10
Stem cells are non-phagocytic and must be induced to incorporate the SPIOs.
This can be accomplished using any of the following approaches.
Magnetofection is a technique where the negatively charged SPIO is mixed
with a positively charged transfection agent. The electrostatic interactions
create a SPIO-Transfection agent complex that is incubated with stem cells
for 24 h allowing migration of the complex into the stem cells through invag-
ination or pinocytosis. The second technique involves use of low voltage
electromagnetic pulses to transfect stem cells with proteins or DNA. It is
called magneto-electroporation (MEP) and is much faster than magnetofec-
tion, allowing stem cell labeling in a minutes and does not need a transfection
agent. Both techniques allow stable uptake of SPIO’s in stem cells. Recently,
a new instant magnetic cell labeling technique, called magnetosonoporation
(MSP) has been described that uses ultrasound waves instead of electric
pulses to temporarily permeabilize cells (sonoporation), enabling intracel-
lular uptake of exogenous compounds [ 18 ]. MSP can instantly label a large
amount of stem cells without use of a transfection agent.
Direct labeling techniques have a drawback of not mirroring the cell
physiology. SPIOs may remain in stem cells even after cell death creating
false impression of cell viability. Some SPIOs may be engulfed by macro-
phages instead of stem cells confounding the data. Also, dilutional effects of
cell division may make labeled cells undetectable after several generations.
In spite of the drawbacks and limitations of direct labeling techniques, SPIO
labeled mesenchymal stem cells have been shown to generate good results
up to 8 weeks in animal models of myocardial infarction [ 19 , 20 ].
Indirect labeling is typically accomplished by binding an MR contrast
agent to a stem cell specific receptor. Receptor based imaging is more sensi-
tive compared to internalization of contrast in the direct methods and also
more specific since the receptor is unique to the target cell. While indirect
labeling techniques hold promise but currently no receptor based cell label-
ing have been successfully applied using MRI.
Reporter gene cell labeling is another method of labeling the stem cells. In
this approach the stem cell is transfected with a gene that expresses an
enzyme, protein or a receptor that can be detected using MRI [ 21 , 22 ]. The
potential advantages of this technique are many including, indefinite tracking
of cells due to lack of dilutional effects from cell division, dead cells don’t
confound results as only viable cells express the signal protein or receptor.
Also, the reporter gene in a stem cell can be programmed to signal only if a
particular condition is met, like when the stem cell undergoes differentiation
to the intended mature cell (stem cell maturing into chondrocyte or cardiac
cell etc.). Most MRI based reporter genes are based on production of intracel-
lular metalloproteins, mostly transferrin, ferritin and tyrosinase. Increasing
the transferrin expression using a reporter gene will lead to accumulation of
intracellular iron that can be detected using MRI by studying changes in the
T2* signal (Fig. 1.2). Similarly, tyrosinase reporter gene produces melanin
that binds iron inside the cells that leads to increased relaxivity. Using ironT. Pa ndey