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Review
. 2016 Sep 22;6(5):234-261.
eCollection 2016.

Genetic engineered molecular imaging probes for applications in cell therapy: emphasis on MRI approach

In K Cho  1 , Silun Wang  2 , Hui Mao  2 , Anthony Ws Chan  1
Affiliations
Review

Genetic engineered molecular imaging probes for applications in cell therapy: emphasis on MRI approach

In K Cho et al. Am J Nucl Med Mol Imaging. .

Abstract

Recent advances in stem cell-based regenerative medicine, cell replacement therapy, and genome editing technologies (i.e. CRISPR-Cas 9) have sparked great interest in in vivo cell monitoring. Molecular imaging promises a unique approach to noninvasively monitor cellular and molecular phenomena, including cell survival, migration, proliferation, and even differentiation at the whole organismal level. Several imaging modalities and strategies have been explored for monitoring cell grafts in vivo. We begin this review with an introduction describing the progress in stem cell technology, with a perspective toward cell replacement therapy. The importance of molecular imaging in reporting and assessing the status of cell grafts and their relation to the local microenvironment is highlighted since the current knowledge gap is one of the major obstacles in clinical translation of stem cell therapy. Based on currently available imaging techniques, we provide a brief discussion on the pros and cons of each imaging modality used for monitoring cell grafts with particular emphasis on magnetic resonance imaging (MRI) and the reporter gene approach. Finally, we conclude with a comprehensive discussion of future directions of applying molecular imaging in regenerative medicine to emphasize further the importance of correlating cell graft conditions and clinical outcomes to advance regenerative medicine.

Keywords: In vivo cell monitoring; cell tracking; longitudinal monitoring; magnetic resonance imaging; molecular imaging; regenerative medicine; reporter gene; stem cell.

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Figures

Figure 1
Figure 1
The number of publications by year. A. The total number of publications by year. The PubMed search was conducted using the terms in vivo cell, imaging, tracking, or monitoring while excluding terms like reviews, methods, and drug delivery. The years when embryonic stem cells (1998) and induced pluripotent stem cells (2006) were developed are indicated by arrows. B. The number of publications broken down into each imaging modality. C. The number of publications using multimodal imaging methods. Abbreviations: PET-positron emission tomography, MRI-magnetic resonance imaging, BLI-bioluminescence imaging, CT-computed tomography, SPECT-single photon emission CT, CEST-chemical exchange saturation transfer.
Figure 2
Figure 2
Different labeling strategies highlighting the main differences among different strategies.
Figure 3
Figure 3
Direct and indirect labeling methods. A. In the direct labeling method, cells are incubated with a contrast agent (e.g. SPIO). Cells actively take up the contrast agent and are harvested after a specific incubation period. The harvested cells are injected into a study subject, and the cells are tracked using the imaging modality of choice. The major limitation of the direct labeling method is the dilution of contrast agent. B. For the indirect labeling method, a reporter gene is transduced into the cells, and cells expressing the reporter gene are injected into a study subject. The major advantage of the indirect labeling method is that the contrast agent does not get diluted by cell division, allowing longitudinal monitoring.
Figure 4
Figure 4
Exogenous and endogenous labeling methods. A. Exogenous labeling methods require injection of a contrast agent. Either binding of the contrast agent or activation by enzyme generates contrast. B, C. Endogenous labeling methods. B. The constitutive expression does not require injection of contrast agent but lacks the mechanism to regulate the expression. C. An inducible promoter allows expression of the reporter gene when monitoring is required.
Figure 5
Figure 5
Exogenous and endogenous genetic reporters.
Figure 6
Figure 6
A current model of the magnetosome formation process. A. The first step of magnetosome formation is invagination and vesicle formation. The initial invagination process is activated by MamI, L, Q, and B. A vesicle is formed by recruiting additional proteins (e.g. MamK and E). B. After the vesicles are formed, they are aligned on a chain via interaction between MamK, cytoskeletal structure, and MamJ, a membrane-bound protein. Mm6 is involved in the biomineralization process. C. A magnetite crystal starts to form with an increase of iron content in the vesicle, with many proteins involved in the process (i.e. MamA, G, F, D, and C). MamP appears to be involved in the size restriction.
Figure 7
Figure 7
MagA structure and longitudinal monitoring of stem cell graft using inducible MagA as a genetic MRI reporter. A. Phyre server-generated MagA 3D structure. B. Superimposed model of MagA and c2k3ca. C. A representative MRI image of a single mouse, with the status of MagA expression at the top of images. MRI images were taken in a 7-day interval. D. Regions of interest (ROIs) analysis of signal intensity showing significant hypointense signal with the “On” states, while no such difference was observed for the “Off” state [119].
Figure 8
Figure 8
CEST images assess cancer treatment response. Farrar et al., [134] evaluate whether the lysine-rich protein (LRP) MRI reporter gene can be engineered into G47Δ, a herpes simplex-derived oncolytic virus that is currently being tested in clinical trials, and can be in vivo tracked by CEST MRI. Representative MTRasym maps (color scale) acquired with a saturation frequency offset of 3.6 ppm and overlaid onto the associated T2-weighted images at baseline (A and C) and 8 hours after injection of G47∆-LRP (B) and G47∆-empty virus (D). A significant increase in MTRasym is observed after virus injection (B) but not in G47∆-empty virus (D).
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