Labeling Cell Surface GPIs and GPI-Anchored Proteins through Metabolic Engineering with Artificial Inositol Derivatives

Sensitive Method To Analyze Cell Surface GPI-Anchored Proteins Using DNA Hybridization Chain Reaction-Mediated Signal Amplification

GPI-anchored proteins (GPI-APs) are ubiquitous and essential but exist in low abundances on the cell surface, making their analysis and investigation especially challenging. To tackle the problem, a new method to detect and study GPI-APs based upon GPI metabolic engineering and DNA-facilitated fluorescence signal amplification was developed. In this context, cell surface GPI-APs were metabolically engineered using azido-inositol derivatives to introduce an azido group. This allowed GPI-AP coupling with alkyne-functionalized multifluorophore DNA assemblies generated by hybridization chain reaction (HCR). It was demonstrated that this approach could significantly improve the detection limit and sensitivity of GPI-APs, thereby enabling various biological studies, including the investigation of live cells. This new, enhanced GPI-AP detection method has been utilized to successfully explore GPI-AP engineering, analyze GPI-APs, and profile GPI-AP expression in different cells.

Probing Glycerolipid Metabolism using a Caged Clickable Glycerol-3-Phosphate Probe

In this study, we present the probe SATE-G3P-N3 as a novel tool for metabolic labeling of glycerolipids (GLs) to investigate lipid metabolism in yeast cells. By introducing a clickable azide handle onto the glycerol backbone, this probe enables general labeling of glycerolipids. Additionally, this probe contains a caged phosphate moiety at the glycerol sn-3 position to not only facilitate probe uptake by masking negative charge but also to bypass the phosphorylation step crucial for initiating phospholipid synthesis, thereby enhancing phospholipid labeling. The metabolic labeling activity of the probe was thoroughly assessed through cellular fluorescence microscopy, mass spectrometry (MS), and thin-layer chromatography (TLC) experiments. Fluorescence microscopy analysis demonstrated successful incorporation of the probe into yeast cells, with labeling predominantly localized at the plasma membrane. LCMS analysis confirmed metabolic labeling of various phospholipid species (PC, PS, PA, PI, and PG) and neutral lipids (MAG, DAG, and TAG), and GL labeling was corroborated by TLC. These results showcased the potential of the SATE-G3P-N3 probe in studying GL metabolism, offering a versatile and valuable approach to explore the intricate dynamics of lipids in yeast cells.

Recent research progress in glycosylphosphatidylinositol-anchored protein biosynthesis, chemical/chemoenzymatic synthesis, and interaction with the cell membrane

Glycosylphosphatidylinositol (GPI) attachment to the C-terminus of proteins is a prevalent posttranslational modification in eukaryotic species, and GPIs help anchor proteins to the cell surface. GPI-anchored proteins (GPI-APs) play a key role in various biological events. However, GPI-APs are difficult to access and investigate. To tackle the problem, chemical and chemoenzymatic methods have been explored for the preparation of GPI-APs, as well as GPI probes that facilitate the study of GPIs on live cells. Substantial progress has also been made regarding GPI-AP biosynthesis, which is helpful for developing new synthetic methods for GPI-APs. This article reviews the recent advancements in the study of GPI-AP biosynthesis, GPI-AP synthesis, and GPI interaction with the cell membrane utilizing synthetic probes.

Display of Polyvalent Hybrid Antibodies on the Cell Surface for Enhanced Cell Recognition

Cell surface engineering with exogeneous receptors holds great promise for various applications. However, current biological methods face problems with safety, antigen escape, and receptor stoichiometry. The purpose of this study is to develop a biochemical method for displaying polyvalent antibodies (PAbs) on the cell surface. The PAbs are synthesized through the self-assembly of DNA-Ab conjugates under physiological conditions without the involvement of any factors harsh to cells. The data show that PAb-functionalized cells can recognize target cells much more effectively than monovalent controls. Moreover, dual Ab incorporation into the same PAb with a defined stoichiometric ratio leads to the formation of a polyvalent hybrid Ab (DPAb). DPAb-functionalized cells can effectively recognize target cell models with antigen escape, which cannot be achieved by PAbs with one type of Ab. Therefore, this work presents a novel biochemical method for Ab display on the cell surface for enhanced cell recognition.

