Strategies for cell membrane functionalization

Abstract

The ability to rationally manipulate and augment the cytoplasmic membrane can be used to overcome many of the challenges faced by conventional cellular therapies and provide innovative opportunities when combined with new biotechnologies. The focus of this review is on emerging strategies used in cell functionalization, highlighting both pioneering approaches and recent developments. These will be discussed within the context of future directions in this rapidly evolving field.

Keywords: Functionalizing; biomaterials; cells; membrane.

Figure 1

Fluorescence microscopy images of functionalized cells. (a) An example of cell surface chemistry, with human foetal osteoblasts (nuclei-labeled blue with DAPI) metabolically labeled with L-azidohomoalanine were conjugated to a biotinylated alkyne that was subsequently visualized using fluorescent streptavidin (labeled red). Reprinted (adapted) with permission from Borcard et al. Bioconjugate Chemistry 22, 1422-32 Copyright 2011 American Chemical Society. (b) An example of non-covalent membrane labeling, in which a polyethylene glycol/oleyl chain was used to anchor proteins such as GFP (labeled green) into NIH3T3 cells. Reproduced with kind permission from John Wiley and Sons: Kato et al. (c) An example of an extended cellular coating, whereby matrix proteins including fibronectin (labeled red) were used to “shrink wrap” C2C12 cells (nuclei-labeled blue with DAPI, actin fibres labeled in green and indicated with arrows). Reproduced with kind permission from Springer Science + Business Media: Palchesko et al., Figure 4(e). (A color version of this figure is available in the online journal.)

Figure 2

Three broad approaches to cell membrane functionalization. (a) The first method is direct surface chemistry, performed on functional groups present on the cell membrane. Here, for instance, amine groups present on membrane proteins have been biotinylated (purple) to allow the addition of streptavidin (yellow). This approach is commonly used to deliver species labeled with streptavidin or biotin. (b) The second method is to increase the cationic surface charge of the exogenous species to facilitate attractive electrostatic interactions with negatively charged moieties present predominantly within the glycocalyx. (c) The third strategy uses hydrophobic interactions between a conjugated lipid tail and the phospholipid bilayer, to anchor the exogenous species to the cell membrane. (A color version of this figure is available in the online journal.)

Figure 3

Metabolic labeling and biorthogonal chemistry. (a) Unnatural biomolecular precursors, included as cell media additives, can be taken up by cells and become incorporated into lipids, carbohydrates or proteins (blue), including those at the cell membrane. (b) Metabolic labeling can be used to present reactive groups that can bind a secondary species (yellow). This is usually mediated by orthogonal click chemistry, in this example, an alkynated secondary species is bound to a cell metabolically labeled with azide groups. (A color version of this figure is available in the online journal.)

Figure 4

Cell paintballing using coacervate microdroplets. Armstrong et al. recently demonstrated that membrane-free coacervate microdroplets can be actively loaded with biomaterial payloads of protein or nucleotides, and then delivered to the cell membrane using optical tweezers. (a)–(e) Time-lapse bright field microscope images showing an optical trap (pink circle) maneuvering a GFP-loaded coacervate microdroplet toward a human mesenchymal stem cell to initiate a targeted fusion event. (f) Fluorescence microscopy revealed fluorescence emission from the GFP payload present at the site of delivery