Diverse mechanisms of polysaccharide biosynthesis, assembly and secretion across kingdoms

Acyltransferases that Modify Cell Surface Polymers Across the Membrane

Cell surface oligosaccharides and related polymers are commonly decorated with acyl esters that alter their structural properties and influence their interactions with other molecules. In many cases, these esters are added to polymers that are already positioned on the extracytoplasmic side of a membrane, presenting cells with a chemical challenge because the high-energy acyl donors used for these modifications are made in the cytoplasm. How activated acyl groups are passed from the cytoplasm to extra-cytoplasmic polymers has been a longstanding question. Recent mechanistic work has shown that many bacterial acyl transfer pathways operate by shuttling acyl groups through two covalent intermediates to their final destination on an extracellular polymer. Key to these and other pathways are cross-membrane acyltransferases─enzymes that catalyze transfer of acyl groups from a donor on one side of the membrane to a recipient on the other side. Here we review what has been learned recently about how cross-membrane acyltransferases in polymer acylation pathways function, highlighting the chemical and biosynthetic logic used by two key protein families, membrane-bound O-acyltransferases (MBOATs) and acyltransferase-3 (AT3) proteins. We also point out outstanding questions and avenues for further exploration.

Recent advances in polysaccharide-dominated extracellular polymeric substances from microalgae: A review

Microalgae are an environmentally friendly and sustainable alternative resource for future food and pharmaceutical production. Microalgal extracellular polymeric substances (EPMS) are polymers consisting of polysaccharides, proteins, lipids and nucleic acids secreted by microalgal cells. This review systematically summarizes the research progress of microalgal EPMS, including its composition, structure, formation, biological activity and application. The diversity of structural units and binding modes confers microalgal EPMS with unique structural and biological activity, which is species-specific. In addition to the polysaccharides with antioxidant, antiviral, and antitumor effects, extracellular vesicles isolated from microalgal EPMS are emerging as new drug carriers. However, challenges such as relatively low yields, complex separation techniques, intricate processing-secretion pathways, and unclear mechanisms of action still hinder the industrial application of microalgal EPMS. By scientifically summarizing the research progress and leveraging strategies such as metabolic regulation, genetic modification, and advanced separation and characterization technologies, microalgal EPMS is expected to see widespread applications in the food, cosmetics, and therapeutic industries.

Structure-function insights of natural Ganoderma polysaccharides: advances in biosynthesis and functional food applications

Ganoderma polysaccharides (GPs), derived from various species of the Ganoderma genus, exhibit diverse bioactivities, including immune modulation, anti-tumor effects, and gut microbiota regulation. These properties position GPs as dual-purpose agents for medicinal and functional food development. This review comprehensively explores the structural complexity of six key GPs and their specific mechanisms of action, such as TLR signaling in immune modulation, apoptosis pathways in anti-tumor activity, and their prebiotic effects on gut microbiota. Additionally, the structure-activity relationships (SARs) of GPs are highlighted to elucidate their biological efficacy. Advances in green extraction techniques, including ultrasonic-assisted and enzymatic methods, are discussed for their roles in enhancing yield and aligning with sustainable production principles. Furthermore, the review addresses biotechnological innovations in polysaccharide biosynthesis, improving production efficiency and making large-scale production feasible. These insights, combined with ongoing research into their bioactivity, provide a solid foundation for developing health-promoting functional food products that incorporate GPs. Furthermore, future research directions are suggested to optimize biosynthesis pathways and fully harness the health benefits of these polysaccharides.

Transcriptome Analysis of Wild Bletilla striata Tubers Across Multiple Years Revealed the Molecular Mechanisms Regulating Polysaccharide Metabolism and Tuber Enlargement

A Bletilla striata (Thunb.) Reichb.f., known as Bai Ji in Chinese, is a plant from the Orchidaceae family that has been used for its medicinal properties for thousands of years in China. B. striata holds significant economic value due to its esteemed medicinal applications. Our study aimed to analyze the transcriptome of wild B. striata tubers across multiple years to understand the molecular mechanisms regulating polysaccharide metabolism and tuber enlargement. We collected wild B. striata samples of different growth ages and analyzed their chemical composition, including total phenols, polysaccharides, alkaloids, and proteins. The results showed that the content of these compounds varied with the growth age of the plants. Our study focused on analyzing the genes associated with growth years and accelerating the seedling growth cycle, which holds immense value for the preservation and optimal utilization of superior B. striata orchid resources. To further investigate the underlying molecular mechanisms, we performed a comprehensive transcriptome analysis to explore gene expression, functional annotation, and regulatory networks related to the development and chemical composition of B. striata tubers. The quality of perennial medicinal herbs is intricately linked to their growth age. Unfortunately, excessive wild resource excavation has resulted in the premature harvesting of these herbs, causing a decline in their overall quality and effectiveness. Our study offers valuable insights into the conservation and utilization of B. striata resources.

