Biomedical Engineering Reference
In-Depth Information
4 Cell Surface Modifications
Cell surfaces can be modified in a number of ways, including covalent conjugation
of polymers to amino groups on membrane proteins [
102
-
105
], electrostatic inter-
action between cationic polymers and a negatively charged cell surface [
106
-
109
],
and hydrophobic interactions that anchor long alkyl chains of amphiphilic polymers
to the lipid bilayer of the cell membrane [
110
-
126
]. These methods have been used
to immobilize various functional groups and bioactive substances onto the cell
surface. The covalent conjugation method is expected to impair membrane protein
functions because polymers are attached by crosslinking to the amino groups on
membrane proteins. Therefore, the extent of this reaction should be carefully
controlled. The electrostatic interaction is performed by simply adding a cationic
polymer solution to a cell suspension. However, most cationic polymers are cyto-
toxic; therefore, this treatment causes deterioration or death to most cell types. In
contrast, the hydrophobic interaction can be performed by simply adding amphi-
philic polymers with long alkyl chains to a cell suspension. This has not caused any
major damage to cell function or integrity. Our group has extensively studied
surface modifications with amphiphilic polymers [
110
-
126
].
4.1 Cell Surface Modifications with Amphiphilic Polymers
Amphiphilic polymers are typically derived by conjugating polyethylene glycol
(PEG) to a phospholipid (PEG-lipid) (Fig.
8
)[
117
,
118
]. When a PEG-lipid solution
is added to a cell suspension, the hydrophobic alkyl chains of the PEG-lipid sponta-
neously form hydrophobic interactions with the lipid bilayer of the cell membrane
(Fig.
8a
). This spontaneous anchoring of the PEG-lipid to a supported lipid mem-
brane was confirmed by observation with surface plasmon resonance (SPR) (Fig.
9a
).
Three kinds of PEG-lipids with different alkyl chain lengths were added to a lipid
membrane that had been created on a SPR sensor surface. The SPR angle rapidly
increased with the spontaneous anchoring of PEG-lipid into the lipid membrane. In
addition, the anchoring rates decreased with increasing alkyl chain lengths. This
spontaneous anchoring was also demonstrated with a human cell line derived from T
cell leukemia cells (CCRF-CEM). A solution of fluorescein isothiocyanate (FITC)-
conjugated PEG-lipid was added to a suspension of CCRF-CEM. Under a confocal
laser scanning microscope, the bright fluorescence from FITC was observed at the
periphery of all cells (Fig.
9b
). This indicated that PEG-lipids had lodged on the cell
surface. The retention time of PEG-lipids on cell membranes can be controlled by
adjusting the length of the lipid alkyl chain. The dissociation rate of PEG-lipid was
much slower with long than with short hydrophobic domains [
111
].
Proteins can be immobilized on the cell surface with the use of a short, single
stranded DNA (ssDNA) attached to the end of a PEG chain (ssDNA-PEG-lipid)
[
114
-
116
,
119
,
125
]. First, an ssDNA-PEG-lipid is prepared by conjugating
