Magnetic Nanoparticles for Labeling Stem Cells
A crucial aspect of successful cell transplantation is tracking and monitoring the grafted cells in the transplant recipient. To screen cells in vivo, several techniques have been described using nanoparticles (quantum dots, pebbles and superparamagnetic iron-oxide [SPIO] nanoparticles),[13,14] but only SPIO nanoparticles visualized by MRI are suitable in human medicine. SPIO nanoparticles are negative contrast agents that selectively shorten T2-relaxation time and, thus, can be detected in the tissue as a hypointense (dark) signal. MRI, as a noninvasive method, may then be used not only to evaluate whether the cells have been successfully engrafted, but also monitor the time course of cell migration and their survival in the targeted tissue. This information may further help to optimize the transplantation procedure in terms of the number of required cells, the method or site of cell administration and the therapeutic time window after injury during which transplantation will be most effective.
Superparamagnetic iron-oxide nanoparticles usually consist of a crystalline iron oxide core and a polymer shell (Figure 1A). In order to prevent aggregation, dextran is the most commonly used surface coating. Dextran-coated SPIO ferumoxide nanoparticles are commercially available and have been approved as contrast agents by the US FDA as Feridex® and Endorem® or as ultrasmall SPIO nanoparticles (Combidex®, Sinerem®). To facilitate the uptake of nanoparticles into cells, a common labeling approach is to combine commercially available contrast agents and transfection agents (e.g., Superfect, poly-L-lysine, Lipofectamine™, Fugene® or protamine). The advantage of Endorem,[7] as well as of carboxydextran-coated ferucarbotran (Resovist®),[15] is that they have been shown to be suitable contrast agents for labeling rat or human MSCs (Figure 1B & C), ESCs and OECs, and do not require the use of a transfection agent, however, they have a lower labeling efficiency than newly developed coatings.[16,17] Various strategies for optimizing the delivery of magnetic nanoparticles into cells have been developed, such as the specific targeting and endocytosis of nanoparticles through the use of transferrin receptors,[18] magnetodendrimers,[19] transduction agents such as HIV-derived TAT protein[20] or by the use of electroporation.[21]
Figure 1.
MRI tracking of SPIO-labeled stem cells. (A) Scheme of an iron nanoparticle. The contrast agent Endorem® consists of a superparamagnetic Fe3O4 core coated by a dextran shell. (B) Rat mesenchymal stem cell (MSC) culture labeled with superparamagnetic iron-oxide nanoparticles and stained for Prussian blue. (C) Transmission-electron micrograph of a cluster of iron nanoparticles surrounded by a cell membrane. (D) Cortical photochemical lesion visible on MRI 2 weeks after induction as a hyperintensive (light) area. (E) Both the cell implant (MSCs in the hemisphere contralateral to the lesion) and the lesion are hypointensive (dark) 2 weeks after implantation. (F) A few cells weakly stained for Prussian blue were found in the photochemical lesion in animals without implanted cells. (G & H) A hypointense signal in the lesion was seen 7 days after the intravenous injection of Endorem-labeled rat MSCs (G) and persisted for 7 weeks (H). Insets show a higher magnification view of the lesion. (I) Massive invasion of rat MSCs (Prussian blue staining counterstained with hematoxylin) into a photochemical lesion 7 weeks after intravenous injection. (J) Longitudinal section of a rat spinal cord compression lesion on MRI 5 weeks after compression. The lesion is seen as a hyperintensive area (arrow). (K) Prussian blue staining of a spinal cord compression lesion (control animal). (L) Longitudinal MRI of a spinal cord compression lesion populated with intravenously injected nanoparticle-labeled MSCs 4 weeks after implantation. The lesion with nanoparticle-labeled cells is visible as a dark hypointensive area (arrow). (M) Prussian blue staining of a spinal cord lesion with intravenously injected nanoparticle-labeled MSCs.
PI: Postimplantation.
Modified with permission from.[7,40]
Superparamagnetic iron-oxide nanoparticles coated with poly-L-lysine,[16,101]D-mannose, or poly(N,N-dimethylacrylamide)[17] display better cell labeling efficiency and easier MRI detection, along with a lower concentration of iron within the cells, when compared with Endorem. SPIO nanoparticles coated with polyvinyl pyrrolidone[22] or mesoporous silica nanoparticles[23] have also been developed as efficient contrast agents.
For clinical use in particular, it needs to be shown that SPIO nanoparticles are nontoxic and biodegradable and do not affect the proliferation and differentiation potential of MSCs or other cells in vitro.[24] The concentration of iron in the culture media has to be optimized for each cell culture, as a lower concentration may result in insufficient cell uptake, whereas a higher concentration may induce the precipitation of complexes or may be toxic to the cells.
In addition to the use of SPIO nanoparticles for labeling transplanted cells, another strategy, the systemic injection of MRI contrast agents and their subsequent preferential phagocytosis by monocytes and macrophages, has been used as a tool for the in vivo visualization of the inflammatory response after stroke and other CNS pathologies.[25] For in vivo monitoring of macrophages after traumatic brain injury or in an ischemic lesion, the intravenous administration of ultra-SPIO (USPIO) particles,[26,27] micron-sized paramagnetic iron-oxide (MPIO) particles[28] as well as SPIO nanoparticles[29] have been used.
Nanomedicine. 2010;5(1):99-108. © 2010 Future Medicine Ltd.
Cite this: Nanotechnology for Treatment of Stroke and Spinal Cord Injury - Medscape - Jan 01, 2010.
