Christian Rathert

Induced pluripotent stem cell (iPSC)-derived Flk-1 progenitor cells engraft, differentiate, and improve heart function in a mouse model of acute myocardial infarction

AIMS Induced pluripotent stem cell (iPSC)-derived cardiovascular progenitor cells represent a suitable autologous cell source for myocardial regeneration as they have the capability to form myocardial cells and to contribute to revascularization. As a first proof of concept we evaluated the potential of a murine iPSC-derived cardiovascular progenitor population, which expresses the surface marker foetal liver kinase-1 (Flk-1), to restore myocardial tissue and improve cardiac function after acute myocardial infarction (MI) in mice. METHODS AND RESULTS iPSC-derived Flk-1(pos) vs. Flk-1(neg) cells were selected by fluorescence activated cell sorting (FACS) and injected into the ischaemic myocardium of left anterior descending coronary artery (LAD)-ligated mice. Addressing safety aspects we used an octamer binding factor 4 (Oct4)-enhanced green fluorescent protein (eGFP) expressing iPSC clone from the transgenic Oct4-eGFP reporter mouse strain OG2 to enable FACS-based depletion of undifferentiated cells prior to transplantation. Infarcted animals were treated with placebo (phosphate-buffered saline, n = 13), Flk-1(neg) cells (n = 14), or Flk-1(pos) cells (n = 11; 5 × 10(5) cells each). Heart function was evaluated by magnetic resonance imaging and conductance catheter analysis 2 weeks postoperatively. Cardiovascular in vitro and in vivo differentiations were investigated by immunofluorescence staining. Treatment with Flk-1(pos) and Flk-1(neg) cells resulted in a favourable myocardial remodelling and improved left ventricular function. Engraftment and functional benefits were superior after transplantation of Flk-1(pos) compared with Flk-1(neg) cells. Furthermore, Flk-1(pos) grafts contained considerably more vascular structures in relation to Flk-1(neg) grafts. CONCLUSION iPSC-derived Flk-1(pos) progenitor cells differentiate into cardiovascular lineages in vitro and in vivo and improve cardiac function after acute MI. This proof of concept study paves the way for an autologous iPSC-based therapy of MI.

Substantial Early Loss of Induced Pluripotent Stem Cells Following Transplantation in Myocardial Infarction

The limited success of cardiac stem cell therapy has lately generated discussion regarding its effectiveness. We hypothesized that immediate cell loss after intramyocardial injection significantly obscures the regenerative potential of stem cell therapy. Therefore, our aim was to assess the distribution and quantity of induced pluripotent stem cells after intramyocardial delivery using in vivo bioluminescence analysis. In this context, we wanted to investigate if the injection of different cell concentrations would exert influence on cardiac cell retention. Murine-induced pluripotent stem cells were transfected for luciferase reporter gene expression and transplanted into infarcted myocardium in mice after left anterior descending coronary artery ligation. Cells were delivered constantly in aqueous media (15 μL) in different cell concentrations (group A, n = 10, 5.0 × 10(5) cells; group B, n = 10, 1.0 × 10(6) cells). Grafts were detected using bioluminescence imaging. Organ explants were imaged 10 min after injection to quantify early cardiac retention and cell biodistribution. Bioluminescence imaging showed a massive early displacement from the injection site to the pulmonary circulation, leading to lung accumulation. Mean cell counts of explanted organs in group A were 7.51 × 10(4) ± 4.09 × 10(3) (heart), 6.44 × 10(4) ± 2.48 × 10(3) (left lung), and 8.06 × 10(5) ± 3.61 × 10(3) (right lung). Respective cell counts in group B explants were 1.69 × 10(5) ± 7.69 × 10(4) (heart), 2.11 × 10(5) ± 4.58 × 10(3) (left lung), and 3.25 × 10(5) ± 9.35 × 10(3) (right lung). Applying bioluminescence imaging, we could unveil and quantify massive early cardiac stem cell loss and pulmonary cell accumulation following intramyocardial injection. Increased injection concentrations led to much higher intracardiac cell counts; however, pulmonary biodistribution of transplanted cells still persisted. Therefore, we recommend applying tissue engineering techniques for cardiac stem cell transplantations in order to improve cardiac retention and limit biodistribution.

Rhesus monkey cardiosphere-derived cells for myocardial restoration

BACKGROUND AIMS Cardiosphere-derived cells (CDC) have been proposed as a promising myocardial stem cell source for cardiac repair. They have been isolated from human, porcine and rodent cardiac biopsies. However, their usefulness for myocardial restoration remains controversial. We aimed to determine the survival, differentiation and functional effects of Rhesus monkey CDC (RhCDC) in a mouse model of myocardial infarction. METHODS RhCDC were isolated and characterized by flow cytometry and reverse transcriptase (RT)-polymerase chain reaction (PCR) and compared with human CDC. They were injected intramyocardially into severe combined immune deficiency (SCID) beige mice after ligature of the left anterior descending artery (LAD). Phosphate-buffered saline (PBS) served as placebo. Medium treatment alone was used to distinguish between cellular and non-cellular effects. Animals were divided into a non-infarcted control group (n = 7), infarct control groups (n = 24), medium-treated infarct groups (n = 35) and RhCDC-treated infarct groups (n = 33). Follow-up was either 1 or 4 weeks. LV function was assessed by pressure-volume loop analysis. Differentiation was analyzed by immunhistochemical profiling and RT-PCR. RESULTS Proliferating RhCDC grafts were detected after transplantation in an acute infarct model. RhCDC as well as medium treatment protected myocardium within the infarct area and improved LV function. RhCDC had a superior regenerative effect than medium alone. CONCLUSIONS For the first time, RhCDC have been used for the restoration of infarcted myocardium. RhCDC proliferated in vivo and positively influenced myocardial remodeling. This effect could be mimicked by treatment with unconditioned medium alone, emphasizing a non-cellular paracrine therapeutic mechanism. However, as a robust cardiac stem cell source, CDC might be useful to evoke prolonged paracrine actions in cardiac stem cell therapy.

