and the synthetic strategies to access HMOs are summarized. Additionally, the potential use and molecular mechanisms of HMOs as a new food bioactive component in different diseases, such as infection, necrotizing enterocolitis, diabetes, and allergy, are critically reviewed. Finally, the current challenges and prospects of HMOs in basic research and application are discussed.

An Overview of Sugar Nucleotide-Dependent Glycosyltransferases for Human Milk Oligosaccharide Synthesis

Human milk oligosaccharides (HMOs) have received increasing attention because of their special effects on infant health and commercial value as the new generation of core components in infant formula. Currently, large-scale production of HMOs is generally based on microbial synthesis using metabolically engineered cell factories. Introduction of the specific glycosyltransferases is essential for the construction of HMO-producing engineered strains in which the HMO-producing glycosyltransferases are generally sugar nucleotide-dependent. Four types of glycosyltransferases have been used for typical glycosylation reactions to synthesize HMOs. Soluble expression, substrate specificity, and regioselectivity are common concerns of these glycosyltransferases in practical applications. Screening of specific glycosyltransferases is an important research topic to solve these problems. Molecular modification has also been performed to enhance the catalytic activity of various HMO-producing glycosyltransferases and to improve the substrate specificity and regioselectivity. In this article, various sugar nucleotide-dependent glycosyltransferases for HMO synthesis were overviewed, common concerns of these glycosyltransferases were described, and the future perspectives of glycosyltransferase-related studies were provided.

Elimination of Byproduct Generation and Enhancement of 2'-Fucosyllactose Synthesis by Expressing a Novel α1,2-Fucosyltransferase in Engineered Escherichia coli

2'-Fucosyllactose (2'-FL) is a kind of fucosylated human milk oligosaccharide (HMO), representing the most abundant oligosaccharide in breast milk. We conducted systematic studies on three canonical α1,2-fucosyltransferases (WbgL, FucT2, and WcfB) to quantify the byproducts in a lacZ- and wcaJ-deleted Escherichia coli BL21(DE3) basic host strain. Further, we screened a highly active α1,2-fucosyltransferase from Helicobacter sp. 11S02629-2 (BKHT), which exhibits high in vivo 2'-FL productivity without the formation of byproducts difucosyl lactose (DFL) and 3-FL. The maximum 2'-FL titer and yield reached 11.13 g/L and 0.98 mol/mol of lactose, respectively, in shake-flask cultivation, both approaching the theoretical maximum value. In a 5 L fed-batch cultivation, the maximum 2'-FL titer reached 94.7 g/L extracellularly with a yield of 0.98 mol of 2'-FL/mol of lactose and productivity of 1.14 g L^-1 h^-1. Our reported 2'-FL yield is the highest from lactose reported to date.

Production of 2’-Fucosyllactose using ⍺1,2-fucosyltransferase from a GRAS bacterial strain

2'-Fucosyllactose (2'-FL) is a major oligosaccharide found in human breast milk. It is produced from GDP-L-fucose and D-lactose by ⍺1,2-fucosyltransferase (⍺1,2-fucT), but the enzyme has been identified mostly in pathogens. In this study, an ⍺1,2-fucT was isolated from a Generally Recognized as Safe (GRAS) Bacillus megaterium strain. The enzyme was successfully expressed in metabolically-engineered Escherichia coli. Furthermore, replacement of non-conserved amino acid residues with conserved ones in the protein led to an increase in the rate of 2'-FL production. As a result, fed-batch fermentation of E. coli produced 30 g/L of 2'-FL from glucose and lactose. Thus, the overproduction of 2'-FL using a novel enzyme from a GRAS bacteria strain was successfully demonstrated.

Enhancing lactose recognition of a key enzyme in 2'-fucosyllactose synthesis: α-1,2-fucosyltransferase

BACKGROUND

2'-Fucosyllactose, a representative oligosaccharide in human milk, is an emerging and promising food and pharmaceutical ingredient due to its powerful health benefits, such as participating in immune regulation, regulation of intestinal flora, etc. To enable economically viable production of 2'-fucosyllactose, different biosynthesis strategies using precursors and pathway enzymes have been developed. The α-1,2-fucosyltransferases are an essential part involved in these strategies, but their strict substrate selectivity and unsatisfactory substrate tolerance are one of the key roadblocks limiting biosynthesis.

RESULTS

To tackle this issue, a semi-rational manipulation combining computer-aided designing and screening with biochemical experiments were adopted. The mutant had a 100-fold increase in catalytic efficiency compared to the wild-type. The highest 2'-fucosyllactose yield was up to 0.65 mol mol^-1 lactose with a productivity of 2.56 g mL^-1  h^-1 performed by enzymatic catalysis in vitro. Further analysis revealed that the interactions between the mutant and substrates were reduced. The crucial contributions of wild-type and mutant to substrate recognition ability were closely related to their distinct phylotypes in terms of amino acid preference.

