What is the purpose of adding the Polyethylene glycol (PEG) to the mixture of cells?

Question

What do you think would be the action of PEG at the molecular level to accomplish this desired outcome?

Answer 1

Hi Angelz,

This is an interesting question, and one which was eagerly researched in the 1980s by numerous groups. At the time, several mechanisms were suggested for how PEG accomplished cell fusion.  These included

(1) increasing surface tension,

(2) absorbing to and cross-linking bilayers,

(3) altering the structure and dielectric properties of bulk water,

(4) altering the molecular order of the bilayer at the point of contact, (5) producing volume-exclusion-induced aggregation and dehydration, (6) induction of non-bilayer structures,

(7) acting as a detergent to disrupt bilayer structure,

(8) inducing phase separation that destabilizes the bilayer,

(9) producing compressive and then, upon dilution, expansive osmotic stress on membrane vesicles,

(10) containing impurities that disrupted membranes.

Most of these proposed mechanisms did not hold up to scrutiny. Early work did show that some of the effect of PEG was due to impurities, but even after purification PEG is still effective in triggering fusion. (Lentz et al. 1992).

The work of Klaus Arnold and colleagues provided crucial information in ultimately defining the mechanism by which PEG promoted membrane fusion.

PEG was known to aggregate membranes, as demonstrated quantitatively by electron microscopy (Saez et al. 1982) and X-ray scattering (Boni et al. 1984).

Some explained this according to surface absorption and cross-linking. Arnold showed that low molecular weight PEG covalently attached to an alkyl chain that would insert into bilayers (the detergent C12E8), pushed bilayers apart (Arnold et al. 1980), rather than drawing them together.

This was extended to phosphatidylethanolamine labeled with PEG-2000 attached to the head group (Kasbauer et al. 1997). This suggested that absorption of PEG to membrane surfaces was not likely to explain PEG-mediated membrane aggregation, as was confirmed by the observation that PEG could be separated from lipid vesicles by a dialysis membrane and still drive membranes into closer contact (Boni et al. 1984).

Ruby MacDonald (1985) then showed that vesicles would still fuse when aggregated by PEG in this manner. In addition, Arnold demonstrated that water was excluded from regions of contact between PEG-aggregated vesicles using NMR (Arnold et al. 1985) and electrophoretic mobility (Arnold et al. 1990).

A theoretical treatment supported these experiments by showing that surface exclusion would be expected to provide a membrane aggregating attraction (Evans and Needham 1988), as was then confirmed by direct experiment (Kuhl et al. 1996).

Today we know that, aside from producing a thermodynamic force driving close contact between membranes, PEG also promotes fusion via a positive osmotic pressure that likely helps stabilize fusion intermediates (Malinin et al. 2002).

This osmotic effect is not required to trigger fusion, so that the main effect of PEG on membrane vesicles is volume-exclusion aggregation of membranes and dehydration in areas of contact.

Hope this helps-

Jim

P.S. Please read the review article cited below if you need more detailed information.

Answer 2

PEG reduces the zeta potential. Reducing the zeta potential allows the cells to come closer together increasing the chance of their membranes interacting.