Development of Biomimetic Substrates for Limbal Epithelial Stem Cells Using Collagen-Based Films, Hyaluronic Acid, Immortalized Cells, and Macromolecular Crowding

Despite the promising potential of cell-based therapies developed using tissue engineering techniques to treat a wide range of diseases, including limbal stem cell deficiency (LSCD), which leads to corneal blindness, their commercialization remains constrained. This is primarily attributable to the limited cell sources, the use of non-standardizable, unscalable, and unsustainable techniques, and the extended manufacturing processes required to produce transplantable tissue-like surrogates. Herein, we present the first demonstration of the potential of a novel approach combining collagen films (CF), hyaluronic acid (HA), human telomerase-immortalized limbal epithelial stem cells (T-LESCs), and macromolecular crowding (MMC) to develop innovative biomimetic substrates for limbal epithelial stem cells (LESCs). The initial step involved the fabrication and characterization of CF and CF enriched with HA (CF-HA). Subsequently, T-LESCs were seeded on CF, CF-HA, and tissue culture plastic (TCP). Thereafter, the effect of these matrices on basic cellular function and tissue-specific extracellular matrix (ECM) deposition with or without MMC was evaluated. The viability and metabolic activity of cells cultured on CF, CF-HA, and TCP were found to be similar, while CF-HA induced the highest (p < 0.05) cell proliferation. It is notable that CF and HA induced cell growth, whereas MMC increased (p < 0.05) the deposition of collagen IV, fibronectin, and laminin in the T-LESC culture. The data highlight the potential of, in particular, immortalized cells and MMC for the development of biomimetic cell culture substrates, which could be utilized in ocular surface reconstruction following further in vitro, in vivo, and clinical validation of the approach.

Genomic and Transcriptomic Analyses Identify Two Key Glycosyltransferase Genes alhH and alhK of Exopolysaccharide Biosynthesis in Pantoea alhagi NX-11

The exopolysaccharide (EPS) produced by Pantoea alhagi NX-11, referred to as alhagan, enhances plant stress resistance, improves soil properties, and exhibits notable rheological properties. Despite these benefits, the exact bio-synthetic process of alhagan by P. alhagi NX-11 remains unclear. This study focused on sequencing the complete genome of P. alhagi NX-11 and identifying an alhagan synthesis gene cluster (LQ939_RS12550 to LQ939_RS12700). Gene annotation revealed that alhagan biosynthesis in P. alhagi NX-11 follows the Wzx/Wzy-dependent pathway. Furthermore, transcriptome analysis of P. alhagi NX-11 highlighted significant upregulation of four glycosyltransferase genes (alhH, wcaJ, alhK, and alhM) within the alhagan synthesis gene cluster. These glycosyltransferases are crucial for alhagan synthesis. To delve deeper into this process, two upregulated and uncharacterized glycosyltransferase genes, alhH and alhK, were knocked out. The resulting mutants, ΔalhH and ΔalhK, showed a notable decrease in EPS yield, reduced molecular weight, and altered monosaccharide compositions. These findings contribute to a better understanding of the alhagan biosynthesis mechanism in P. alhagi NX-11.

Deciphering the Roles of Extracellular Polymeric Substances (EPS) in Shaping Disinfection Kinetics through Permanent Removal via Genetic Disruption

Extracellular polymeric substances (EPS) ubiquitously encapsulate microbes and play crucial roles in various environmental processes. However, understanding their complex interactions with dynamic bacterial behaviors, especially during the disinfection process, remains very limited. In this work, we investigated the impact of EPS on bacterial disinfection kinetics by developing a permanent EPS removal strategy. We genetically disrupted the synthesis of exopolysaccharides, the structural components of EPS, in Pseudomonas aeruginosa, a well-known EPS-producing opportunistic pathogen found in diverse environments, creating an EPS-deficient strain. This method ensured a lasting absence of EPS while maintaining bacterial integrity and viability, allowing for real-time in situ investigations of the roles of EPS in disinfection. Our findings indicate that removing EPS from bacteria substantially lowered their susceptibility threshold to disinfectants such as ozone, chloramine B, and free chlorine. This removal also substantially accelerated disinfection kinetics, shortened the resistance time, and increased disinfection efficiency, thereby enhancing the overall bactericidal effect. The absence of EPS was found to enhance bacterial motility and increase bacterial cell vulnerability to disinfectants, resulting in greater membrane damage and intensified reactive oxygen species (ROS) production upon exposure to disinfectants. These insights highlight the central role of EPS in bacterial defenses and offer promising implications for developing more effective disinfection strategies.