CONCLUSION

It is envisioned that the engineered α-1,2-fucosyltransferase could be harnessed to relieve constraints imposed on the bioproduction of 2'-fucosyllactose and lay a theoretical foundation for elucidating the substrate recognition mechanisms of fucosyltransferases. © 2022 Society of Chemical Industry.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2017-2018

This review is the tenth update of the original article published in 1999 on the application of matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2018. Also included are papers that describe methods appropriate to glycan and glycoprotein analysis by MALDI, such as sample preparation techniques, even though the ionization method is not MALDI. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, new methods, matrices, derivatization, MALDI imaging, fragmentation and the use of arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Most of the applications are presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. The reported work shows increasing use of combined new techniques such as ion mobility and highlights the impact that MALDI imaging is having across a range of diciplines. MALDI is still an ideal technique for carbohydrate analysis and advancements in the technique and the range of applications continue steady progress.

Biosynthesis of human milk oligosaccharides via metabolic engineering approaches: current advances and challenges

Human milk oligosaccharides (HMOs) are structurally complex unconjugated glycans that are the third largest solid component in human milk. HMOs have drawn increasing attention because of their beneficial effects to infant health. Of the more than 200 HMOs, only less than 10 have been used in medical or food industries. Although HMO research has been becoming increasingly intensive and booming, the limited availability of HMOs still cannot meet the demand in health effect research and large-scale application. Therefore, efficient synthetic approaches and strategies for HMO production are urgently needed. The goal of this review is to highlight recent advances in microbial cell factory development for HMO biosynthesis. Key challenges in representative HMO production are also highlighted. The further perspectives in general HMO biosynthesis are discussed.

Multi-level metabolic engineering of Escherichia coli for high-titer biosynthesis of 2'-fucosyllactose and 3-fucosyllactose

Fucosyllactoses (FL), including 2'-fucosyllactose (2'-FL) and 3-fucosyllactose (3-FL), have garnered considerable interest for their value in newborn formula and pharmaceuticals. In this study, an engineered Escherichia coli was developed for high-titer FL biosynthesis by introducing multi-level metabolic engineering strategies, including (1) individual construction of the 2'/3-FL-producing strains through gene combination optimization of the GDP-L-fucose module; (2) screening of rate-limiting enzymes (α-1,2-fucosyltransferase and α-1,3-fucosyltransferase); (3) analysis of critical intermediates and inactivation of competing pathways to redirect carbon fluxes to FL biosynthesis; (4) enhancement of the catalytic performance of rate-limiting enzymes by the RBS screening, fusion peptides and multi-copy gene cloning. The final strains EC49 and EM47 produced 9.36 g/L for 2'-FL and 6.28 g/L for 3-FL in shake flasks with a modified-M9CA medium. Fed-batch cultivations of the two strains generated 64.62 g/L of 2'-FL and 40.68 g/L of 3-FL in the 3-L bioreactors, with yields of 0.65 mol 2'-FL/mol lactose and 0.67 mol 3-FL/mol lactose, respectively. This research provides a viable platform for other high-value-added compounds production in microbial cell factories.

Module-Guided Metabolic Rewiring for Fucosyllactose Biosynthesis in Engineered Escherichia coli with Lactose De Novo Pathway

Fucosyllactose (FL) has garnered considerable attention for its benefits on infant health. In this study, we report an efficient E. coli cell factory to produce 2'/3-fucosyllactose (2'/3-FL) with lactose de novo pathway through metabolic network remodeling, including (1) modification of the PTS^Glc system to enhance glucose internalization efficiency; (2) screening for β-1,4-galactosyltransferase (β-1,4-GalT) and introduction of lactose synthesis pathway; (3) eliminating inhibition of byproduct pathways; (4) constructing antibiotic-free and inducer-free FL strains; and (5) up-regulating the expression of genes in the GDP-l-fucose module. The final engineered strains BP10-3 and BP11-3 produced 4.36 g/L for 2'-FL and 3.23 g/L for 3-FL in shake flasks. In 3 L bioreactors, fed-batch cultivations of the two strains produced 40.44 g/L for 2'-FL and 30.42 g/L for 3-FL, yielding 0.63 and 0.69 g/g glucose, respectively. The strategy described in this work will help to engineer E. coli as a safe chassis for other lactose-independent HMOs production.

