Myocardial regeneration with stem-cell transplantation is a possible treatment option to reverse deleterious effects that occur after myocardial infarction. Since little is known about stem cell survival after transplantation, developing techniques for "tracking" cells would be desirable. Iron-oxide-labeled stem cells have been used for in vivo tracking using MRI but produce negative contrast images that are difficult to interpret. The aim of the current study was to test a positive contrast MR technique using reduced z-gradient rephasing (GRASP) to aid in dynamically tracking stem cells in an in vivo model of mouse myocardial infraction. Ferumoxides and protamine sulfate were complexed and used to magnetically label embryonic stem cellderived cardiac-precursor-cells (ES-CPCs). A total of 500,000 ES-CPCs were injected in the border zone of infarcted mice and MR imaging was performed on a 9.4T scanner using T 2 *-GRE sequences (negative contrast) and positive contrast GRASP technique before, 24 hours, and 1 week after ES-CPC implantation. Following imaging, mice were sacrificed for histology and Perl's staining was used to confirm iron within myocardium. Good correlation was observed between signal loss seen on conventional

Objective The aim of the current study is to test the ability to label and detect murine embryonic stem cell-derived cardiovascular progenitor cells (ES-CPCs) with Magnetic Resonance Imaging (MRI) using the novel contrast agent Gadofluorine M-Cy3 (GdFM-Cy3). Background Cell therapy shows great promise for the treatment of cardiovascular disease. An important limitation to previous clinical studies is the inability to accurately identify transplanted cells. GdFM-Cy3 is a lipophilic paramagnetic contrast agent that contains a perfluorinated side chain and an amphiphilic character that allows for micelle formation in an aqueous solution. Previous studies reported that it is easily taken up and stored within the cytosol of mesenchymal stem cells, thereby allowing for paramagnetic cell labeling. Investigators in our laboratory have recently developed techniques for the robust generation of ES-CPCs. We reasoned that GdFM-Cy3 would be a promising agent for the in vivo detection of these cells after cardiac cell transplantation. Methods ES-CPCs were labeled with GdFM-Cy3 by incubation. In vitro studies were performed to assess the impact of GdFM-Cy3 on cell function and survival. 500,000 GdFM-Cy3-labeled ES-CPCs or control ES-CPCs were injected into the myocardium of mice with and without myocardial infarction. Mice were imaged (9.4T) before and over a two-week time interval after stem cell transplantation. Mice were then euthanized and their hearts were sectioned for fluorescence microscopy. Results In vitro studies demonstrated that GdFM-Cy3 was easily transfectable, non-toxic, stayed within cells after labeling and could be visualized using MRI and fluorescence microscopy. In vivo studies confirmed the efficacy of the agent for the detection of cells transplanted into the hearts of mice after myocardial infarction. A correspondence between MRI and histology was observed. Conclusions The results of the current study suggest that it is possible to identify and potentially track GdFM-Cy3-labeled ES-CPCs in murine infarct models via MRI.

We recently described a murine embryonic stem cell (ESC) line engineered to express the activated Notch 4 receptor in a tetracycline (doxcycline; Dox) regulated fashion (tet-notch4 ESCs). Notch 4 induction in Flk1(+) hematopoietic and vascular progenitors from this line respecified them to a cardiovascular fate. We reasoned that these cells would be ideal for evaluating the contribution of the cardiomyocyte and vascular lineages to the functional improvement noted following stem cell transplantation in infarcted hearts. Flk-1(+) Tet-notch4 cells from d 3 embryoid bodies exposed to doxycycline (Dox(+)) were compared to uninduced (Dox(-)) Flk-1(+) cells. Mice underwent transplantation of 5 x 10(5) Dox(+) cells, Dox(-)cells, or an equal volume of serum-free medium after surgically induced myocardial infarction. The mean ejection fraction was 59 + or - 15, 46 + or - 17, and 39 + or - 13% in the Dox(+), Dox(-), and serum-free medium groups, respectively (P<0.05 for the differences among all 3 groups). Immunohistochemistry of hearts injected with Dox(+) grafts expressed myocardial and vascular markers, whereas grafts of Dox(-) cells expressed primarily vascular markers. We conclude that cardiovascular progenitors are more effective than vascular progenitors in improving function after myocardial infarction. The transplantation of appropriate cell types is critical for maximizing the benefit of cardiovascular cell therapy.-Adler, E. D., Chen, V. C., Bystrup, A., Kaplan, A. D., Giovannone, S., Briley-Saebo, K., Young, W., Kattman, S., Mani, V., Laflamme, M., Zhu, W.-Z., Fayad, Z., Keller, G. The cardiomyocyte lineage is critical for optimization of stem cell therapy in a mouse model of myocardial infarction.

BACKGROUND: Intramyocardial transplantation of stem cells improves left ventricular ejection fraction (EF) in animal studies and preliminary clinical trials. The mechanism may involve either replacement of myocytes or improved vascular supply to existing myocytes. We recently identified an Embyronic Stem cell derived cardiovascular progenitor cell (ES-CPC) that is the common precursor of cardiomyocyte and vascular cell lineages. To determine whether myocyte transplantation improves myocardial function more than angiogenesis alone does, we compared the effect of ES-CPCs to hemangioblasts (vascular/hematopoetic progenitor cells) on EF in a mouse model of myocardial infarction. METHODS: ES-CPC and hemangioblasts were isolated from a doxycycline-responsive, Notch-inducible ES cell line containing Notch 4 cDNA under the control of a tetracycline-inducible promoter. Notch induction of mesoderm-derived ES cells resulted in a CPC phenotype, whereas non-induced cells developed into hemangioblasts. Mice underwent transplantation of 500,000 ES-CPC (n=20), hemangioblasts (n=16), or an equal volume of serum-free media (n=12) 30 minutes after surgically-induced myocardial infarction. All cell lines constitutively expressed green fluorescent protein (GFP). EF was assessed two weeks post-transplantation using 9.4 Tesla MRI. Mice were then euthanized and frozen heart sections were examined using fluorescent microscopy. RESULTS: The mean EF was 59Â ± 15, 46Â ± 17, and 39Â ± 13% in the ES-CPC, hemangioblast, and control groups, respectively (p<0.05 for the differences among all 3 groups; ANOVA). GFP + cells were detected in frozen sections of both the ES-CPC and hemangioblast groups. GFP + cells in ES-CPC treated hearts expressed markers associated with both cardiomyocyte and vascular phenotypes, whereas the GFP + cells in the hemangioblast group expressed markers associated with vascular phenotypes. CONCLUSIONS: Both hemangioblast and ES-CPC transplantation improves EF in a mouse model of myocardial infarction, but ES-CPC transplantation was more effective. This suggests that enhancement of myocardial function by transplantation of both cardiomyocyte and vascular phenotypes exceeds that with vascular phenotypes alone.