Glycosylation-on-a-Chip: A Flow-Based Microfluidic System for Cell-Free Glycoprotein Biosynthesis

Microfluidic mimicry of the Golgi-linked N-glycosylation machinery

The complexity of the eukaryotic glycosylation machinery hinders the development of cell-free protein glycosylation since in vitro methods struggle to simulate the natural environment of the glycosylation machinery. Microfluidic technologies have the potential to address this limitation due to their ability to control glycosylation parameters, such as enzyme/substrate concentrations and fluxes, in a rapid and precise manner. However, due to the complexity and sensitivity of the numerous components of the glycosylation machinery, very few "glycobiology-on-a-chip" systems have been proposed or reported in the literature. Herein, we describe the design, fabrication and proof-of-concept of a droplet-based microfluidic platform able to mimic N-linked glycan processing along the secretory pathway. Within a single microfluidic device, glycoproteins and glycosylation enzymes are encapsulated and incubated in water-in-oil droplets. Additional glycosylation enzymes are subsequently supplied to these droplets via picoinjection, allowing further glycoprotein processing in a user-defined manner. After system validation, the platform is used to perform two spatiotemporally separated consecutive enzymatic N-glycan modifications, mirroring the transition between the endoplasmic reticulum and early Golgi.

Cell-free synthesis system: An accessible platform from biosensing to biomanufacturing

The fundamental aspect of cell-free synthesis systems is the in vitro transcription-translation process. By artificially providing the components required for protein expression, in vitro protein production alleviates various limitations tied to in vivo production, such as oxygen supply and nutrient constraints, thus showcasing substantial potential in engineering applications. This article presents a comprehensive review of cell-free synthesis systems, with a primary focus on biosensing and biomanufacturing. In terms of biosensing, it summarizes the recognition-response mechanisms and key advantages of cell-free biosensors. Moreover, it examines the strategies for the cell-free production of intricate proteins, including membrane proteins and glycoproteins. Additionally, the integration of cell-free metabolic engineering approaches with cell-free synthesis systems in biomanufacturing is thoroughly discussed, with the expectation that biotechnology will embrace greater prosperity.

Advancements in Electrochemical Biosensors for Comprehensive Glycosylation Assessment of Biotherapeutics

Proteins represent a significant portion of the global therapeutics market, surpassing hundreds of billions of dollars annually. Among the various post-translational modifications, glycosylation plays a crucial role in influencing protein structure, stability, and function. This modification is especially important in biotherapeutics, where the precise characterization of glycans is vital for ensuring product efficacy and safety. Although mass spectrometry-based techniques have become essential tools for glycomic analysis due to their high sensitivity and resolution, their complexity and lengthy processing times limit their practical application. In contrast, electrochemical methods provide a rapid, cost-effective, and sensitive alternative for glycosylation assessment, enabling the real-time analysis of glycan structures on biotherapeutic proteins. These electrochemical techniques, often used in conjunction with complementary methods, offer valuable insights into the glycosylation profiles of both isolated glycoproteins and intact cells. This review examines the latest advancements in electrochemical biosensors for glycosylation analysis, highlighting their potential in enhancing the characterization of biotherapeutics and advancing the field of precision medicine.

Cell-Free Gene Expression: Methods and Applications

Cell-free gene expression (CFE) systems empower synthetic biologists to build biological molecules and processes outside of living intact cells. The foundational principle is that precise, complex biomolecular transformations can be conducted in purified enzyme or crude cell lysate systems. This concept circumvents mechanisms that have evolved to facilitate species survival, bypasses limitations on molecular transport across the cell wall, and provides a significant departure from traditional, cell-based processes that rely on microscopic cellular "reactors." In addition, cell-free systems are inherently distributable through freeze-drying, which allows simple distribution before rehydration at the point-of-use. Furthermore, as cell-free systems are nonliving, they provide built-in safeguards for biocontainment without the constraints attendant on genetically modified organisms. These features have led to a significant increase in the development and use of CFE systems over the past two decades. Here, we discuss recent advances in CFE systems and highlight how they are transforming efforts to build cells, control genetic networks, and manufacture biobased products.

Establishing a Cell-Free Glycoprotein Synthesis System for Enzymatic N-GlcNAcylation

N-linked glycosylation plays a key role in the efficacy of many therapeutic proteins. One limitation to the bacterial glycoengineering of human N-linked glycans is the difficulty of installing a single N-acetylglucosamine (GlcNAc), the reducing end sugar of many human-type glycans, onto asparagine in a single step (N-GlcNAcylation). Here, we develop an in vitro method for N-GlcNAcylating proteins using the oligosaccharyltransferase PglB from Campylobacter jejuni. We use cell-free protein synthesis (CFPS) to test promiscuous PglB variants previously reported in the literature for the ability to produce N-GlcNAc and successfully determine that PglB with an N311V mutation (PglBN311V) exhibits increased GlcNAc transferase activity relative to the wild-type enzyme. We then improve the transfer efficiency by producing CFPS extracts enriched with PglBN311V and further optimize the reaction conditions, achieving a 98.6 ± 0.5% glycosylation efficiency. We anticipate this method will expand the glycoengineering toolbox for therapeutic research and biomanufacturing.