Strategies for Enhancing Microbial Production of 2'-Fucosyllactose, the Most Abundant Human Milk Oligosaccharide

Human milk oligosaccharides (HMOs), a group of structurally diverse unconjugated glycans in breast milk, act as important prebiotics and have plenty of unique health effects for growing infants. 2'-Fucosyllactose (2'-FL) is the most abundant HMO, accounting for approximately 30%, among approximately 200 identified HMOs with different structures. 2'-FL can be enzymatically produced by α1,2-fucosyltransferase, using GDP-l-fucose as donor and lactose as acceptor. Metabolic engineering strategies have been widely used for enhancement of GDP-l-fucose supply and microbial production of 2'-FL with high productivity. GDP-l-fucose supply can be enhanced by two main pathways, including de novo and salvage pathways. 2'-FL-producing α1,2-fucosyltransferases have widely been identified from various microorganisms. Metabolic pathways for 2'-FL synthesis can be basically constructed by enhancing GDP-l-fucose supply and introducing α1,2-fucosyltransferase. Various strategies have been attempted to enhance 2'-FL production, such as acceptor enhancement, donor enhancement, and improvement of the functional expression of α1,2-fucosyltransferase. In this review, current progress in GDP-l-fucose synthesis and bacterial α1,2-fucosyltransferases is described in detail, various metabolic engineering strategies for enhancing 2'-FL production are comprehensively reviewed, and future research focuses in biotechnological production of 2'-FL are suggested.

Enhancing heterologous expression of a key enzyme for the biosynthesis of 2'-fucosyllactose

BACKGROUND

2'-Fucosyllactose (2'-FL) is the most abundant human milk oligosaccharide (HMO) in human milk and has important physiological functions. The market demand of 2'-FL is continuing to grow, but high production cost has limited its availability. To solve the dilemma, biosynthesis of 2'-FL has been proposed and is considered the most promising pathway for massive production. α-1,2-Fucosyltransferase is one of the key elements involved in its biosynthesis, but the limited intracellular accumulation and unstable properties of α-1,2-fucosyltransferases when expressed in host strains have become a major hurdle for the effective biosynthesis of 2'-FL.

RESULTS

A combinatorial engineering strategy of synergic modification of ribosome binding site, fusion peptide and enzyme gene was leveraged to enhance the soluble expression of α-1,2-fucosyltransferases and promote enzyme activity. The preferable combination was to employ an optimized ribosome binding site region to drive 3 × FLAG as a fusion partner along with the α-1,2-fucosyltransferase for expression in Escherichia coli (DE3) PlySs, and protein yield and enzyme activity were remarkably improved by 11.51-fold and 13.72-fold, respectively.

CONCLUSION

After finely tuning the synergy among different elements, the abundant protein yield and high enzyme activity confirmed that the drawbacks of heterologous expression in α-1,2-fucosyltransferase had been properly addressed. A suitable external environment further drives the efficient synthesis of α-1,2-fucosyltransferases. To our knowledge, this is the first report of a systematic and effective modification of α-1,2-fucosyltransferase expression, which could potentially serve as a guideline for industrial application. © 2022 Society of Chemical Industry.

Human milk oligosaccharide 2'-fucosyllactose promotes melanin degradation via the autophagic AMPK-ULK1 signaling axis

There is still an unmet need for development of safer antimelanogenic or melanin-degrading agents for skin hyperpigmentation, induced by intrinsic or extrinsic factors including aging or ultraviolet irradiation. Owing to the relatively low cytotoxicity compared with other chemical materials, several studies have explored the role of 2'-fucosyllactose (2'-FL), the most dominant component of human milk oligosaccharides. Here, we showed that 2'-FL reduced melanin levels in both melanocytic cells and a human skin equivalent three-dimensional in vitro model. Regarding the cellular and molecular mechanism, 2'-FL induced LC3I conversion into LC3II, an autophagy activation marker, followed by the formation of LC3II⁺/PMEL⁺ autophagosomes. Comparative transcriptome analysis provided a comprehensive understanding for the up- and downstream cellular processes and signaling pathways of the AMPK–ULK1 signaling axis triggered by 2'-FL treatment. Moreover, 2'-FL activated the phosphorylation of AMPK at Thr172 and of ULK1 at Ser555, which were readily reversed in the presence of dorsomorphin, a specific AMPK inhibitor, with consequent reduction of the 2'-FL-mediated hypopigmentation. Taken together, these findings demonstrate that 2'-FL promotes melanin degradation by inducing autophagy through the AMPK–ULK1 axis. Hence, 2'-FL may represent a new natural melanin-degrading agent for hyperpigmentation.

Metabolic engineering strategies of de novo pathway for enhancing 2'-fucosyllactose synthesis in Escherichia coli

2′‐Fucosyllactose (2′‐FL), one of the most abundant human milk oligosaccharides (HMOs), is used as a promising infant formula ingredient owing to its multiple health benefits for newborns. However, limited availability and high‐cost preparation have restricted its extensive use and intensive research on its potential functions. In this work, a powerful Escherichia coli cell factory was developed to ulteriorly increase 2′‐FL production. Initially, a modular pathway engineering was strengthened to balance the synthesis pathway through different plasmid combinations with a resulting maximum 2′‐FL titre of 1.45 g l⁻¹. To further facilitate the metabolic flux from GDP‐l‐fucose towards 2′‐FL, the CRISPR‐Cas9 system was utilized to inactivate the genes including lacZ and wcaJ, increasing the titre by 6.59‐fold. Notably, the co‐introduction of NADPH and GTP regeneration pathways was confirmed to be more conducive to 2′‐FL formation, achieving a 2′‐FL titre of 2.24 g l⁻¹. Moreover, comparisons of various exogenous α1,2‐fucosyltransferase candidates revealed that futC from Helicobacter pylori generated the highest titre of 2′‐FL. Finally, the viability of scaled‐up production of 2′‐FL was evidenced in a 3 l bioreactor with a maximum titre of 22.3 g l⁻¹ 2′‐FL and a yield of 0.53 mole 2′‐FL mole⁻¹ lactose.

