TEM
Transmission electron microscopy
w DNA
Weight fraction of DNA
z
+
/z
À
Molar charge ratio polycation/polyanion
1 Exploring Cell Interactions of Dendritic
and Protein Polyelectrolytes
1.1 Introduction
Ionic interactions play a crucial role in many cellular processes such as membrane
permeation or regulation of gene transcription [1]. The cell membrane, with its
bilayer and negatively charged surface, serves as a protective barrier for the living
cell towards its environment. Hydrophobic or positively charged molecules can
interact with the cell membrane and can enter cells via various mechanisms [2]. In
the cell nucleus, polycationic proteins and histones form complexes with DNA and
regulate transcription through alteration of the charge densities of nuclear proteins
and DNA [3]. Inspired by these polyelectrolytic interactions in nature, synthetic
nanosized polyelectrolytes have been designed that contain multiple electrolyte
groups, i.e., polycations or polyanions. Some of these polyelectrolyte carriers have
already been successfully explored for biomedical applications such as gene or
cancer therapy [4, 5]. With greater emphasis on defined macromolecular architectures
of the materials, regularly branched cascade molecules (so-called dendrimers) and
protein-based polyelectrolytes have emerged over the past decade. Their interactions
with cells have been analyzed to elucidate the role of polyelectrolyte interactions in
nature and to apply this knowledge for the design of efficient and biocompatible
nanotransporters for the delivery of cargoes such as drugs, proteins, or nucleic acids
into cells and subcellular compartments [6, 7].
Endocytosis is essential for uptake of macromolecules and nanoparticles. Thereby,
the physical and chemical characteristics of the cargo and transporter complex
determine the nature of the cell uptake mechanism. Particles larger than 500 nm are
typically phagocytosed or macropinocytosed, whereas smaller hydrophilic molecules
are internalized by any of the various endocytotic processes [2, 8]. Often, nanometersized transporter molecules or particles accomplish cell uptake simultaneously by
several endocytotic mechanisms [9]. Clathrin-dependent endocytosis is the most
widely understood pathway, which is present in nearly all mammalian cells. The
clathrin pathway is considered the most important uptake mechanism for several
polymers such as poly(ethylene glycol)-polylactides, poly(lactide-co-glycolide)
(PLGA), silica-based nanoparticles, and chitosan nanoparticles [8, 10–12].
Besides clathrin-mediated cell uptake, caveolae-mediated endocytosis is another
well-investigated uptake mechanism. The caveolae pathway is particularly attractive
for the delivery of proteins and nucleic acids because these vesicles have neutral pH
and the lysosomes, where degradation takes place, are bypassed [8, 13]. Nanoparticles
Polymer Complexes in Biological Applications
213
Transmission electron microscopy
w DNA
Weight fraction of DNA
z
+
/z
À
Molar charge ratio polycation/polyanion
1 Exploring Cell Interactions of Dendritic
and Protein Polyelectrolytes
1.1 Introduction
Ionic interactions play a crucial role in many cellular processes such as membrane
permeation or regulation of gene transcription [1]. The cell membrane, with its
bilayer and negatively charged surface, serves as a protective barrier for the living
cell towards its environment. Hydrophobic or positively charged molecules can
interact with the cell membrane and can enter cells via various mechanisms [2]. In
the cell nucleus, polycationic proteins and histones form complexes with DNA and
regulate transcription through alteration of the charge densities of nuclear proteins
and DNA [3]. Inspired by these polyelectrolytic interactions in nature, synthetic
nanosized polyelectrolytes have been designed that contain multiple electrolyte
groups, i.e., polycations or polyanions. Some of these polyelectrolyte carriers have
already been successfully explored for biomedical applications such as gene or
cancer therapy [4, 5]. With greater emphasis on defined macromolecular architectures
of the materials, regularly branched cascade molecules (so-called dendrimers) and
protein-based polyelectrolytes have emerged over the past decade. Their interactions
with cells have been analyzed to elucidate the role of polyelectrolyte interactions in
nature and to apply this knowledge for the design of efficient and biocompatible
nanotransporters for the delivery of cargoes such as drugs, proteins, or nucleic acids
into cells and subcellular compartments [6, 7].
Endocytosis is essential for uptake of macromolecules and nanoparticles. Thereby,
the physical and chemical characteristics of the cargo and transporter complex
determine the nature of the cell uptake mechanism. Particles larger than 500 nm are
typically phagocytosed or macropinocytosed, whereas smaller hydrophilic molecules
are internalized by any of the various endocytotic processes [2, 8]. Often, nanometersized transporter molecules or particles accomplish cell uptake simultaneously by
several endocytotic mechanisms [9]. Clathrin-dependent endocytosis is the most
widely understood pathway, which is present in nearly all mammalian cells. The
clathrin pathway is considered the most important uptake mechanism for several
polymers such as poly(ethylene glycol)-polylactides, poly(lactide-co-glycolide)
(PLGA), silica-based nanoparticles, and chitosan nanoparticles [8, 10–12].
Besides clathrin-mediated cell uptake, caveolae-mediated endocytosis is another
well-investigated uptake mechanism. The caveolae pathway is particularly attractive
for the delivery of proteins and nucleic acids because these vesicles have neutral pH
and the lysosomes, where degradation takes place, are bypassed [8, 13]. Nanoparticles
Polymer Complexes in Biological Applications
213