Chemical Approaches for Measuring and Manipulating Lipids at the Organelle Level

As the products of complex and often redundant metabolic pathways, lipids are challenging to measure and perturb using genetic tools. Yet by virtue of being the major constituents of cellular membranes, lipids are highly regulated in space and time. Chemists have stepped into this methodological void, developing an array of techniques for the precise quantification and manipulation of lipids at the subcellular, organelle level. Here, we survey the landscape of these methods. For measuring lipids, we summarize the use of metabolic labeling and click chemistry tagging, photoaffinity labeling, isotopic tagging for Raman microscopy, and chemoenzymatic labeling for tracking lipid production and interorganelle transport. For perturbing lipids, we describe synthetic photocaged lipids and membrane editing approaches using optogenetic enzymes for precise manipulation of lipid signaling. Collectively, these chemical and biochemical tools are revealing phenomena and mechanisms underlying lipid functions at the subcellular level.

Harnessing Clickable Acylated Glycerol Probes as Chemical Tools for Tracking Glycerolipid Metabolism

We report the use of clickable monoacylglycerol (MAG) analogs as probes for the labeling of glycerolipids during lipid metabolism. Incorporation of azide tags onto the glycerol region was pursued to develop probes that would label glycerolipids, in which the click tag would not be removed through processes including acyl chain and headgroup remodeling. Analysis of clickable MAG probes containing acyl chains of different length resulted in widely variable cell imaging and cytotoxicity profiles. Based on these results, we focused on a probe bearing a short acyl chain (C4 -MAG-N3 ) that was found to infiltrate natural lipid biosynthetic pathways to produce click-tagged versions of both neutral and phospholipid products. Alternatively, strategic blocking of the glycerol sn-3 position in probe C4 -MEG-N3 served to deactivate phospholipid tagging and focus labeling on neutral lipids. This work shows that lipid metabolic labeling profiles can be tuned based on probe structures and provides valuable tools for evaluating alterations to lipid metabolism in cells.

Labeling cell surface glycosylphosphatidylinositol-anchored proteins through metabolic engineering using an azide-modified phosphatidylinositol

Glycosylphosphatidylinositol (GPI) anchorage is one of the most common mechanisms to attach proteins to the plasma membrane of eukaryotic cells. GPI-anchored proteins (GPI-APs) play a critical role in many biological processes but are difficult to study. Here, a new method was developed for the effective and selective metabolic engineering and labeling of cell surface GPI-APs with an azide-modified phosphatidylinositol (PI) as the biosynthetic precursor of GPIs. It was demonstrated that this azido-PI derivative was taken up by HeLa cells and incorporated into the biosynthetic pathway of GPIs to present azide-labeled GPI-APs on the live cell surface. The azido group was used as a molecular handle to install other labels through a biocompatible click reaction to enable various biological studies, e.g., fluorescent imaging and protein pull-down, which can help explore the functions of GPI-APs and discover new GPI-APs.

Cellular Labeling of Phosphatidylserine Using Clickable Serine Probes

Phosphatidylserine (PS) is a key lipid that plays important roles in disease-related biological processes, and therefore, the means to track PS in live cells are invaluable. Herein, we describe the metabolic labeling of PS in Saccharomyces cerevisiae cells using analogues of serine, a PS precursor, derivatized with azide moieties at either the amino (N-l-SerN3) or carbonyl (C-l-SerN3) groups. The conservative click tag modification enabled these compounds to infiltrate normal lipid biosynthetic pathways, thereby producing tagged PS molecules as supported by mass spectrometry studies, thin-layer chromatography (TLC) analysis, and further derivatization with fluorescent reporters via click chemistry to enable imaging in yeast cells. This approach shows strong prospects for elucidating the complex biosynthetic and trafficking pathways involving PS.

Photoreactive bioorthogonal lipid probes and their applications in mammalian biology

Lipids are an important class of biological molecules that possess many critical physiological functions, which enable the optimal survival of all organisms, including humans. While the role of lipids in the formation of biological cellular membranes and as a source of energy is fairly well understood, the cellular signalling pathways that lipids modulate in mammals are, in comparison, poorly characterized mechanistically and/or largely unknown. In an effort to dissect these mammalian cellular pathways regulated by signalling lipids and map hitherto unknown protein-lipid interactions, the last two decades have seen tremendous progress in the development of multifunctional lipid probes that, in conjunction with well-established bioorthogonal chemistries and chemoproteomics platforms, has almost exponentially expanded our knowledge in this field. In this review, we focus on the various photoreactive bioorthogonal lipid probes described in the literature, and briefly summarize the different photo-crosslinking groups and bioorthogonal chemistries used by them. Furthermore, we report specific case examples of such photoreactive bioorthogonal lipid probes, and discuss the new biological pathways and insights that have emerged from their use through chemoproteomics in mammalian cells. Finally, we highlight the challenges associated with the use of lipid probes in biological systems, and highlight their importance in the discovery and mechanistic understanding of lipid signalling pathways in the years to come.

