Physics And Chemistry Basis Of Biotechnology - De Cuyper - tiera.ru

Molecular and cellular magnetic resonance contrast agents
In this study, oligodendrocyte progenitors were first incubated with iron oxide particles
that were covalently linked to anti-transferrin receptor(Tfr) mabs (See Fig. 1). Upon
binding of the construct, the Tfr is being crosslinked by the mab which results in an
internalising signal. The receptor-mediated endocytosis then stashes away the iron
oxide particles in endosomes. Following transplantation of only 5 x 10 4 magnetically
tagged cells, the dark areas on the MR images corresponded to the cellular migration
mainly within the dorsal column, and corresponded to the areas of new myelination
(see Fig.2). While the imaging in that study was performed at high resolution ex vivo,
we have since demonstrated that these and similar cells can be monitored serially in
vivo, even using lower resolution clinical MRI systems [77]. In parallel with these new
technologies, another breakthrough development occurred that will have profound
implications for the use of cellular therapies, namely the isolation and successful
propagation of human embryonic stem (ES) cells [78, 79]. Using mouse ES cells, a
number of different groups have shown the near unlimited potential of these cells to
become differentiated, specific cell types that can be used to repair defunct or damaged
tissue. Magnetic particles that can label cells non-specifically, regardless of tissue
origin or animal species, yet show high cellular uptake ratios are highly desirable to
further develop this field of cellular MR imaging. In collaboration with chemists at
Temple University, our group has developed dendrimer-coated iron oxide particles or
“magnetodendrimers” as a new type of non-specific magnetic tag that shows excellent
cellular uptake and relaxation enhancement [80].
While superparamagnetic iron oxides appear, for good reason, the primary magnetic
label of choice for tagging cells, paramagnetic chelates may also be used. It was shown
that by injecting Gd-DTPA-dextran in just one single cell of an early (16-cell) stage
developing frog embryo, the embryonic cell lineages and movement of differentiating
cells could be followed by MR microscopic imaging [81]. Transferrin conjugated to
poly-L-lysine has been used to co-transfer Gd-chelates and DNA into cells [82], and
using the HIV tat-peptide described earlier cells have been tagged with Gd- and Dychelates
[83]. The advantage of using Gd-chelates is that the inner-sphere water coordination
may be manipulated in order to create “on-off’ cellular switches; our group
is currently pursuing this approach using paramagnetically labelled dendrimers [84].
5. Axonal and neuronal tracing
Nerves are normally isointense with the surrounding tissue and are therefore difficult to
detect individually by MR imaging. Following injection of wheat germ agglutinin
(WGA)-conjugated dextran-coated iron oxide particles into the rabbit forearm muscle,
the particles are taken up and transported by median nerves, which can then be easily
detected as separate structures on the images [85]. Similarly, using a rat model of focal
crush injury to the sciatic nerve, it was shown that these nerves can become traceable
following injection of MION particles directly at the site of injury [86]. Binding and
transport of the iron oxides were comparable to slow axonal transport with a speed of
about 5 mm a day. Interestingly, although WGA is being reported to have a high
specific affinity for neurons, similar results were obtained for WGA-coated and non-
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