| Aims:Bone marrow mesenchymal stem cells (MSCs) are multipotent cells that can differentiate into a variety of cell types and are ideal donor cells for treatment of acute myocardial infarction (AMI). Published clinical studies demonstrate that MSCs can attenuate cardiac dysfunction and left ventricular remodeling after AMI. Experimental models of cell-based therapy for myocardial infarction are needed to identify the underlying mechanisms and the fate of the transplanted cells. The rat myocardial infarcted model induced by permanent ligation of the anterior descending coronary artery has been widely used in stem cell studies. It is crucial to select appropriate methods for assessing the cardiac function and tracking the grafted cells. Magnetic resonance imaging (MRI) has excellent interstudy reproducibility and has been mostly used in clinical stem cell trials. Due to the high spatial resolution, MRI can clearly show the anatomy of the heart and accurately evaluate the overall and local cardiac function. However, it is challenge to apply MRI in rats due to the extremely high heart rate (370-580beats/min) and respiratory rate (60-120times/min), which require much higher time and spatial resolutions. Hence, we aim to investigate the feasibility of ultra-high field strength (7.0T) MRI for evaluation of cardiac function in myocardial infarcted rat models and in vivo tracking MSCs after transplanted into the infarcted hearts.Methods:MSCs were isolated and cultured from bone marrow of male Sprague-Dawley (SD) rats, and doubly labeled with micron-sized particles of iron oxide (MPIO) and DM-DiI. AMI was experimentally induced by permanent ligation of the anterior descending coronary artery in female SD rats. MRI was performed to assess the cardiac function before and1week after the surgery, respectively. Left ventricular ejection fraction (LVEF^end diastolic volume (EDV)、end systolic volume (ESV) and left ventricular mass (LVM) were measured. Then the chests of MI rats were reopened in order to receive injections of2×106MSCs into the border zone of infarcted myocardium.3days after the transplantation, MRI was performed to in vivo tracking the grafted cells. After the examination, the animals were sacrificed for the histopathological test.Results:Cine MR imaging showed that the infarcted myocardium mainly located at the LV anterior wall, the infarcted wall became thinner and the contraction weakened or disappeared. The cardiac function decreased significantly at1week after MI than before MI (LVEF:52.45±7.32%vs.65.84±8.34%, P<0.001; EDV:2.71±0.58ml vs.2.52±0.42ml, P<0.05; ESV:1.31±0.15ml vs.0.86±0.13ml, P<0.05).3days after the MSCs transplantation, both the cine MR imaging and T2*weighted imaging detected the hypo intensities caused by the MPIO. The borders of the hypo intensities were much clearer on T2*weighted images. Histopathological test (HE staining and Masson’s staining) showed that the infarcted wall became thinner, the myocardium was replaced by fibrosis and ventricular muscles were divided into more branches. The detection of dual-labeled MSCs (fluorescence microscopy) and iron-positive cells (Prussian blue staining) demonstrated that the MR hypo intensities arose from the grafted MSCs.Conclusions:Cine MR imaging can be used for the assessment of cardiac functional parameters in rat models of myocardial infarction. With intracellular contrast agent MPIO, T2*weighted imaging can in vivo tracking the MSCs after transplanted into the infarcted hearts. Aims:We aim to evaluate the outcomes of iron oxide labeled bone marrow mesenchymal stem cells (MSCs) after transplantation in rat models of myocardial infarction, and to test the accuracy of tracking the long-term fate of the transplanted MSCs using a7.0T magnetic resonance imaging (MRI). Besides, with the help of histopathology and molecular biology experiments, to analyze the underlying mechanisms and the fate of the transplanted MSCs in rat infarcted hearts.Methods:MSCs were isolated and cultured from bone marrow of male Sprague-Dawley (SD) rats, and doubly labeled with micron-sized particles of iron oxide (MPIO) and DM-Dil. Myocardial infarction (MI) was experimentally induced by permanent ligation of the anterior descending coronary artery in female SD rats. Seven days after MI, rats were randomized to injections of dual-labeled MSCs2×106/50μl (MSCs group) or PBS50ul (control group) into the border zone of infarcted myocardium. MRI was used to evaluate stem cell migration, signal intensity changes and cardiac function at baseline (1day before transplantation),3days,2weeks and4weeks after transplantation, respectively. At each time point after MRI test,5-8animals were sacrificed for postmortem analyses.Results:Except3rats from MSCs group, hypointensities caused by the MPIO particles were detected at all times in the peri-infarct region of MSCs injection on T2*-weighted images. As time progressed, the signal gradually weakened and the area shrank. By quantitative analysis, there was a significant decrease both in signal contrast (%) and size (mm2) between day3and week4(95.27±20.10%vs.62.14±13.58%and5.34±0.80mm2vs.2.53±0.93mm2, P<0.05, respectively). No MR hypointensities were detected in the control group. Cine MR imaging showed that the infarcted wall became thinner; the contraction weakened or disappeared, LVEF decreased, EDV and ESV increased. As time progressed, cardiac function in control rats deteriorated. However, there was an improvement in MSCs-treated rats. By4weeks, control heart LVEF had decreased by20%(EF41±5%vs. baseline52±7%, P<0.05) and was significantly lower than that of MSC-treated hearts (49±65vs.51±6%, P<0.05); The EDV and ESV in MSC-treated hearts were significantly smaller (3.6±0.4ml vs.4.8±0.4ml,P<0.05and1.8±0.4ml vs.2.8±0.3ml, P<0.05, respectively); Relative infarct size had increased significantly in control hearts but not in MSC-treated hearts (18±2%vs.14±2%, P<0.05). The histology analysis showed that MSC-treated hearts had significantly increased capillary density (CD31staining) in the peri-infarct region, and lower cardiomyocytes apoptosis (TUNEL staining) and collagen deposition (Masson’s staining). In addition, at4weeks after transplantation, the number of grafted MSCs in the heart was extremely low (RT-PCR), and most of the iron-positive cells (Prussian blue staining) were cardiac macrophages (CD68staining), indicating that the MR hypo intensities mainly arose from cardiac macrophage that engulfed the MPIO particles.Conclusions:At4weeks after injection of MPIO-labeled MSCs, most of the transplanted cells are not present in the peri-infarcted myocardium and the MRI hypo intensities arise from cardiac macrophages that engulfed the MPIOs. This finding suggests that iron nanoparticles are not a reliable marker to monitor transplanted stem cell traffic and survival. Despite cell loss, MPIO-labeled MSCs maintain their protective effect against progressive LV dilatation and dysfunction, probably attributable to paracrine effects, enhanced angiogenesis and inhibition of host cell apoptosis and fibrosis. |
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