Protein-protein interactions determine cellular functions driven by electrostatic and other noncovalent forces. As an example of negatively charged macromolecule, heparan sulfate interaction with the alarmin high mobility group box 1 (HMGB1) is critical for receptor for advanced glycation end products (RAGE)-mediated signaling. Here we investigated the hypothesis that dendritic polyglycerol (dPG) dendrimers could replace heparan sulfate (i) in functional interactions with HMGB1 and (ii) in regulating interleukin-33 (IL-33) interactions with its receptors to modulate immune responses. To determine potential effects on immune cells in the brain, we investigated the charge-dependent effects of negatively charged sulfated dPG (dPGS), positively charged aminated dPG (dPGA) and hybrid dPG (sulfated and aminated dPGSA) dendrimers on IL-33 and HMGB1 in microglia. We used structural modeling to predict the protein interfaces mediating electrostatic interactions resulting in ligand-receptor complex formation. In addition, proximity ligation assays were performed to verify the binding of HMGB1 and IL-33 to ST2, RAGE and Toll-like receptor 4. Our results show that IL-33, an alarmin of the interleukin-1 family, cross-talks with ST2 and RAGE in lipopolysaccharide-activated human microglia. Notably, dPGS inhibited IL-33 interactions with ST2 but not RAGE, whereas dPGSA inhibited both IL-33 and HMGB1 interactions with their receptors. Thus, this study provides insights in the mechanisms of alarmin-mediated molecular cross-talks in human microglia that are driven by electrostatic interactions and the modulation by differentially charged dendrimers in microglia activated by the pro-inflammagen lipopolysaccharide.
Zhang et al. (Mon,) studied this question.