2'-Fucosyllactose production in engineered Escherichia coli with deletion of waaF and wcaJ and overexpression of FucT2

2'-Fucosyllactose (2'-FL), a major oligosaccharide of human breast milk, and is currently supplemented into infant formula. For the overproduction of 2'-FL via fucosylation of lactose, conventional approaches have focused on the episomal overexpression of de novo or salvage GDP-L-fucose biosynthetic pathway and α-1,2-fucosyltransferase (FucT2) through T7 RNA polymerase expression system in engineered E. coli. However, these approaches have drawbacks of metabolic burden, plasmid instability, and inclusion body formation. In this study, a deletion mutant of waaF coding for ADP-heptose:LPS heptosyltransferase II was employed for 2'-FL production. As the waaF deletion induces accumulation of colanic acid, additional deletion of wcaJ coding for UDP-glucose-1-phosphate transferase in the waaF deletion mutant resulted in enhanced accumulation of GDP-L-fucose. Besides, 2'-FL yields and titers were drastically improved when T7 promoter was replaced with Trc promoter for α-1,2 fucosyltransferase expressions in the waaF and wcaJ deleted strain. As a result, when FucT2 was expressed under Trc promoter in the E. coli JM109(DE3) ΔwaaFΔwcaJ, 14.7 g/L of 2'-FL was produced with a productivity of 0.31 g/L/h in a fed-batch fermentation. We envision that the deletion-based metabolic design and decreased promoter strength for fucosyltransferase expression can resolve the drawbacks of T7 RNA polymerase-based expression design for 2'-FL production in E. coli.

Improved production of 2'-fucosyllactose in engineered Saccharomyces cerevisiae expressing a putative α-1, 2-fucosyltransferase from Bacillus cereus

Background

2′-fucosyllactose (2′-FL) is one of the most abundant oligosaccharides in human milk. It constitutes an authorized functional additive to improve infant nutrition and health in manufactured infant formulations. As a result, a cost-effective method for mass production of 2′-FL is highly desirable.

Results

A microbial cell factory for 2′-FL production was constructed in Saccharomyces cerevisiae by expressing a putative α-1, 2-fucosyltransferase from Bacillus cereus (FutBc) and enhancing the de novo GDP-l-fucose biosynthesis. When enabled lactose uptake, this system produced 2.54 g/L of 2′-FL with a batch flask cultivation using galactose as inducer and carbon source, representing a 1.8-fold increase compared with the commonly used α-1, 2-fucosyltransferase from Helicobacter pylori (FutC). The production of 2′-FL was further increased to 3.45 g/L by fortifying GDP-mannose synthesis. Further deleting gal80 enabled the engineered strain to produce 26.63 g/L of 2′-FL with a yield of 0.85 mol/mol from lactose with sucrose as a carbon source in a fed-batch fermentation.

Conclusion

FutBc combined with the other reported engineering strategies holds great potential for developing commercial scale processes for economic 2′-FL production using a food-grade microbial cell factory.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01657-5.

Synthesis as an Expanding Resource in Human Milk Science

Few classes of natural products rival the structural audacity of oligosaccharides. Their complexity, however, has stood as an immense roadblock to translational research, as access to homogeneous material from nature is challenging. Thus, while carbohydrates are critical to the myriad functional and structural aspects of the biological sciences, their behavior is largely descriptive. This challenge presents an attractive opportunity for synthetic chemistry, particularly in the area of human milk science. First, there is an inordinate need for synthesizing homogeneous human milk oligosaccharides (HMOs). Superimposed on this goal is the mission of conducting syntheses at scale to enable animal studies. Herein, we present a personalized rumination of our involvement, and that of our colleagues, which has led to the synthesis and characterization of HMOs and mechanistic probes. Along the way, we highlight chemical, chemoenzymatic, and synthetic biology based approaches. We close with a discussion on emergent challenges and opportunities for synthesis, broadly defined, in human milk science.

