Nonmetabolic radiolabeling and tagging of glycoconjugates

Click hybridization of immune cells and polyamidoamine dendrimers

Immobilizing highly branched polyamidoamine (PAMAM) dendrimers to the cell surface represents an innovative method of enhancing cell surface loading capacity to deliver therapeutic and imaging agents. In this work, hybridized immune cells, that is, macrophage RAW264.7 (RAW), with PAMAM dendrimer G4.0 (DEN) on the basis of bioorthogonal chemistry are clicked. Efficient and selective cell surface immobilization of dendrimers is confirmed by confocal microscopy. Viability and motility of RAW-DEN hybrids remain the same as untreated RAW cells according to WST-1 assay and wound closure assay. Furthermore, Western blot analysis reveals that there are no significant alterations in the expression levels of signaling molecules AKT, p38, and NFκB (p65) and their corresponding activated (phosphorylated) forms in RAW cells treated with azido sugar and dendrimer, indicating that the hybridization process neither induced cell stress response nor altered normal signaling pathways. Taken together, this work shows the feasibility of applying bioorthogonal chemistry to create cell-nanoparticle hybrids and demonstrates the noninvasiveness of this cell surface engineering approach.

Surface engineering of living myoblasts via selective periodate oxidation

Cell surface molecules are vital for normal cell activity. To study the functions of these molecules or manipulate cell behavior, the ability to decorate cell surfaces with bioactive molecules of our choosing is a potentially powerful technique. Here, we describe the molecular engineering of living L6 myoblast monolayers via selective periodate oxidation of sialic acid residues and the application of this surface modification in the artificial aggregation of cells. The aldehyde groups generated by this reaction were used to selectively ligate a model molecule, biotin hydrazide, to the cell surfaces. Flow cytometry analysis after staining with fluorescently conjugated avidin revealed a concentration-dependent increase in fluorescence compared to untreated cells with a maximal shift of 345.1 +/- 27.4-fold and an EC(50) of 17.4 +/- 1.1 microM. This mild oxidation reaction did not affect cell number, viability, or morphology. We then compared this chemical technique with the metabolic incorporation of reactive cell surface ketone groups using N-levulinoylmannosamine (ManLev). In this cell line, only a 22.3-fold fluorescence shift was observed compared to untreated cells when myoblasts were incubated with a high concentration of ManLev for 48 hours. Periodate oxidation was then used to modify myoblast surfaces to induce cell aggregation. Crosslinking biotinylated myoblasts, which do not spontaneously aggregate in culture, with avidin resulted in the rapid formation of millimeter-sized, multicellular structures. These data indicate that sodium periodate treatment is an effective, noncytotoxic method for the in vitro molecular engineering of living cell surfaces with the potential for cell biology and tissue engineering applications.

Expression and stabilization of galactose oxidase in Escherichia coli by directed evolution

We have used directed evolution methods to express a fungal enzyme, galactose oxidase (GOase), in functional form in Escherichia coli. The evolved enzymes retain the activity and substrate specificity of the native fungal oxidase, but are more thermostable, are expressed at a much higher level (up to 10.8 mg/l of purified GOase), and have reduced negative charge compared to wild type, all properties which are expected to facilitate applications and further evolution of the enzyme. Spectroscopic characterization of the recombinant enzymes reveals a tyrosyl radical of comparable stability to the native GOase from Fusarium.

Metabolic delivery of ketone groups to sialic acid residues. Application To cell surface glycoform engineering

The development of chemical strategies for decorating cells with defined carbohydrate epitopes would greatly facilitate studies of carbohydrate-mediated cell surface interactions. This report describes a general strategy for engineering the display of chemically defined oligosaccharides on cell surfaces that combines the concepts of metabolic engineering and selective chemical reactivity. Using a recently described method (Mahal, L. K., Yarema, K. J., and Bertozzi, C. R. (1997) Science 276, 1125-1128), we delivered a uniquely reactive ketone group to endogenous cell surface sialic acid residues by treating cells with the ketone-bearing metabolic precursor N-levulinoylmannosamine (ManLev). The ketone undergoes highly selective condensation reactions with complementary nucleophiles such as aminooxy and hydrazide groups. The detailed quantitative parameters of ManLev metabolism in human and nonhuman-derived cell lines were determined to establish a foundation for the modification of cell surfaces with novel epitopes at defined cell-surface densities. Ketones within the glycoconjugates on ManLev-treated cells were then reacted with synthetic aminooxy and hydrazide-functionalized carbohydrates. The remodeled cells were endowed with novel lectin binding profiles as determined by flow cytometry analysis. The simplicity and generality of this method make it well suited for use in the study of carbohydrate-mediated cell surface interactions.