Magnetic nanoparticles for oligodendrocyte precursor cell transplantation therapies: progress and challenges

Oligodendrocyte precursor cells (OPCs) have shown high promise as a transplant population to promote regeneration in the central nervous system, specifically, for the production of myelin – the protective sheath around nerve fibers. While clinical trials for these cells have commenced in some areas, there are currently key barriers to the translation of neural cell therapies. These include the ability to (a) image transplant populations in vivo; (b) genetically engineer transplant cells to augment their repair potential; and (c) safely target cells to sites of pathology. Here, we review the evidence that magnetic nanoparticles (MNPs) are a ‘multifunctional nanoplatform’ that can aid in safely addressing these translational challenges in neural cell/OPC therapy: by facilitating real-time and post-mortem assessment of transplant cell biodistribution, and biomolecule delivery to transplant cells, as well as non-invasive ‘magnetic cell targeting’ to injury sites by application of high gradient fields. We identify key issues relating to the standardization and reporting of physicochemical and biological data in the field; we consider that it will be essential to systematically address these issues in order to fully evaluate the utility of the MNP platform for neural cell transplantation, and to develop efficacious neurocompatible particles for translational applications. Electronic supplementary material The online version of this article (doi:10.1186/2052-8426-2-23) contains supplementary material, which is available to authorized users.


Fe3O4-PEI-RITC MNPs:
The Fe3O4-PEI-RITC particles comprise a magnetite core, surrounded by a covalently attached polyethyleneimine (PEI) layer, onto which a red dye (rhodamine B isothiocyanate, RITC) is bound. The synthesis of these particles and their characterization have been described elsewhere, with TEM analyses indicating a uniform spherical shape and a core diameter of 24.3 ± 5.7 nm [4].

Fe3O4-PEI-RITC uptake experiments:
At 24 h after plating, OPCs were incubated with 20 µg/ml MNPs for 24 h. Samples were washed with phosphate buffered saline (PBS), then fixed and either immunostained, or processed for Perls' Prussian blue histochemical staining. Fluorescence microscopy: Samples were imaged using an Axio Scope A1 fluorescence microscope (Carl Zeiss MicroImaging GmbH, Goettingen, Germany), and the images merged using Adobe Photoshop CS3 (version 10.0.1).

Z-stack fluorescence microscopy:
Z-stack fluorescence micrographs were created using a Nikon Eclipse 80i microscope fitted with a CA742-95 camera (Hamamatsu Photonics, Hamamatsu, Japan), with manual focus stepping at 0.5 µm, and the image manipulations performed using NIS Elements (Nikon, version 3.00).

Perls' Prussian blue iron staining:
Fixed OPCs were incubated with 2% potassium ferricyanide in 2% HCl for 10 min, washed three times with distilled water and then mounted in glycerol-based mounting medium without DAPI.
Calculations to convert data presented as nmol Fe/mg cellular protein to pg Fe/cell: The Dringen group at Bremen University have published OLN-93 MNP uptake data as nmol Fe/mg cellular protein [5][6][7][8]. This is difficult to compare with other studies, which typically report pg Fe/cell. However, this group have previously published data which can be analysed to determine the average quantity of protein per OLN-93 cell [9]. Figure 1, page 139, shows cellular protein content (µg/well) from a 12 well plate (~22.1 mm diameter, ~384 mm 2 area) and number of cell nuclei per mm 2 at 24, 48 and 72 h. These approximate values were derived from the bar graphs in this Figure, and the three timepoints compared (Table S1).  (Table S2).