Engineered Microbial Routes for Human Milk Oligosaccharides Synthesis

Human milk oligosaccharides (HMOs) are one of the important ingredients in human milk, which have attracted great interest due to their beneficial effect on the health of newborns. The large-scale production of HMOs has been researched using engineered microbial routes due to the availability, safety, and low cost of host strains. In addition, the development of molecular biology technology and metabolic engineering has promoted the effectiveness of HMOs production. According to current reports, 2'-fucosyllactose (2'-FL), 3-fucosyllactose (3-FL), lacto-N-tetraose (LNT), lacto-N-neotetraose (LNnT), 3'-sialyllactose (3'-SL), 6'-sialyllactose (6'-SL), and some fucosylated HMOs with complex structures have been produced via the engineered microbial route, with 2'-FL having been produced the most. However, due to the uncertainty of metabolic patterns, the selection of host strains has certain limitations. Aside from that, the expression of appropriate glycosyltransferase in microbes is key to the synthesis of different HMOs. Therefore, finding a safe and efficient glycosyltransferase has to be addressed when using engineered microbial pathways. In this review, the latest research on the production of HMOs using engineered microbial routes is reported. The selection of host strains and adapting different metabolic pathways helped researchers designing engineered microbial routes that are more conducive to HMOs production.

Biotechnological Production of 2'-Fucosyllactose: A Prevalent Fucosylated Human Milk Oligosaccharide

Human milk oligosaccharide (HMO) is a key component of human milk carbohydrates and is closely related to the nutrition and health benefits of breastfeeding in infants. 2'-Fucosyllactose (2'-FL) is the most abundant fucosylated HMO, which has remarkable value in nutrition and medicine, such as suppressing pathogen infection, regulating intestinal flora, and boosting immunity. However, 2'-FL production via the method of extraction or chemical synthesis cannot meet its large demand, and as a result, environmentally friendly and efficient biotechnological approaches, including in vitro enzymatic synthesis and microbial cell factory production, have been developed and applied to its commercialized production. This review introduces, summarizes, and discusses the recent advances in the biotechnological production of 2'-FL. Furthermore, future research directions for the biotechnological production of 2'-FL as well as the strategies to further improve its concentration are highlighted and discussed.

Pathway Optimization of 2'-Fucosyllactose Production in Engineered Escherichia coli

2'-Fucosyllactose (2'-FL), one of the most valuable oligosaccharides in human milk, is used as an emerging food ingredient in the nutraceutical and food industries due to its numerous health benefits. Herein, the de novo and salvage pathways for GDP-fucose synthesis were engineered and optimized in Escherichia coli BL21 (DE3) to improve the production of 2'-FL. The de novo pathway genes encoding phosphomannomutase (ManB), mannose-1-phosphate guanyltransferase (ManC), GDP-d-mannose-4,6-dehydratase (Gmd), and GDP-l-fucose synthase (WcaG) combined with the gene from the salvage pathway encoding fucose kinase/fucose-1-phosphate guanylyltransferase (Fkp) were reconstructed in two vectors to evaluate the GDP-fucose biosynthesis. Then, the fucT2 gene, encoding α1,2-fucosyltransferase, was introduced into the GDP-fucose-overproducing strains to realize 2'-FL biosynthesis. Furthermore, the genes in bypass pathways, including lacZ, fucI, fucK, and wcaJ, were inactivated to improve 2'-FL production. In addition, the two GDP-fucose synthesis pathways, along with fucT2, were transcriptionally fine-tuned to efficiently increase 2'-FL production. The final metabolically engineered E. coli produced 2.62 and 14.1 g/L in shake-flask and fed-batch cultivations, respectively.

Multi-Path Optimization for Efficient Production of 2'-Fucosyllactose in an Engineered Escherichia coli C41 (DE3) Derivative

2′-fucosyllactose (2′-FL), one of the simplest but most abundant oligosaccharides in human milk, has been demonstrated to have many positive benefits for the healthy development of newborns. However, the high-cost production and limited availability restrict its widespread use in infant nutrition and further research on its potential functions. In this study, on the basis of previous achievements, we developed a powerful cell factory by using a lacZ-mutant Escherichia coli C41 (DE3)ΔZ to ulteriorly increase 2′-FL production by feeding inexpensive glycerol. Initially, we co-expressed the genes for GDP-L-fucose biosynthesis and heterologous α-1,2-fucosyltransferase in C41(DE3)ΔZ through different plasmid-based expression combinations, functionally constructing a preferred route for 2′-FL biosynthesis. To further boost the carbon flux from GDP-L-fucose toward 2′-FL synthesis, deletion of chromosomal genes (wcaJ, nudD, and nudK) involved in the degradation of the precursors GDP-L-fucose and GDP-mannose were performed. Notably, the co-introduction of two heterologous positive regulators, RcsA and RcsB, was confirmed to be more conducive to GDP-L-fucose formation and thus 2′-FL production. Further a genomic integration of an individual copy of α-1,2-fucosyltransferase gene, as well as the preliminary optimization of fermentation conditions enabled the resulting engineered strain to achieve a high titer and yield. By collectively taking into account the intracellular lactose utilization, GDP-L-fucose availability, and fucosylation activity for 2′-FL production, ultimately a highest titer of 2′-FL in our optimized conditions reached 6.86 g/L with a yield of 0.92 mol/mol from lactose in the batch fermentation. Moreover, the feasibility of mass production was demonstrated in a 50-L fed-batch fermentation system in which a maximum titer of 66.80 g/L 2′-FL was achieved with a yield of 0.89 mol 2′-FL/mol lactose and a productivity of approximately 0.95 g/L/h 2′-FL. As a proof of concept, our preliminary 2′-FL production demonstrated a superior production performance, which will provide a promising candidate process for further industrial production.

