Biocompatible Azide-Alkyne “Click” Reactions for Surface Decoration of Glyco-Engineered Cells

Apoptosis or programmed cell death is a key regulator of physiological growth control and regulation of tissue homeostasis. One of the most important advances in cancer research in recent years is the recognition that cell death mostly by apoptosis is crucially involved in the regulation of tumor formation and also critically determines treatment response. Killing of tumor cells by most anticancer strategies currently used in clinical oncology, for example, chemotherapy, gamma-irradiation, suicide gene therapy or immunotherapy, has been linked to activation of apoptosis signal transduction pathways in cancer cells such as the intrinsic and/or extrinsic pathway. Thus, failure to undergo apoptosis may result in treatment resistance. Understanding the molecular events that regulate apoptosis in response to anticancer chemotherapy, and how cancer cells evade apoptotic death, provides novel opportunities for a more rational approach to develop molecular-targeted therapies for combating cancer.

Selective chemical reactions enacted within a cellular environment can be powerful tools for elucidating biological processes or engineering novel interactions. A chemical transformation that permits the selective formation of covalent adducts among richly functionalized biopolymers within a cellular context is presented. A ligation modeled after the Staudinger reaction forms an amide bond by coupling of an azide and a specifically engineered triarylphosphine. Both reactive partners are abiotic and chemically orthogonal to native cellular components. Azides installed within cell surface glycoconjugates by metabolism of a synthetic azidosugar were reacted with a biotinylated triarylphosphine to produce stable cell-surface adducts. The tremendous selectivity of the transformation should permit its execution within a cell's interior, offering new possibilities for probing intracellular interactions.

Dissecting complex cellular processes requires the ability to track biomolecules as they function within their native habitat. Although genetically encoded tags such as GFP are widely used to monitor discrete proteins, they can cause significant perturbations to a protein's structure and have no direct extension to other classes of biomolecules such as glycans, lipids, nucleic acids and secondary metabolites. In recent years, an alternative tool for tagging biomolecules has emerged from the chemical biology community--the bioorthogonal chemical reporter. In a prototypical experiment, a unique chemical motif, often as small as a single functional group, is incorporated into the target biomolecule using the cell's own biosynthetic machinery. The chemical reporter is then covalently modified in a highly selective fashion with an exogenously delivered probe. This review highlights the development of bioorthogonal chemical reporters and reactions and their application in living systems.