Metabolic Glycoengineering in hMSC-TERT as a Model for Skeletal Precursors by Using Modified Azide/Alkyne Monosaccharides

Clickable X-ray Nanoprobes for Nanoscopic Bioimaging of Cellular Structures

Synchrotron-based X-ray microscopy (XRM) has garnered widespread attention from researchers due to its high spatial resolution and excellent energy (element) resolution. Existing molecular probes suitable for XRM include immune probes and genetic labeling probes, enabling the precise imaging of various biological targets within cells. However, immune labeling techniques are prone to cross-interference between antigens and antibodies. Genetic labeling technologies have limited systems that allow express markers independently, and moreover, genetically encoded labels based on catalytic polymerization lack a fixed morphology. When applied to cell imaging, this can result in reduced localization accuracy due to the diffusion of labels within the cells. Therefore, both techniques face challenges in simultaneously labeling multiple biotargets within cells and achieving high-precision imaging. In this work, we applied the click reaction and developed a third category of imaging probes suitable for XRM, termed clickable X-ray nanoprobes (Click-XRN). Click-XRN consists of two components: an X-ray-sensitive multicolor imaging module and a particle-size-controllable morphology module. Efficient identification of intra- and extracellular biotargets is achieved through click reactions between the probe and biomolecules. Click-XRN possesses a controllable particle size, and its loading of various metal ions provides distinctive signals for imaging under XRM. Based on this, we optimized the imaging energy of Click-XRN with different particle sizes, enabling single-color and two-color imaging of the cell membrane, cell nucleus, and mitochondria with nanoscale spatial nanometers. Our work provides a potent molecular tool for investigating cellular activities through XRM.

SiaNAl can be efficiently incorporated in glycoproteins of human mesenchymal stromal cells by metabolic glycoengineering

Metabolic glycoengineering involves the stimulation of cells with functionalized monosaccharides. Glucosamine, galactosamine, and mannosamine derivatives are commercially available, but their application may lead to undirected (i.e., chemical) incorporation into proteins. However, sialic acids are attached to the ends of complex sugar chains of glycoproteins, which might be beneficial for cell surface modification via click chemistry. Thus, we studied the incorporation of chemically synthesized unnatural alkyne modified sialic acid (SiaNAl) into glycoproteins of human telomerase-immortalized mesenchymal stromal cells (hMSC-TERT) and we show that SiaNAl can be efficiently incorporated in glycoproteins involved in signal transduction and cell junction.

Development of an Effective Functional Lipid Anchor for Membranes (FLAME) for the Bioorthogonal Modification of the Lipid Bilayer of Mesenchymal Stromal Cells

The glycosylation of cellular membranes is crucial for the survival and communication of cells. As our target is the engineering of the glycocalyx, we designed a functionalized lipid anchor for the introduction into cellular membranes called Functional Lipid Anchor for MEmbranes (FLAME). Since cholesterol incorporates very effectively into membranes, we developed a twice cholesterol-substituted anchor in a total synthesis by applying protecting group chemistry. We labeled the compound with a fluorescent dye, which allows cell visualization. FLAME was successfully incorporated in the membranes of living human mesenchymal stromal cells (hMSC), acting as a temporary, nontoxic marker. The availability of an azido function─a bioorthogonal reacting group within the compound─enables the convenient coupling of alkyne-functionalized molecules, such as fluorophores or saccharides. After the incorporation of FLAME into the plasma membrane of living hMSC, we were able to successfully couple our molecule with an alkyne-tagged fluorophore via click reaction. This suggests that FLAME is useful for the modification of the membrane surface. Coupling FLAME with a galactosamine derivative yielded FLAME-GalNAc, which was incorporated into U2OS cells as well as in giant unilamellar vesicles (GUVs) and cell-derived giant plasma membrane vesicles (GPMVs). With this, we have shown that FLAME-GalNAc is a useful tool for studying the partitioning in the liquid-ordered (Lo) and the liquid-disordered (Ld) phases. The molecular tool can also be used to analyze the diffusion behavior in the model and the cell membranes by fluorescence correlation spectroscopy (FCS).