Recent advances on 2'-fucosyllactose: physiological properties, applications, and production approaches

The trisaccharide, 2'-fucosyllactose (Fucα1-2Galβ1-4Glc; 2'-FL), is the most abundant oligosaccharide in human milk. It has numerous significant biological properties including prebiotics, antibacterial, antiviral, and immunomodulating effects, and has been approved as "generally recognized as safe" (GRAS) by the Food and Drug Administration (FDA) and as a novel food (NF) by the European Food Safety Authority (EFSA). 2'-FL not only serves as a food ingredient added in infant formula, but also as a dietary supplement and medical food material in food bioprocesses. There is considerable commercial interest in 2'-FL for its irreplaceable nutritional applications. This review aims at systematically elaborating key functional properties of 2'-FL as well as its applications. In addition, several approaches for 2'-FL production are described in this review, including chemical, chemo-enzymatical, and cell factory approaches, and the pivotal research results also have been summarized. With the rapid development of metabolic engineering and synthetic biology strategies, using the engineered cell factory for 2'-FL large-scale production might be a promising approach. From an economic and safety point of view, microbial selection for cell factory engineering in 2'-FL bioprocess also should be taken into consideration.

Development of fluorescent Escherichia coli for a whole-cell sensor of 2'-fucosyllactose

2′-Fucosyllactose (2′-FL), a major component of fucosylated human milk oligosaccharides, is beneficial to human health in various ways like prebiotic effect, protection from pathogens, anti-inflammatory activity and reduction of the risk of neurodegeneration. Here, a whole-cell fluorescence biosensor for 2′-FL was developed. Escherichia coli (E. coli) was engineered to catalyse the cleavage of 2′-FL into l-fucose and lactose by constitutively expressing α-l-fucosidase. Escherichia coli ∆L YA, in which lacZ is deleted and lacY is retained, was employed to disable lactose consumption. E. coli ∆L YA constitutively co-expressing α-l-fucosidase and a red fluorescence protein (RFP) exhibited increased fluorescence intensity in media containing 2′-FL. However, the presence of 50 g/L lactose reduced the RFP intensity due to lactose-induced cytotoxicity. Preadaptation of bacterial strains to fucose alleviated growth hindrance by lactose and partially recovered the fluorescence intensity. The fluorescence intensity of the cell was linearly proportional to 1–5 g/L 2′-FL. The whole-cell sensor will be versatile in developing a 2′-FL detection system.

2'-fucosyllactose inhibits imiquimod-induced psoriasis in mice by regulating Th17 cell response via the STAT3 signaling pathway

Psoriasis is a chronic immune-mediated inflammatory cutaneous disorder with Th17 cells and Th17-related cytokines playing an important role in its development. 2'-FL (2'-fucosyllactose), which makes up about 30% of all HMOs (human milk oligosaccharides) in blood type secretor positive maternal milk, plays an essential role in supporting aspects of immune development and regulation. To explore the immunomodulatory effect of 2'-FL in psoriasis, we employed the imiquimod (IMQ)-induced psoriasis-like mouse model. Our data showed that mice administered with 2'-FL exhibited attenuated skin damage and inflammation, characterized by significantly decreased erythema and thickness and reduced recruitment of pro-inflammatory cytokines, when compared to control mice. The alleviated skin inflammation in 2'-FL treated mice was associated with a reduced proportion of Th17 cells and decreased production of Th17-related cytokines. Furthermore, we have demonstrated that 2'-FL reduced the phosphorylation of STAT3 in the skin tissue from mice with IMQ stimulation, which could account for the decreasing recruitment of Th17 cells. In vitro studies showed that 2'-FL inhibited differentiation of Th17 cells, phosphorylation of STAT3, and RORγt mRNA levels in T cells under Th17 polarization. Our results indicate that 2'-FL ameliorates IMQ-induced psoriasis by inhibiting Th17 cell immune response and Th17-related cytokine secretion via modulation of the STAT3 signaling pathway.