Recent advances in the biosynthesis of fungal glucan structural diversity

Glucans are the most abundant class of macromolecule polymers in fungi, which are commonly found in Ascomycota and Basidiomycota. Fungal glucans are not only essential for cell integrity and function but also crucial for the immense industrial interest in high value applications. They present a variety of structural characteristics at the nanoscale due to the high regulation of genes and the involvement of stochastic processes in synthesis. However, although recent findings have demonstrated the genes of glucans synthesis are relatively conserved across diverse fungi, the formation and organization of diverse glucan structures is still unclear in fungi. Here, we summarize the structural features of fungal glucans and the recent developments in the mechanisms of glucans biosynthesis. Furthermore, we propose the engineering strategies of targeted glucan synthesis and point out the remaining challenges in the synthetic process. Understanding the synthesis process of diverse glucans is necessary for tailoring high value glucan towards specific applications. This engineering strategy contributes to enable the sustainable and efficient production of glucan diversity.

Regulation of extracellular polymers based on quorum sensing in wastewater biological treatment from mechanisms to applications: A critical review

Extracellular polymeric substances (EPS) regulated by quorum sensing (QS) could directly mediate adhesion between microorganisms and form tight microbial aggregates. Besides, EPS have redox properties, which can facilitate electron transfer for promoting electroactive bacteria. Currently, the applications research on improving wastewater biological treatment performance based on QS regulated EPS have been widely reported, but reviews on the level of QS regulated EPS to enhance EPS function in microbial systems are still lacking. This work proposes the potential mechanisms of EPS synthesis by QS regulation from the viewpoint of material metabolism and energy metabolism, and summarizes the effects of QS on EPS synthesis. By synthesizing the role of QS in EPS regulation, we further point out the applications of QS-regulated EPS in wastewater biological treatment, which involve a series of aspects such as strengthening microbial colonization, mitigating membrane biofouling, improving the shock resistance of microbial metabolic systems, and strengthening the electron transfer capacity of microbial metabolic systems. According to this comprehensive review, future research on QS-regulated EPS should focus on the exploration of the micro-mechanisms, and economic regulation strategies for QS-regulated EPS should be developed, while the stability of QS-regulated EPS in long-term production experimental research should be further demonstrated.

The Gram-positive bacterium Romboutsia ilealis harbors a polysaccharide synthase that can produce (1,3;1,4)-β-D-glucans

(1,3;1,4)-β-D-Glucans are widely distributed in the cell walls of grasses (family Poaceae) and closely related families, as well as some other vascular plants. Additionally, they have been found in other organisms, including fungi, lichens, brown algae, charophycean green algae, and the bacterium Sinorhizobium meliloti. Only three members of the Cellulose Synthase-Like (CSL) genes in the families CSLF, CSLH, and CSLJ are implicated in (1,3;1,4)-β-D-glucan biosynthesis in grasses. Little is known about the enzymes responsible for synthesizing (1,3;1,4)-β-D-glucans outside the grasses. In the present study, we report the presence of (1,3;1,4)-β-D-glucans in the exopolysaccharides of the Gram-positive bacterium Romboutsia ilealis CRIB^T. We also report that RiGT2 is the candidate gene of R. ilealis that encodes (1,3;1,4)-β-D-glucan synthase. RiGT2 has conserved glycosyltransferase family 2 (GT2) motifs, including D, D, D, QXXRW, and a C-terminal PilZ domain that resembles the C-terminal domain of bacteria cellulose synthase, BcsA. Using a direct gain-of-function approach, we insert RiGT2 into Saccharomyces cerevisiae, and (1,3;1,4)-β-D-glucans are produced with structures similar to those of the (1,3;1,4)-β-D-glucans of the lichen Cetraria islandica. Phylogenetic analysis reveals that putative (1,3;1,4)-β-D-glucan synthase candidate genes in several other bacterial species support the finding of (1,3;1,4)-β-D-glucans in these species.

Recent Progress in 1,2-cis glycosylation for Glucan Synthesis

Controlling the stereoselectivity of 1,2-cis glycosylation is one of the most challenging tasks in the chemical synthesis of glycans. There are various 1,2-cis glycosides in nature, such as α-glucoside and β-mannoside in glycoproteins, glycolipids, proteoglycans, microbial polysaccharides, and bioactive natural products. In the structure of polysaccharides such as α-glucan, 1,2-cis α-glucosides were found to be the major linkage between the glucopyranosides. Various regioisomeric linkages, 1→3, 1→4, and 1→6 for the backbone structure, and 1→2/3/4/6 for branching in the polysaccharide as well as in the oligosaccharides were identified. To achieve highly stereoselective 1,2-cis glycosylation, including α-glucosylation, a number of strategies using inter- and intra-molecular methodologies have been explored. Recently, Zn salt-mediated cis glycosylation has been developed and applied to the synthesis of various 1,2-cis linkages, such as α-glucoside and β-mannoside, via the 1,2-cis glycosylation pathway and β-galactoside 1,4/6-cis induction. Furthermore, the synthesis of various structures of α-glucans has been achieved using the recent progressive stereoselective 1,2-cis glycosylation reactions. In this review, recent advances in stereoselective 1,2-cis glycosylation, particularly focused on α-glucosylation, and their applications in the construction of linear and branched α-glucans are summarized.