Stealth Nanoparticles Grafted with Dense Polymer Brushes Display Adsorption of Serum Protein Investigated by Isothermal Titration Calorimetry

Core–shell nanoparticles receive much attention for their current and potential applications in life sciences. Commonly, a dense shell of hydrated polymer, a polymer brush, is grafted to improve colloidal stability of functional nanoparticles and to prevent protein adsorption, aggregation, cell recognition, and uptake. Until recently, it was widely assumed that a polymer brush shell indeed prevents strong association of proteins and that this leads to their superior “stealth” properties in vitro and in vivo. We show using <i>T</i>-dependent isothermal titration calorimetry on well-characterized monodisperse superparamagnetic iron oxide nanoparticles with controlled dense stealth polymer brush shells that “stealth” core–shell nanoparticles display significant attractive exothermic and enthalpic interactions with serum proteins, despite having excellent colloidal stability and negligible nonspecific cell uptake. This observation is at room temperature shown to depend only weakly on variation of iron oxide core diameter and type of grafted stealth polymer: poly­(ethylene glycol), poly­(ethyl oxazoline), poly­(isopropyl oxazoline), and poly­(<i>N</i>-isopropyl acrylamide). Polymer brush shells with a critical solution temperature close to body temperature showed a strong temperature dependence in their interactions with proteins with a significant increase in protein binding energy with increased temperature. The stoichiometry of interaction is estimated to be near 1:1 for PEGylated nanoparticles and up to 10:1 for larger thermoresponsive nanoparticles, whereas the average free energy of interaction is enthalpically driven and comparable to a weak hydrogen bond

Influence of Grafted Block Copolymer Structure on Thermoresponsiveness of Superparamagnetic Core–Shell Nanoparticles

The morphology and topology of thermoresponsive polymers have a strong impact on their responsive properties. Grafting onto spherical particles has been shown to reduce responsiveness and transition temperatures; grafting of block copolymers has shown that switchable or retained wettability of a surface or particle during desolvation of one block can take place. Here, doubly thermoresponsive block copolymers were grafted onto spherical, monodisperse, and superparamagnetic iron oxide nanoparticles to investigate the effect of thermal desolvation on spherical brushes of block copolymers. By inverting the block order, the influence of core proximity on the responsive properties of the individual blocks could be studied as well as their relative influence on the nanoparticle colloidal stability. The inner block was shown to experience a stronger reduction in transition temperature and transition enthalpy compared to the outer block. Still, the outer block also experiences a significant reduction in responsiveness due to the restricted environment in the nanoparticle shell compared to that of the free polymer state. The demonstrated pronounced distance dependence importantly implies the possibility, but also the necessity, to radially tailor polymer hydration transitions for applications such as drug delivery, hyperthermia, and biotechnological separation for which thermally responsive nanoparticles are being developed

Lipid Bilayers Significantly Modulate Cross-Fibrillation of Two Distinct Amyloidogenic Peptides

Amyloid plaques comprising misfolded proteins are the hallmark of several incurable diseases, including Alzheimer’s disease, type-II diabetes, Jacob–Creutzfeld disease, and others. While the exact molecular mechanisms underlying protein misfolding diseases are still unknown, several theories account for amyloid fiber formation and their toxic significance. Prominent among those is the “prion hypothesis” stipulating that misfolded protein seeds act as “infectious agents” propagating aggregation of nominally healthy, native proteins. Recent studies, in fact, have reported that interactions between different amyloid peptides that are partly sequence-related might also affect fibrillation pathways and pathogenicity. Here, we present evidence that two structurally and physiologically unrelated amyloidogenic peptides, the islet amyloid polypeptide (IAPP, the peptide comprising the amyloid aggregates in type II diabetes) and an amyloidogenic determinant of the prion protein (PrP), give rise to a significantly distinct fibrillation pathway when they are incubated together in the presence of membrane bilayers. In particular, the experimental data demonstrate that the lipid bilayer environment is instrumental in initiating and promoting the assembly of morphologically distinct fibrillar species. Moreover, cross-fibrillation produced peptide species exhibiting significantly altered membrane interaction profiles, as compared to the scenario where the two peptides aggregated separately. Overall, our data demonstrate that membranes constitute a critical surface-active medium for promoting interactions between disparate amyloidogenic peptides, modulating both fibrillation pathways as well as the biophysical properties of the peptide aggregates. This work hints that membrane-induced cross-fibrillation of unrelated amyloidogenic peptides might play an insidious role in the molecular pathologies of protein misfolding diseases

Interaction of Size-Tailored PEGylated Iron Oxide Nanoparticles with Lipid Membranes and Cells

Targeted nanomedicine builds on the concept that nanoparticles can be directed to specific tissues while remaining inert to others organs. Many studies have been performed on the synthesis and cellular interactions of core–shell nanoparticles, in which a functional inorganic core is coated with a biocompatible polymer layer that should reduce nonspecific uptake and cytotoxicity. However, work is lacking that relates structural parameters of the core–shell structure and colloidal properties directly to interactions with cell membranes and further correlates these interactions to cell uptake. We have synthesized monodisperse (SD < 10%), single-crystalline, and superparamagnetic iron oxide nanoparticles (SPION) of different core size (3–8 nm) that are densely grafted with nitrodopamine-poly­(ethylene glycol) (NDA-PEG­(5 kDa)) brushes. We investigated the interactions of the PEGylated SPION with biomimetic membranes and cancer and kidney cells. It is shown that a dense homogeneous PEG shell suppresses membrane interactions and cell uptake but that nanoparticle curvature can influence membrane interactions for similarly grafted nanoparticles. Weak adsorption to anionic lipid membranes is shown to correlate with eukaryote cell uptake and is attributed to double-layer interactions without direct membrane penetration. This attraction is strongly suppressed during physiological conditions and leads to unprecedented low cell uptake and full cell viability when compared to those of traditional dextran-coated SPION. Less curved (larger core) PEGylated SPION show weaker membrane adsorption and lower cell uptake due to effectively denser shells. These results provide a better understanding of design criteria for core–shell nanoparticles in terms of avoiding nonspecific uptake by cells, reducing toxicity, and increasing circulation time