In contrast, polyanionic core–shell dendrimers such as 5 (Fig. 3a) did not display
any cellular uptake and, therefore, it was not feasible to investigate the fate of these
polyanions inside viable cells [18]. However, applying them to fixed cells allowed
cell membrane uptake and specific staining of the cell nuclei in Drosophila tissue
(Fig. 3b) [20]. In the cell nucleus, DNA wraps around histone proteins forming
nucleosomes. The role of histone proteins is to pack and order DNA into structural
units based on electrostatic interactions with DNA, which has an important impact
for gene transcription. In the presence of the polyanionic core–shell macromolecule
5, bearing high numbers of negatively charged carboxylate groups, DNA is most
probably displaced from the complexes and polyelectrolyte complexes between
5 and the histone H1 proteins are formed due to tight interactions (Fig. 3c). 5 specifically stains the cell nucleus by binding to the positively charged nuclear proteins
(Fig. 3d), and it might therefore serve as an attractive alternative to conventional
fluorescent antibodies. Polyanionic core–shell dendrimers combine unique
properties such as water solubility, high photochemical stability, narrow emission
spectra, and cell-specific binding, making them attractive for cytochemical and
histochemical studies.
1.3 Polycationic Serum Albumin Proteins for Gene Delivery
As discussed above, many synthetic polycations exhibit high cellular toxicities, even
at low concentrations. Interestingly, polycationic proteins such as nuclear histone
proteins are often much less cytotoxic, which might be due to the presence of many
other functional groups within the polypeptide scaffold that potentially “dilute” the
effect of the positive charges. In order to assess the impact of the positive charges of
polycationic proteins on cell uptake and cell toxicity, polycationic derivatives of the
abundant blood plasma protein transporter serum albumin were prepared [6]. Serum
albumin is well suited to impart chemical surface modifications due to the high
number of negatively charged amino acids (e.g., 100 glutamate and aspartate groups),
its diameter of 4.7 nm calculated from the crystal structure [6], and the presence of
lipophilic pockets that can accommodate lipophilic guest molecules. Albumin
polycations have been achieved by successive conversion of carboxylic acid side
chains of the negatively charged amino acids of aspartate and glutamate residues into
primary amino groups (Fig. 4) [6]. Stepwise reaction of the carboxylic acid groups
with increasing amounts of ethylenediamine and catalyst allowed the synthesis of
albumin polycations with varying charge densities. The albumin polycations are
denoted according to their modification, e.g., cBSA-147 is cationized bovine serum
albumin with 147 additional primary amino groups that have been introduced by
chemical modification.
With increasing charge density, more efficient cellular uptake of the polycationic
BSA derivatives has been found and the highest polycationic albumin cBSA-147
revealed the most efficient uptake into A549 cells [6]. Interestingly, even at high
cBSA-147 concentrations only low cytotoxicity has been found. Cell uptake proceeds
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J. Hedrich et al.
any cellular uptake and, therefore, it was not feasible to investigate the fate of these
polyanions inside viable cells [18]. However, applying them to fixed cells allowed
cell membrane uptake and specific staining of the cell nuclei in Drosophila tissue
(Fig. 3b) [20]. In the cell nucleus, DNA wraps around histone proteins forming
nucleosomes. The role of histone proteins is to pack and order DNA into structural
units based on electrostatic interactions with DNA, which has an important impact
for gene transcription. In the presence of the polyanionic core–shell macromolecule
5, bearing high numbers of negatively charged carboxylate groups, DNA is most
probably displaced from the complexes and polyelectrolyte complexes between
5 and the histone H1 proteins are formed due to tight interactions (Fig. 3c). 5 specifically stains the cell nucleus by binding to the positively charged nuclear proteins
(Fig. 3d), and it might therefore serve as an attractive alternative to conventional
fluorescent antibodies. Polyanionic core–shell dendrimers combine unique
properties such as water solubility, high photochemical stability, narrow emission
spectra, and cell-specific binding, making them attractive for cytochemical and
histochemical studies.
1.3 Polycationic Serum Albumin Proteins for Gene Delivery
As discussed above, many synthetic polycations exhibit high cellular toxicities, even
at low concentrations. Interestingly, polycationic proteins such as nuclear histone
proteins are often much less cytotoxic, which might be due to the presence of many
other functional groups within the polypeptide scaffold that potentially “dilute” the
effect of the positive charges. In order to assess the impact of the positive charges of
polycationic proteins on cell uptake and cell toxicity, polycationic derivatives of the
abundant blood plasma protein transporter serum albumin were prepared [6]. Serum
albumin is well suited to impart chemical surface modifications due to the high
number of negatively charged amino acids (e.g., 100 glutamate and aspartate groups),
its diameter of 4.7 nm calculated from the crystal structure [6], and the presence of
lipophilic pockets that can accommodate lipophilic guest molecules. Albumin
polycations have been achieved by successive conversion of carboxylic acid side
chains of the negatively charged amino acids of aspartate and glutamate residues into
primary amino groups (Fig. 4) [6]. Stepwise reaction of the carboxylic acid groups
with increasing amounts of ethylenediamine and catalyst allowed the synthesis of
albumin polycations with varying charge densities. The albumin polycations are
denoted according to their modification, e.g., cBSA-147 is cationized bovine serum
albumin with 147 additional primary amino groups that have been introduced by
chemical modification.
With increasing charge density, more efficient cellular uptake of the polycationic
BSA derivatives has been found and the highest polycationic albumin cBSA-147
revealed the most efficient uptake into A549 cells [6]. Interestingly, even at high
cBSA-147 concentrations only low cytotoxicity has been found. Cell uptake proceeds
218
J. Hedrich et al.