The Applications of Metabolic Glycoengineering

Mammalian cell membranes are decorated by the glycocalyx, which offer versatile means of generating biochemical signals. By manipulating the set of glycans displayed on cell surface, it is vital for gaining insight into the cellular behavior modulation and medical and biotechnological adhibition. Although genetic engineering is proven to be an effective approach for cell surface modification, the technique is only suitable for natural and genetically encoded molecules. To circumvent these limitations, non-genetic approaches are developed for modifying cell surfaces with unnatural but functional groups. Here, we review latest development of metabolic glycoengineering (MGE), which enriches the chemical functions of the cell surface and is becoming an intriguing new tool for regenerative medicine and tissue engineering. Particular emphasis of this review is placed on discussing current applications and perspectives of MGE.

Fe(III)-complex mediated bacterial cell surface immobilization of eGFP and enzymes

We report a facile and reversible method to immobilize a broad range of His6-tagged proteins on the E. coli cell surface through Fe(iii)-metal complexes. A His6-tagged eGFP and four His6-tagged enzymes were successfully immobilized on the cell surface. Additionally, a hydrogel sheath around E. coli cells was generated by immobilized His6-tagged HRP.

Chemical probes for tagging mycobacterial lipids

Mycobacteria, which cause tuberculosis and related diseases, possess a diverse set of complex envelope lipids that provide remarkable tolerance to antibiotics and are major virulence factors that drive pathogenesis. Recently, metabolic labeling and bio-orthogonal chemistry have been harnessed to develop chemical probes for tagging specific lipids in live mycobacteria, enabling a range of new basic and translational research avenues. A toolbox of probes has been developed for labeling mycolic acids and their derivatives, including trehalose-, arabinogalactan-, and protein-linked mycolates, as well as newer probes for labeling phthiocerol dimycocerosates (PDIMs) and potentially other envelope lipids. These lipid-centric tools have yielded fresh insights into mycobacterial growth and host interactions, provided new avenues for drug target discovery and characterization, and inspired innovative diagnostic and therapeutic strategies.

A Not-So-Ancient Grease History: Click Chemistry and Protein Lipid Modifications

Protein lipid modification involves the attachment of hydrophobic groups to proteins via ester, thioester, amide, or thioether linkages. In this review, the specific click chemical reactions that have been employed to study protein lipid modification and their use for specific labeling applications are first described. This is followed by an introduction to the different types of protein lipid modifications that occur in biology. Next, the roles of click chemistry in elucidating specific biological features including the identification of lipid-modified proteins, studies of their regulation, and their role in diseases are presented. A description of the use of protein-lipid modifying enzymes for specific labeling applications including protein immobilization, fluorescent labeling, nanostructure assembly, and the construction of protein-drug conjugates is presented next. Concluding remarks and future directions are presented in the final section.

Glycoengineering: scratching the surface

At the surface of many cells is a compendium of glycoconjugates that form an interface between the cell and its surroundings; the glycocalyx. The glycocalyx serves several functions that have captivated the interest of many groups. Given its privileged residence, this meshwork of sugar-rich biomolecules is poised to transmit signals across the cellular membrane, facilitating communication with the extracellular matrix and mediating important signalling cascades. As a product of the glycan biosynthetic machinery, the glycocalyx can serve as a partial mirror that reports on the cell's glycosylation status. The glycocalyx can also serve as an information-rich barrier, withholding the entry of pathogens into the underlying plasma membrane through glycan-rich molecular messages. In this review, we provide an overview of the different approaches devised to engineer glycans at the cell surface, highlighting considerations of each, as well as illuminating the grand challenges that face the next era of ‘glyco-engineers’. While we have learned much from these techniques, it is evident that much is left to be unearthed.