Transglycosylating β-d-galactosidase and α-l-fucosidase from Paenibacillus sp. 3179 from a hot spring in East Greenland

Thermal springs are excellent locations for discovery of thermostable microorganisms and enzymes. In this study, we identify a novel thermotolerant bacterial strain related to Paenibacillus dendritiformis, denoted Paenibacillus sp. 3179, which was isolated from a thermal spring in East Greenland. A functional expression library of the strain was constructed, and the library screened for β-d-galactosidase and α-l-fucosidase activities on chromogenic substrates. This identified two genes encoding a β-d-galactosidase and an α-l-fucosidase, respectively. The enzymes were recombinantly expressed, purified, and characterized using oNPG (2-nitrophenyl-β-d-galactopyranoside) and pNP-fucose (4-nitrophenyl-α-l-fucopyranoside), respectively. The enzymes were shown to have optimal activity at 50°C and pH 7-8, and they were able to hydrolyze as well as transglycosylate natural carbohydrates. The transglycosylation activities were investigated using TLC and HPLC, and the β-d-galactosidase was shown to produce the galactooligosaccharides (GOS) 6'-O-galactosyllactose and 3'-O-galactosyllactose using lactose as substrate, whereas the α-l-fucosidase was able to transfer the fucose moiety from pNP-fuc to lactose, thereby forming 2'-O-fucosyllactose. Since enzymes that are able to transglycosylate carbohydrates at elevated temperature are desirable in many industrial processes, including food and dairy production, we foresee the potential use of enzymes from Paenibacillus sp. 3179 in the production of, for example, instant formula.

Engineering the Substrate Transport and Cofactor Regeneration Systems for Enhancing 2'-Fucosyllactose Synthesis in Bacillus subtilis

Human milk oligosaccharides (HMOs) have been proven to be beneficial to infants' intestinal health and immune systems. 2'-Fucosyllactose (2'-FL) is the most abundant and thoroughly studied HMO and has been approved to be an additive of infant formula. How to construct efficient and safe microbial cell factories for the production of 2'-FL attracts increasing attention. In this work, we engineered the Bacillus subtilis as an efficient 2'-FL producer by engineering the substrate transport and cofactor guanosine 5'-triphosphate (GTP) regeneration systems. First, we constructed a synthesis pathway for the 2'-FL precursor guanosine 5'-diphosphate-l-fucose (GDP-l-fucose) by introducing the salvage pathway gene fkp from Bacteriodes fragilis and improved the fucose importation by overexpressing the transporters. Then, the complete synthesis pathway of 2'-FL was constructed by introducing the heterologous fucosyltransferases from different sources, and it was found that the gene from Helicobacter pylori was the best one for 2'-FL synthesis. We also improved the substrate lactose importation by introducing heterologous lactose permeases and eliminated endogenous β-galactosidase (yesZ) to block the lactose degradation. Next, the production of 2'-FL and GDP-l-fucose was improved by fine-tuning the expression of cofactor guanosine 5'-triphosphate regeneration module genes gmd, ndk, guaA, guaC, ykfN, deoD, and xpt. Finally, a 3 L fed-batch fermentation was performed, and the highest 2'-FL titer reached 5.01 g/L with a yield up to 0.85 mol/mol fucose. We optimized the synthesis modules of 2'-FL in B. subtilis, and this provides a good starting point for metabolic engineering to further improve 2'-FL production in the future.

Engineering two species of yeast as cell factories for 2'-fucosyllactose

Oligosaccharides present in human breast milk have been linked to beneficial effects on infant health. Inclusion of these human milk oligosaccharides (HMOs) in infant formula can recapitulate these health benefits. As a result, there is substantial commercial interest in a cost-effective source of HMOs as infant formula ingredients. Here we demonstrate that the yeast species Saccharomyces cerevisiae and Yarrowia lipolytica both can be engineered to produce 2'-fucosyllactose (2'FL), which is the most abundant oligosaccharide in human breast milk, at high titer and productivity. Both yeast species were modified to enable uptake of lactose and synthesis of GDP-fucose - the two precursors of 2'FL - by installing a lactose transporter and enzymes that convert GDP-mannose to GDP-fucose. Production of 2'FL was then enabled by expression of α-1,2-fucosyltransferases from various organisms. By screening candidate transporters from a variety of sources, we identified transporters capable of exporting 2'FL from yeast, which is a key consideration for any biocatalyst for 2'FL production. In particular, we identified CDT2 from Neurospora crassa as a promising target for further engineering to improve 2'FL efflux. Finally, we demonstrated production of 2'FL in fermenters at rates and titers that indicate the potential of engineered S. cerevisiae and Y. lipolytica strains for commercial 2'FL production.

