Rapid cellular uptake of citrate-coated iron oxide nanoparticles unaffected by cell-surface glycosaminoglycans

Citrate-coated iron oxide nanoparticles, specifically Synomag®-COOH (SynC), are promising tracers in magnetic particle imaging (MPI) due to their high magnetic moments and rapid cellular uptake. The mechanisms driving efficient SynC uptake remain unclear. Previous observations suggest a role of the extracellular glycocalyx during nanoparticle uptake. Here, we ascertain whether the cell-surface glycosaminoglycans (GAGs) regulate the uptake of SynC. Using transmission electron microscopy (TEM), we visualized SynC uptake by THP-1 cells, a human acute monocytic leukemia cell line. We investigated the interaction of SynC with GAGs in living cells using click-chemistry-based labeling. Upon treating THP-1 cells with chondroitinase or hyaluronidase and with a xylosyltransferase-deficient cell line, we quantified SynC uptake and measured interactions of SynC with cells in real time using magnetic particle spectroscopy (MPS). The THP-1 cell membrane engulfed or formed extensions around SynC, indicating uptake through pinocytosis and phagocytosis. We measured an increased MPS signal of SynC within seconds of cell contact, suggesting an interaction with extracellular components like the glycocalyx. Upon adding SynC to THP-1 cells, we could not observe disruption of fluorescently labeled GAGs or an enhanced intracellular fluorescence, implying that SynC does not accelerate the turnover of GAGs by binding. Lack of chondroitin sulfate, heparan sulfate, and hyaluronic acid did not affect the rapid magnetic behavior increase of SynC upon cell contact. Accordingly, we measured no significant differences in SynC uptake between wild type cells and our GAG-deficient models. These findings suggest that GAGs act as a permeable bandpass for SynC nanoparticles with a minor negative surface charge of -13.8 mV. This finding has significant implications for MPI-based cell tracking because it facilitates efficient tracking of cell types that lack a strong repulsion by cell-surface GAGs. It will be crucial to investigate whether the rapid uptake of SynC is cell-type specific and influenced by different extracellular matrix compositions.