Integrating microfluidics and synthetic biology: advancements and diverse applications across organisms

Synthetic biology is the design and modification of biological systems for specific functions, integrating several disciplines like engineering, genetics, and computer science. The field of synthetic biology is to understand biological processes within host organisms through the manipulation and regulation of their genetic pathways and the addition of biocontrol circuits to enhance their production capabilities. This pursuit serves to address global challenges spanning diverse domains that are difficult to tackle through conventional routes of production. Despite its impact, achieving precise, dynamic, and high-throughput manipulation of biological processes is still challenging. Microfluidics offers a solution to those challenges, enabling controlled fluid handling at the microscale, offering lower reagent consumption, faster analysis of biochemical reactions, automation, and high throughput screening. In this review, we diverge from conventional focus on automating the synthetic biology design-build-test-learn cycle, and instead, focus on microfluidic platforms and their role in advancing synthetic biology through its integration with host organisms - bacterial cells, yeast, fungi, animal cells - and cell-free systems. The review illustrates how microfluidic devices have been instrumental in understanding biological systems by showcasing microfluidics as an essential tool to create synthetic genetic circuits, pathways, and organisms within controlled environments. In conclusion, we show how microfluidics expedite synthetic biology applications across diverse domains including but not limited to personalized medicine, bioenergy, and agriculture.

Cell-Free Systems for the Production of Glycoproteins

Cell-free protein synthesis (CFPS), whereby cell lysates are used to produce proteins from a genetic template, has matured as an attractive alternative to standard biomanufacturing modalities due to its high volumetric productivity contained within a distributable platform. Initially, cell-free lysates produced from Escherichia coli, which are both simple to produce and cost-effective for the production of a wide variety of proteins, were unable to produce glycosylated proteins as E. coli lacks native glycosylation machinery. With many important therapeutic proteins possessing asparagine-linked glycans that are critical for structure and function, this gap in CFPS production capabilities was addressed with the development of cell-free expression of glycoproteins (glycoCFE), which uses the supplementation of extracted lipid-linked oligosaccharides and purified oligosaccharyltransferases to enable glycoprotein production in the CFPS reaction environment. In this chapter, we highlight the basic methods for the preparation of reagents for glycoCFE and the protocol for expression and glycosylation of a model protein using a more productive, yet simplified, glycoCFE setup. Beyond this initial protocol, we also highlight how this protocol can be extended to a wide range of alternative glycan structures, oligosaccharyltransferases, and acceptor proteins as well as to a one-pot cell-free glycoprotein synthesis reaction.

Membrane protein synthesis: no cells required

Despite advances in membrane protein (MP) structural biology and a growing interest in their applications, these proteins remain challenging to study. Progress has been hindered by the complex nature of MPs and innovative methods will be required to circumvent technical hurdles. Cell-free protein synthesis (CFPS) is a burgeoning technique for synthesizing MPs directly into a membrane environment using reconstituted components of the cellular transcription and translation machinery in vitro. We provide an overview of CFPS and how this technique can be applied to the synthesis and study of MPs. We highlight numerous strategies including synthesis methods and folding environments, each with advantages and limitations, to provide a survey of how CFPS techniques can advance the study of MPs.

Small tools for sweet challenges: advances in microfluidic technologies for glycan synthesis

Glycans, including oligosaccharides and glycoconjugates, play an integral role in modulating the biological functions of macromolecules. Many physiological and pathological processes are mediated by interactions between glycans, which has led to the use of glycans as biosensors for pathogen and biomarker detection. Elucidating the relationship between glycan structure and biological function is critical for advancing our understanding of the impact glycans have on human health and disease and for expanding the repertoire of glycans available for bioanalysis, especially for diagnostics. Such efforts have been limited by the difficulty in obtaining sufficient quantities of homogenous glycan samples needed to resolve the exact relationships between glycan structure and their structural or modulatory functions on a given glycoconjugate. Synthetic strategies offer a viable route for overcoming these technical hurdles. In recent years, microfluidics have emerged as powerful tools for realizing high-throughput and reproducible syntheses of homogenous glycans for the potential use in functional studies. This critical review provides readers with an overview of the microfluidic technologies that have been developed for chemical and enzymatic glycan synthesis. The advantages and limitations associated with using microreactor platforms to improve the scalability, productivity, and selectivity of glycosylation reactions will be discussed, as well as suggested future work that can address certain pitfalls.