A challenging approach to the application of multivalent interactions is the mimicry of functional biomacromolecules with therapeutic relevance. Several attempts have been made to mimic specific proteins, e.g., histones or polysaccharides like heparin. In these cases, mimicry is mostly based on the surface charge of the polymer molecules (Figure 1). In the particular case of dendritic PG, polyanionic derivates present similar activities to negatively charged polysaccharides, e.g., heparin, polysialic acid. The amine terminated PGs can act like histones to bind and compact DNA. Applications range from protein resistant coatings (neutral species) to DNA-transfection agents (polycationic systems), and anticoagulating and anti-inflammatory drugs (polyanionic systems).
The research goal is to establish by varying the nanoparticle size, degree of branching, and functionalization the systematic structure activity relationships between dendritic polyanions and polycations as mimics of proteins activity.
Similarities between dendritic polyanions or cations and globular proteins with corresponding surface charges.
In biomineralization, for example, small quantities of carboxylate rich proteins (Figure 2) play an important role because they influence nucleation and crystal growth of inorganic compounds, e.g., CaCO3 and as a result enable formation of inorganic-organic hybrid materials which are much more superior to synthetic ones. Through special interactions growth can also be influenced by polyanionic polymers. Although polyacryl acids and unfunctionalized polyglycerol generate calcite, which is the thermodynamically most stable modification, crystals of the unstable calcium carbonate modifications of vaterite and aragonite are generated in the presence of polyglycerine carboxylates (see Figure 2).
CaCO3 crystallization without (left) and in the presence of polyglycerol carboxylate (middle/right).
Lectins are multivalent carbohydrate-binding proteins which specifically bind different sugar structures. They have received considerable attention recently due to their importance in cell surface interaction and biological recognition. Multivalency in lectins has been discussed in detail and is considered to be a good model for studying multivalency of dendritic polymer derivatives. Our group recently explored the use of multivalent polyglycerol sulfates (see also multivalent research page) for the inhibition of L- and P-selectins, a general class of receptors which displays a selective adhesion and includes a lectin-like domain. Selectin inhibition studies carried out with SPR measurements indicated a clearly enhanced effect due to multivalency. Using L-selectin, the nanomolar binding affinity of PG–galactose was observed. Notably, sulfated dendritic galactose showed further enhancement with an IC50 of 1 nM. This indicates the importance of negatively charged sulfate groups on the surface of these polysaccharide analogues.
Amine functionalized dendrimers have gained special significance for in vitro gene transfection as nonviral polycationic vectors. Analog to polycationic protein complexes (histones), the complexation of DNA molecules with, for example, polyanim dendrimers is based on the interaction between dendrimers that are polycationic under physiological conditions and anionic phosphate groups of the DNA backbone. Because of their compact, neutral structure the resulting dendrimer-DNA complexes can be easily taken up into the cell by endocytosis (Figure 3).
Our transfection experiments showed that incompletely branched polyamines have greater transfection efficiency than completely branched (dendritic) structures. Additionally dendritic polyamine was observed to be clearly dependent upon molecular weight (particle diameter), depending on the size of the biological histones. Currently experiments are in progress in order to reduce the in vivo toxicity of these polycationic vectors by further optimizing the shell. Several hyperbranched PG derivates were post-modified with different amine moieties. These dendritic polyamine compounds were clearly multivalent in gene complexation. The affinity of the amine moieties increased significantly after their attachment onto the PG scaffold. Preliminary cytotoxicity studies showed that the dendritic nanocarriers exhibited a safe profile at the required concentrations and have decreased bioluminescence in a U87-luciferase cell line by 50-80%. In a preliminary in vivo experiment significant gene silencing of some of the structures tested was observed.