Lipid-assisted microinjection: introducing material into the cytosol and membranes of small cells

The microinjection of synthetic molecules, proteins, and nucleic acids into the cytosol of living cells is a powerful technique in cell biology. However, the insertion of a glass micropipette into the cell is a potentially damaging event, which presents significant problems, especially for small mammalian cells (spherical diameter = 2-15 micron), especially if they are only loosely adherent. The current technique is therefore limited to cells that are both sufficiently large or robust and firmly attached to a substrate. We describe here a modification of the standard technique that overcomes some of the problems associated with conventional microinjection but that does not involve the insertion of a micropipette deep into the cell cytoplasm. Instead, this method depends on lipid fusion at the micropipette tip to form a continuous but temporary conductance pathway between the interiors of the micropipette and cell. This technique thus also provides a novel method of transferring lipids and lipid-associated molecules to the plasma membrane of cells.