Magnetic Resonance Tracking of Stem Cells with Iron Oxide Particles - Intracellular Delivery: Fundamentals and Applications - page 472

this technology is the use of these proteins as markers of expression during
gene therapy. These proteins can also be applied to the imaging of endogenous gene
expression during development and pathogenesis in transgenic animals.

5.1 Transferrin

Free, systemic iron is captured by the plasma protein transferrin (Tf), while iron-
loaded Tf is transferred into cells after binding to the transferrin receptor (TfR).
This receptor is naturally abundant on some cell types and has been utilised to
improve the uptake of Tf-conjugated SPIO (Daldrup-Link et al. 2003 ). When
human Tf-conjugated MION were administered IV, the particles bound specifically
to rat gliosarcoma cells transfected to express human TfR (Weissleder et al. 2000 ;
Moore et al. 2001 ). Therefore, TfR can be a model marker for MR visualisation of
in vivo gene expression.

5.2 Ferritin

Ferritin is a ubiquitous and highly-conserved iron-binding protein (Harrison et al.
1996 ). In vertebrates, cytosolic ferritin is a heteropolymer composed of variable
proportions of H and L subunits. The H subunit has ferroxidase activity that promotes
iron oxidation and incorporation, while the L subunit facilitates the activity of the
H-chain by offering sites for iron nucleation and mineralization. Twenty-four
ferritin subunits assemble to form the apoferritin shell. Each apoferritin molecule
of 450 kDa can sequester up to 4,500 iron atoms, depending on the tissue type
and physiologic status of the cell. Abnormality of iron metabolism has been
detected by MRI due to the elevation in ferritin and iron storage (Grabill et al.
2003 ). The T2 relaxation properties of iron-loaded ferritin have also been studied
in vitro (Gossuin et al. 2000 ).
An early study on the transfection of cells to overexpress H-chain ferritin showed
upregulation of TfR accompanied with increased intracellular iron (Cozzi et al.
2000 ). In a separate study, the change in cellular iron was detected as a 7% change
in MR relaxation rate when the ferritin gene was switched on (Cohen et al. 2005 ).
Inducible ferritin expression was also detected in the liver and heart of transgenic
fetal mice, with 25% change in relaxation rates (Cohen et al. 2007 ).
Poor sensitivity is a limitation of these attempts to image gene expression, as
turning on the ferritin gene increases cellular iron content by only 0.2 pg/cell (Cohen
et al. 2005 ). Sensitivity has not been improved by the co-transfection of both ferritin
and TrR genes which yielded an iron content increase of only 14 fg/cell (Deans et al.
2006 ). However, studies are ongoing to improve the sensitivity of imaging ferritin
expression by increase relaxation rate for the same iron quantity per cell through
controlled aggregation of ferritin (Bennett et al. 2008 ).