Metabolic labeling of glycerophospholipids via clickable analogs derivatized at the lipid headgroup

Metabolic labeling, in which substrate analogs containing diminutive tags can infiltrate biosynthetic pathways and generate labeled products in cells, has led to dramatic advancements in the means by which complex biomolecules can be detected and biological processes can be elucidated. Within this realm, metabolic labeling of lipid products, particularly in a manner that is headgroup-specific, brings about a number of technical challenges including the complexity of lipid metabolic pathways as well as the simplicity of biosynthetic precursors to headgroup functionality. As such, only a handful of strategies for metabolic labeling of lipids have thus far been reported. However, these approaches provide enticing examples of how strategic modifications to substrate structures, particularly by introducing clickable moieties, can enable the hijacking of lipid biosynthesis. Furthermore, early work in this field has led to an explosion in diverse applications by which these techniques have been exploited to answer key biological questions or detect and track various lipid-containing biological entities. In this article, we review these efforts and emphasize recent advancements in the development and application of lipid metabolic labeling strategies.

Preparation of Complex Glycans From Natural Sources for Functional Study

One major barrier in glycoscience is the lack of diverse and biomedically relevant complex glycans in sufficient quantities for functional study. Complex glycans from natural sources serve as an important source of these glycans and an alternative to challenging chemoenzymatic synthesis. This review discusses preparation of complex glycans from several classes of glycoconjugates using both enzymatic and chemical release approaches. Novel technologies have been developed to advance the large-scale preparation of complex glycans from natural sources. We also highlight recent approaches and methods developed in functional and fluorescent tagging and high-performance liquid chromatography (HPLC) isolation of released glycans.

Synthesis and evaluation of Nα,Nε-diacetyl-l-lysine-inositol conjugates as cancer-selective probes for metabolic engineering of GPIs and GPI-anchored proteins

Two myo-inositol derivatives having an Nα,Nε-diacetyl-l-lysine (Ac2Lys) moiety linked to the inositol 1-O-position through a self-cleavable linker and a metabolically stable 2-azidoethyl group linked to the inositol 3-O- and 4-O-positions, respectively, were designed and synthesized. The Ac2Lys moiety blocking the inositol 1-O-position required for GPI biosynthesis was expected to be removable by a combination of two enzymes, histone deacetylase (HDAC) and cathepsin L (CTSL), abundantly expressed in cancer cells, but not in normal cells, to transform these inositol derivatives into biosynthetically useful products with a free 1-O-position. As a result, it was found that these inositol derivatives could be incorporated into the glycosylphosphatidylinositol (GPI) biosynthetic pathway by cancer cells, but not by normal cells, to express azide-labeled GPIs and GPI-anchored proteins on cell surfaces. Consequently, this study has established a novel strategy and new molecular tools for selective metabolic labeling of cancer cells, which should be useful for various biological studies and applications.

A Membrane-Anchored Short-Peptide Fusion Inhibitor Fully Protects Target Cells from Infections of Human Immunodeficiency Virus Type 1 (HIV-1), HIV-2, and Simian Immunodeficiency Virus

Emerging studies demonstrate that the antiviral activity of viral fusion inhibitor peptides can be dramatically improved when being chemically or genetically anchored to the cell membrane, where viral entry occurs. We previously reported that the short-peptide fusion inhibitor 2P23 and its lipid derivative possess highly potent antiviral activities against human immunodeficiency virus type 1 (HIV-1), HIV-2, and simian immunodeficiency virus (SIV). To develop a sterilizing or functional-cure strategy, here we genetically linked 2P23 and two control peptides (HIV-1 fusion inhibitor C34 and hepatitis B virus [HBV] entry inhibitor 4B10) with a glycosylphosphatidylinositol (GPI) attachment signal. As expected, GPI-anchored inhibitors were efficiently expressed on the plasma membrane of transduced TZM-bl cells and primarily directed to the lipid raft site without interfering with the expression of CD4, CCR5, and CXCR4. GPI-anchored 2P23 (GPI-2P23) completely protected TZM-bl cells from infections of divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide (T20)-resistant mutants. GPI-2P23 also rendered the cells resistant to viral envelope-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission. Moreover, GPI-2P23-modified human CD4⁺ T cells (CEMss-CCR5) fully blocked both R5- and X4-tropic HIV-1 isolates and displayed a robust survival advantage over unmodified cells during HIV-1 infection. In contrast, it was found that GPI-anchored C34 was much less effective in inhibiting HIV-2, SIV, and T20-resistant HIV-1 mutants. Therefore, our studies have demonstrated that genetically anchoring a short-peptide fusion inhibitor to the target cell membrane is a viable strategy for gene therapy of both HIV-1 and HIV-2 infections.IMPORTANCE Antiretroviral therapy with multiple drugs in combination can efficiently suppress HIV replication and dramatically reduce the morbidity and mortality associated with AIDS-related illness; however, antiretroviral therapy cannot eradiate the HIV reservoirs, and lifelong treatment is required, which often results in cumulative toxicities, drug resistance, and a multitude of complications, thus necessitating the development of sterilizing-cure or functional-cure strategies. Here, we report that genetically anchoring the short-peptide fusion inhibitor 2P23 to the cell membrane can fully prevent infections from divergent HIV-1, HIV-2, and SIV isolates as well as a panel of enfuvirtide-resistant mutants. Membrane-bound 2P23 also effectively blocks HIV-1 Env-mediated cell-cell fusion and cell-associated virion-mediated cell-cell transmission, renders CD4⁺ T cells nonpermissive to infection, and confers a robust survival advantage over unmodified cells. Thus, our studies verify a powerful strategy to generate resistant cells for gene therapy of both the HIV-1 and HIV-2 infections.