Shaping the Infant Microbiome With Non-digestible Carbohydrates

Natural polysaccharides with health benefits are characterized by a large structural diversity and differ in building blocks, linkages, and lengths. They contribute to human health by functioning as anti-adhesives preventing pathogen adhesion, stimulate immune maturation and gut barrier function, and serve as fermentable substrates for gut bacteria. Examples of such beneficial carbohydrates include the human milk oligosaccharides (HMOs). Also, specific non-digestible carbohydrates (NDCs), such as galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS) are being produced with this purpose in mind, and are currently added to infant formula to stimulate the healthy development of the newborn. They mimic some functions of HMO, but not all. Therefore, many research efforts focus on identification and production of novel types of NDCs. In this review, we give an overview of the few NDCs currently available [GOS, FOS, polydextrose (PDX)], and outline the potential of alternative oligosaccharides, such as pectins, (arabino)xylo-oligosaccharides, and microbial exopolysaccharides (EPS). Moreover, state-of-the-art techniques to generate novel types of dietary glycans, including sialylated GOS (Sia-GOS) and galactosylated chitin, are presented as a way to obtain novel prebiotic NDCs that help shaping the infant microbiome.

L-Fucose production by engineered Escherichia coli

L-Fucose (6-deoxy-L-galactose) is a major constituent of glycans and glycolipids in mammals. Fucosylation of glycans can confer unique functional properties and may be an economical way to manufacture L-fucose. Research can extract L-fucose directly from brown algae, or by enzymatic hydrolysis of L-fucose-rich microbial exopolysaccharides. However, these L-fucose production methods are not economical or scalable for various applications. We engineered an Escherichia coli strain to produce L-fucose. Specifically, we modified the strain genome to eliminate endogenous L-fucose and lactose metabolism, produce 2'-fucosyllactose (2'-FL), and to liberate L-fucose from 2'-FL. This E. coli strain produced 16.7 g/L of L-fucose with productivity of 0.1 g·L^-1 ·h^-1 in a fed-batch fermentation. This study presents an efficient one-pot biosynthesis strategy to produce a monomeric form of L-fucose by microbial fermentation, making large-scale industrial production of L-fucose feasible.

Biosynthesis of a Functional Human Milk Oligosaccharide, 2'-Fucosyllactose, and l-Fucose Using Engineered Saccharomyces cerevisiae

2'-fucosyllactose (2-FL), one of the most abundant human milk oligosaccharides (HMOs), has received much attention due to its health-promoting activities, such as stimulating the growth of beneficial gut microorganisms, inhibiting pathogen infection, and enhancing the host immune system. Consequently, large quantities of 2-FL are on demand for food applications as well as in-depth investigation of its biological properties. Biosynthesis of 2-FL has been attempted primarily in Escherichia coli, which might not be the best option to produce food and cosmetic ingredients due to the presence of endotoxins on the cell surface. In this study, an alternative route to produce 2-FL via a de novo pathway using a food-grade microorganism,  Saccharomyces cerevisiae, has been devised. Specifically, heterologous genes, which are necessary to achieve the production of 2-FL from a mixture of glucose and lactose, were introduced into S. cerevisiae. When the lactose transporter (Lac12), de novo GDP-l-fucose pathway (consisting of GDP-d-mannose-4,6-dehydratase (Gmd) and GDP-4-keto-6-deoxymannose-3,5-epimerase-4-reductase (WcaG)), and α1,2-fucosyltransferase (FucT2) were introduced, the resulting engineered strain (D452L-gwf) produced 0.51 g/L of 2-FL from a batch fermentation. In addition, 0.41 g/L of l-fucose was produced when α-l-fucosidase was additionally expressed in the 2-FL producing strain (D452L-gwf). To our knowledge, this is the first report of 2-FL and l-fucose production in engineered S. cerevisiae via the de novo pathway. This study provides the possibility of producing HMOs by a food-grade microorganism S. cerevisiae and paves the way for more HMO production in the future.

Production of a human milk oligosaccharide 2'-fucosyllactose by metabolically engineered Saccharomyces cerevisiae

BACKGROUND

2'-Fucosyllactose (2-FL), one of the most abundant oligosaccharides in human milk, has potential applications in foods due to its health benefits such as the selective promotion of bifidobacterial growth and the inhibition of pathogenic microbial binding to the human gut. Owing to the limited amounts of 2-FL in human milk, alternative microbial production of 2-FL is considered promising. To date, microbial production of 2-FL has been studied mostly in Escherichia coli. In this study, 2-FL was produced alternatively by using a yeast Saccharomyces cerevisiae, which may have advantages over E. coli.

RESULTS

Fucose and lactose were used as the substrates for the salvage pathway which was constructed with fkp coding for a bifunctional enzyme exhibiting L-fucokinase and guanosine 5'-diphosphate-L-fucose phosphorylase activities, fucT2 coding for α-1,2-fucosyltransferase, and LAC12 coding for lactose permease. Production of 2-FL by the resulting engineered yeast was verified by mass spectrometry. 2-FL titers of 92 and 503 mg/L were achieved from 48-h batch fermentation and 120-h fed-batch fermentation fed with ethanol as a carbon source, respectively.

CONCLUSIONS

This is the first report on 2-FL production by using yeast S. cerevisiae. These results suggest that S. cerevisiae can be considered as a host engineered for producing 2-FL via the salvage pathway.