© GEORGE RETSECK
One of the main goals of cell encapsulation techniques is to protect therapeutically important transplanted cell populations from the host’s immune system and thus prolong their viability and effects. But this increasingly popular approach has also led to single-cell encapsulation techniques aimed at basic research applications. The idea is that individual cells in protective yet permeable shells are easier to handle, can better withstand a variety of procedures, and can be stimulated and analyzed in three dimensions, explains Shinji Sakai of Osaka University in Japan.
A recently described method for encapsulating individual animal cells involves coating them with alternating layers of positively and negatively charged polymers (Langmuir, 26:5670–78, 2010). However, says Sakai, few charged polymers are compatible with cell viability, and the resulting shells tend to be weak because they are thin and lack covalent bonds.
Now, Sakai is giving cells sturdy, snug-fitting coats that can be made from a wide variety of materials. He attaches to the cells’ membranes an enzyme, horseradish peroxidase (HRP), that can stitch together (covalently cross-link) almost any polymer to form a hydrogel—as long as the polymer contains enzymatically crosslinkable moieties (either intrinsic or added). Cells can thus be encased in a variety of hydrogel shells, including ones made of polysaccharide derivatives that can be easily and harmlessly degraded.
The variety of possible coatings and, therefore, of possible applications—such as functional analyses and three-dimensional tissue fabrication—is probably the best thing about the technique, says João Mano of the University of Minho in Portugal who adds that he admires the “elegant chemistry” used to make the coats.
Sakai plans to stretch the versatility of the technique even further by finding different ways to attach HRP to specific cell types. “We want to prepare a 3-D tissue with . . . different suits tailor-made for individual cell types,” he says. (ACS Macro Lett, 3:972-75, 2014)
|TECHNIQUE||HOW IT WORKS||PREP TIME||THICKNESS||ENCAPSULATION MATERIALS|
|Layer-by-layer single cell coating||Cell suspensions are repeatedly transferred between solutions of positively or negatively charged polymers to build a layered shell.||Approximately 10 minutes per layer; typically at least five layers||
Each layer is just
a few nanometers. Final shells are 10–100 nm thick.
Frequently used anionic polymers include alginate, pectin, carrageenan, and carboxymethyl cellulose. Cationic polymers include
polylysine, chitosan, and polyethyleneimine.
|Cell surface polymer cross-linking||Membrane-targeted polymer-stitching enzyme (horseradish peroxidase) is added to a cell suspension followed by hydrogen peroxide and the polymer of interest.||Roughly 30 minutes||Approximately 1000 nm||Any water-soluble polymer with enzymatically cross-linkable moieties, either introduced or naturally occurring, e.g. sugar-beet pectin|