Greasing the Wheels of Lipid Biology with Chemical Tools

Biological lipids are a structurally diverse and historically vexing group of hydrophobic metabolites. Here, we review recent advances in chemical imaging techniques that reveal changes in lipid biosynthesis, metabolism, dynamics, and interactions. We highlight tools for tagging many lipid classes via metabolic incorporation of bioorthogonally functionalized precursors, detectable via click chemistry, and photocaged, photoswitchable, and photocrosslinkable variants of different lipids. Certain lipid probes can supplant traditional protein-based markers of organelle membranes in super-resolution microscopy, and emerging vibrational imaging methods, such as stimulated Raman spectroscopy (SRS), enable simultaneous imaging of more than a dozen different types of target molecule, including lipids. Collectively, these chemical imaging techniques will illuminate, in living color, previously hidden aspects of lipid biology.

Labeling of Phosphatidylinositol Lipid Products in Cells through Metabolic Engineering by Using a Clickable myo-Inositol Probe

Phosphatidylinositol (PI) lipids control critical biological processes, so aberrant biosynthesis often leads to disease. As a result, the capability to track the production and localization of these compounds in cells is vital for elucidating their complex roles. Herein, we report the design, synthesis, and application of clickable myo-inositol probe 1 a for bioorthogonal labeling of PI products. To validate this platform, we initially conducted PI synthase assays to show that 1 a inhibits PI production in vitro. Fluorescence microscopy experiments next showed probe-dependent imaging in T-24 human bladder cancer and Candida albicans cells. Growth studies in the latter showed that replacement of myo-inositol with probe 1 a led to an enhancement in cell growth. Finally, fluorescence-based TLC analysis and mass spectrometry experiments support the labeling of PI lipids. This approach provides a promising means for tracking the complex biosynthesis and trafficking of these lipids in cells.

Programming Cell-Cell Interactions through Non-genetic Membrane Engineering

The ability to direct targeted intercellular interactions has the potential to enable and expand the use of cell-based therapies for regenerative medicine, tissue engineering, and immunotherapy. While genetic engineering approaches have proven effective, these techniques are not amenable to all cell types and often yield permanent modifications with potentially long-lasting adverse effects, restricting their application. To circumvent these limitations, there is intense interest in developing non-genetic methods to modify cell membranes with functional groups that will enable the recognition of target cells. While many such techniques have been developed, relatively few have been applied to directing specific cell-cell interactions. This review details these non-genetic membrane engineering approaches-namely, hydrophobic membrane insertion, chemical modification, liposome fusion, metabolic engineering, and enzymatic remodeling-and summarizes their major applications. Based on this analysis, perspective is provided on the ideal features of these systems with an emphasis on the potential for clinical translation.