Multimodal Imaging for In Vivo Evaluation of Induced Pluripotent Stem Cells in a Murine Model of Heart Failure

Myocardial stem cell therapy in heart failure is strongly dependent on successful cellular transfer, engraftment, and survival. Moreover, massive cell loss directly after intramyocardial injection is commonly observed, generating the need for efficient longitudinal monitoring of transplanted cells in order to develop more efficient transplantation techniques. Therefore, the aim of the present study was to assess viability and cardiac retention of induced pluripotent stem cells after intramyocardial delivery using in vivo bioluminescence analysis (BLI) and magnetic resonance imaging (MRI). Murine induced pluripotent stem cells (iPSCs) were transfected for luciferase reporter gene expression and labeled intracellularly with supraparamagnetic iron oxide particles. Consequently, 5 × 105 cells were transplanted intramyocardially following left anterior descending coronary artery ligation in mice. Cardiac iPSCs were detected using BLI and serial T2* sequences by MRI in a 14-day follow-up. Additionally, infarct extension and left ventricular (LV) function were assessed by MRI. Controls received the same surgical procedure without cell injection. MRI sequences showed a strong MRI signal of labeled iPSCs correlating with myocardial late enhancement, demonstrating engraftment in the infarcted area. Mean iPSC volumes were 4.2 ± 0.4 mm3 at Day 0; 3.1 ± 0.4 mm3 at Day 7; and 5.1 ± 0.8 mm3 after 2 weeks. Thoracic BLI radiance decreased directly after injection from 1.0 × 106  ± 4.2 × 104 (p/s/cm2 /sr) to 1.0 × 105  ± 4.9 × 103 (p/s/cm2 /sr) on Day 1. Afterward, BLI radiance increased to 1.1 × 106  ± 4.2 × 104 (p/s/cm2 /sr) 2 weeks after injection. Cardiac graft localization was confirmed by ex vivo BLI analysis and histology. Left ventricular ejection fraction was higher in the iPSC group (30.9 ± 0.9%) compared to infarct controls (24.0 ± 2.1%; P < 0.05). The combination of MRI and BLI assesses stem cell fate in vivo, enabling cardiac graft localization with evaluation of LV function in myocardial infarction.

Macroscopic fluorescence imaging: a novel technique to monitor retention and distribution of injected microspheres in an experimental model of ischemic heart failure.

Background The limited effectiveness of cardiac cell therapy has generated concern regarding its clinical relevance. Experimental studies show that cell retention and engraftment are low after injection into ischemic myocardium, which may restrict therapy effectiveness significantly. Surgical aspects and mechanical loss are suspected to be the main culprits behind this phenomenon. As current techniques of monitoring intramyocardial injections are complex and time-consuming, the aim of the study was to develop a fast and simple model to study cardiac retention and distribution following intramyocardial injections. For this purpose, our main hypothesis was that macroscopic fluorescence imaging could adequately serve as a detection method for intramyocardial injections. Methods and Results A total of 20 mice underwent ligation of the left anterior descending artery (LAD) for myocardial infarction. Fluorescent microspheres with cellular dimensions were used as cell surrogates. Particles (5×105) were injected into the infarcted area of explanted resting hearts (Ex vivo myocardial injetions EVMI, n = 10) and in vivo into beating hearts (In vivo myocardial injections IVMI, n = 10). Microsphere quantification was performed by fluorescence imaging of explanted organs. Measurements were repeated after a reduction to homogenate dilutions. Cardiac microsphere retention was 2.78×105±0.31×105 in the EVMI group. In the IVMI group, cardiac retention of microspheres was significantly lower (0.74×105±0.18×105; p<0.05). Direct fluorescence imaging revealed venous drainage through the coronary sinus, resulting in a microsphere accumulation in the left (0.90×105±0.20×105) and the right (1.07×105±0.17×105) lung. Processing to homogenates involved further particle loss (p<0.05) in both groups. Conclusions We developed a fast and simple direct fluorescence imaging method for biodistribution analysis which enabled the quantification of fluorescent microspheres after intramyocardial delivery using macroscopic fluorescence imaging. This new technique showed massive early particle loss and venous drainage into the right atrium leading to substantial accumulation of graft particles in both lungs.

IMPROVEMENT OF BIODISTRIBUTION IN CARDIAC STEM CELL THERAPY USING FIBRINOGEN

Results: Compared to ex situ injection, the in situ delivery in PBS showed a 90% decline in particle retention (group A = 3,2*10^5 vs. group B = 0,3*105; p<0,01), indicating immediate loss during injection mainly through venous drainage into the lungs. Further particle loss into the lungs was observed until POD3 (p<0,05). Cardiac particle retention could be improved using fibrinogen as a carrier substance (group D = 1,1*10^5; D vs. B: p<0,05).