Live-cell intracellular immunofluorescence

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ABSTRACT Immunofluorescence (IF) is one of the most important tools for both basic biology research and clinical sample imaging due to the broad availability of antibodies and the resolution and sensitivity of fluorescence imaging. Despite the tremendous amount of data produced by IF in the past ~60 years since its initial invention, IF suffers from a fundamental limitation: incapability of interrogating intracellular targets in live cells (cells have to be fixed first, thus only providing a snapshot of the dynamic cell signaling process). Considering the 3D structure of cells, there are far more intracellular targets with biological significance than their cell-membrane counterparts, but immunological agents such as antibodies, antibody fragments, peptides, nanobodies, and scFvs, being highly hydrophilic macromolecules, cannot spontaneously cross the cell membranes. To overcome the barriers of cell membrane and endocytosis that are extremely effective in preventing intact biomolecules to enter the cytosol, we propose to develop an intracellular protein delivery technology building on a unique concept: non- covalent cholesterol tagging. In contrast to conventional delivery technologies that are mostly based on endocytosis (inappropriate for live cell imaging due to the high background generated by imaging agents trapped in endosomes), our small-molecule tag enables proteins to permeate through the cell membrane. More importantly, this is achieved without generating pores in the membrane that cause cytotoxicity. If successful, this platform technology should open a whole new dimension for molecular biology, drug development, and cell engineering.