Biomarker-Based Metabolic Labeling for Redirected and Enhanced Immune Response

Installation of an antibody-recruiting moiety on the surface of disease-relevant cells can lead to the selective destruction of targets by the immune system. Such an approach can be an alternative strategy to traditional chemotherapeutics in cancer therapy and possibly other diseases. Herein we describe the development of a new strategy to selectively label targets with an antibody-recruiting moiety through its covalent and stable installation, complementing existing methods of employing reversible binding. This is achieved through selective delivery of 1,3,4- O-acetyl- N-azidoacetylmannosamine (Ac₃ManNAz) to folate receptor-overexpressing cells using an Ac₃ManNAz-folate conjugate via a cleavable linker. As such, Ac₃ManNAz is converted to cell surface glycan bearing an azido group, which serves as an anchor to introduce l-rhamnose (Rha), a hapten, via a click reaction with aza-dibenzocyclooctyne (DBCO)-Rha. We tested this method in several cell lines including KB, HEK-293, and MCF7 and were able to demonstrate the following: 1) Rha can be selectively installed to the folate receptor overexpressing cell surface and 2) the Rha installed on the target surface can recruit anti-rhamnose (anti-Rha) antibodies, leading to the destruction of target cells via complement-dependent cytotoxicity (CDC) and antibody-dependent cellular phagocytosis (ADCP).

Synthesis of biotin-labelled core glycans of GPI anchors and their application in the study of GPI interaction with pore-forming bacterial toxins

A convergent strategy was developed for the first-time synthesis of biotin-labeled GPI core glycans. These GPI conjugates are useful for various biological studies showcased by their application in the scrutiny of pore-forming bacterial toxin-GPI interaction, revealing that the phosphate group at the GPI inositol 1-O-position had a significant impact on GPI-toxin binding.

Molecular switching system using glycosylphosphatidylinositol to select cells highly expressing recombinant proteins

Although many pharmaceutical proteins are produced in mammalian cells, there remains a challenge to select cell lines that express recombinant proteins with high productivity. Since most biopharmaceutical proteins are secreted by cells into the medium, it is difficult to select cell lines that produce large amounts of the target protein. To address this issue, a new protein expression system using the glycosylphosphatidylinositol (GPI)-anchor was developed. PGAP2 is involved in processing GPI-anchored proteins (GPI-APs) during transport. In PGAP2 mutant cells, most GPI-APs are secreted into the medium. Here, we established a HEK293 cell line where endogenous PGAP2 was knocked out and exogenous PGAP2 was inserted with a piggyBac transposon in the genome. Using these cells, human lysosomal acid lipase (LIPA) and α-galactosidase A (GLA) were expressed as GPI-anchored forms (LIPA-GPI and GLA-GPI) and cells expressing high levels of LIPA-GPI or GLA-GPI on the cell surface were enriched. Removal of the PGAP2 gene by piggyBac transposase or FLP recombinase converted LIPA-GPI and GLA-GPI from membrane-bound to the secreted forms. Thus, cells expressing LIPA or GLA in large amounts could be enriched using this approach. The GPI-based molecular switching system is an efficient approach to isolate cells expressing recombinant proteins with high productivity.

Two-Step Chemoenzymatic Detection of N-Acetylneuraminic Acid-α(2-3)-Galactose Glycans

Sialic acids are typically linked α(2-3) or α(2-6) to the galactose that located at the non-reducing terminal end of glycans, playing important but distinct roles in a variety of biological and pathological processes. However, details about their respective roles are still largely unknown due to the lack of an effective analytical technique. Herein, a two-step chemoenzymatic approach for the rapid and sensitive detection of N-acetylneuraminic acid-α(2-3)-galactose glycans is described.

Metabolic Glycoengineering with N-Acyl Side Chain Modified Mannosamines

In metabolic glycoengineering (MGE), cells or animals are treated with unnatural derivatives of monosaccharides. After entering the cytosol, these sugar analogues are metabolized and subsequently expressed on newly synthesized glycoconjugates. The feasibility of MGE was first discovered for sialylated glycans, by using N-acyl-modified mannosamines as precursor molecules for unnatural sialic acids. Prerequisite is the promiscuity of the enzymes of the Roseman-Warren biosynthetic pathway. These enzymes were shown to tolerate specific modifications of the N-acyl side chain of mannosamine analogues, for example, elongation by one or more methylene groups (aliphatic modifications) or by insertion of reactive groups (bioorthogonal modifications). Unnatural sialic acids are incorporated into glycoconjugates of cells and organs. MGE has intriguing biological consequences for treated cells (aliphatic MGE) and offers the opportunity to visualize the topography and dynamics of sialylated glycans in vitro, ex vivo, and in vivo (bioorthogonal MGE).