Metabolic glycoengineering - exploring glycosylation with bioorthogonal chemistry

Glycans are involved in numerous biological recognition events. Being secondary gene products, their labeling by genetic methods - comparable to GFP labeling of proteins - is not possible. To overcome this limitation, metabolic glycoengineering (MGE, also known as metabolic oligosaccharide engineering, MOE) has been developed. In this approach, cells or organisms are treated with synthetic carbohydrate derivatives that are modified with a chemical reporter group. In the cytosol, the compounds are metabolized and incorporated into newly synthesized glycoconjugates. Subsequently, the reporter groups can be further derivatized in a bioorthogonal ligation reaction. In this way, glycans can be visualized or isolated. Furthermore, diverse targeting strategies have been developed to direct drugs, nanoparticles, or whole cells to a desired location. This review summarizes research in the field of MGE carried out in recent years. After an introduction to the bioorthogonal ligation reactions that have been used in in connection with MGE, an overview on carbohydrate derivatives for MGE is given. The last part of the review focuses on the many applications of MGE starting from mammalian cells to experiments with animals and other organisms.

Improved Protocol to Study Osteoblast and Adipocyte Differentiation Balance

Adipogenesis-osteoblastogenesis balance-rupture is relevant in multiple diseases. Current human mesenchymal stem cells (hMSCs) in vitro differentiation models are expensive, and are hardly reproducible. Their scarcity and variability make an affordable and reliable method to study adipocyte-osteoblast-equilibrium difficult. Moreover, media composition has been inconstant throughout the literature. Our aims were to compare improved differentiation lab-made media with consensus/commercial media, and to identify a cell-line to simultaneously evaluate both MSCs differentiations. Lab-made media were compared with consensus and commercial media in C3H10T1/2 and hMSC, respectively. Lab-made media were tested on aged women primary pre-osteoblast-like cells. To determine the optimum cell line, C3H10T1/2 and hMSC-TERT cells were differentiated to both cell fates. Differentiation processes were evaluated by adipocytic and osteoblastic gene-markers expression and staining. Lab-made media significantly increased consensus medium induction and overcame commercial media in hMSCs differentiation to adipocytes and osteoblasts. Pre-osteoblast-like cells only properly differentiate to adipocyte. Lab-made media promoted adipocyte gene-markers expression in C3H10T1/2 and hMSC-TERT, and osteoblast gene-markers in C3H10T1/2. Oil Red O and Alizarin Red staining supported these findings. Optimized lab-made media were better at differentiating MSCs compared to consensus/commercial media, and evidenced the adipogenic commitment of pre-osteoblast-like cells from aged-women. C3H10T1/2 is an optimum MSC line by which to study adipocyte-osteoblast differentiation balance.

Switching azide and alkyne tags on bioorthogonal reporters in metabolic labeling of sialylatedglycoconjugates: a comparative study

Sialylation of cell surface glycans plays an essential role in cell-cell interaction and communication of cells with their microenvironment. Among the tools that have been developed for the study of sialylation in living cells, metabolic oligosaccharide engineering (MOE) exploits the biosynthetic pathway of sialic acid (Sia) to incorporate unnatural monosaccharides into nascent sialylatedglycoconjugates, followed by their detection by a bioorthogonal ligation of a molecular probe. Among bioorthogonal reactions, the copper-catalyzed azide-alkyne cycloaddition (CuAAC) is the only ligation where both reactive tags can be switched on the chemical reporter or on the probe, making this reaction very flexible and adaptable to various labeling strategies. Azide- and alkyne-modified ManNAc and Sia reporters have been widely used, but per-O-acetylated ManNAz (Ac₄ManNAz) remains the most popular choice so far for tracking intracellular processing of sialoglycans and cell surface sialylation in various cells. Taking advantage of CuAAC, we compared the metabolic incorporation of ManNAl, ManNAz, SiaNAl, SiaNAz and Ac₄ManNAz in the human colon cell lines CCD841CoN, HT29 and HCT116, and in the two gold standard cell lines, HEK293 and HeLa. Using complementary approaches, we showed marked differences in the efficiency of labeling of sialoglycoproteins between the different chemical reporters in a given cell line, and that switching the azide and alkyne bioorthogonal tags on the analogs highly impacted their metabolic incorporation in the human colon cell lines. Our results also indicated that ManNAz was the most promiscuous metabolized reporter to study sialylation in these cells.

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.