Positron emission tomography based in-vivo imaging of early phase stem cell retention after intramyocardial delivery in the mouse model

New Approaches in Heart Research: Prevention Instead of Cardiomyoplasty?

Cardiovascular diseases are the leading cause of death in industrialized nations. Due to the high number of patients and expensive treatments, according to the Federal Statistical Office (2017) in Germany, cardiovascular diseases account for around 15% of total health costs. Advanced coronary artery disease is mainly the result of chronic disorders such as high blood pressure, diabetes, and dyslipidemia. In the modern obesogenic environment, many people are at greater risk of being overweight or obese. The hemodynamic load on the heart is influenced by extreme obesity, which often leads to myocardial infarction (MI), cardiac arrhythmias, and heart failure. In addition, obesity leads to a chronic inflammatory state and negatively affects the wound-healing process. It has been known for many years that lifestyle interventions such as exercise, healthy nutrition, and smoking cessation drastically reduce cardiovascular risk and have a preventive effect against disorders in the healing process. However, little is known about the underlying mechanisms, and there is significantly less high-quality evidence compared to pharmacological intervention studies. Due to the immense potential of prevention in heart research, the cardiologic societies are calling for research work to be intensified, from basic understanding to clinical application. The topicality and high relevance of this research area are also evident from the fact that in March 2018, a one-week conference on this topic with contributions from top international scientists took place as part of the renowned "Keystone Symposia" ("New Insights into the Biology of Exercise"). Consistent with the link between obesity, exercise, and cardiovascular disease, this review attempts to draw lessons from stem-cell transplantation and preventive exercise. The application of state-of-the-art techniques for transcriptome analysis has opened new avenues for tailoring targeted interventions to very individual risk factors.

Cardiac cell therapies for the treatment of acute myocardial infarction in mice: systematic review and meta-analysis

Backgound Aims: This meta-analysis aims at summarizing the whole body of research on cell therapies for acute myocardial infarction (MI) in the mouse model to bring forward ongoing research in this field of regenerative medicine. Despite rather modest effects in clinical trials, pre-clinical studies continue to report beneficial effects of cardiac cell therapies for cardiac repair following acute ischemic injury. Results: The authors' meta-analysis of data from 166 mouse studies comprising 257 experimental groups demonstrated a significant improvement in left ventricular ejection fraction of 10.21% after cell therapy compared with control animals. Subgroup analysis indicated that second-generation cell therapies such as cardiac progenitor cells and pluripotent stem cell derivatives had the highest therapeutic potential for minimizing myocardial damage post-MI. Conclusions: Whereas the vision of functional tissue replacement has been replaced by the concept of regional scar modulation in most of the investigated studies, rather basic methods for assessing cardiac function were most frequently used. Hence, future studies will highly benefit from integrating methods for assessment of regional wall properties to evolve a deeper understanding of how to modulate cardiac healing after acute MI.

Comparison of metabolic and functional parameters using cardiac 18F-FDG-PET in early to mid-adulthood male and female mice

BACKGROUND

In this descriptive study of male and female mice at different weeks of age, we use serial non-invasive cardiac 18F-FDG-PET scans to follow up on metabolic alterations, heart function parameters, and the ECG of both sexes in early to mid-adulthood.

METHODS

ECG-gated 18F-FDG-PET scans were performed in mice on 10, 14, and 18 weeks of age, using a dedicated small-animal PET scanner. The percentage of the injected activity per gram (%IA/g) in the heart, left ventricular metabolic volume (LVMV), myocardial viability and left ventricular function parameters: end-diastolic (EDV), end-systolic (ESV), stroke volume (SV), and the ejection fraction (EF%) were estimated.

RESULTS

Compared to their age-matched female counterpart, male mice showed a constant increase in LVMV and ventricular volume during the follow-up. In contrast, female mice remain stable after ten weeks of age. Furthermore, male mice showed lower heart rates, positive correlation with cardiac %IA/g, and negative correlation with LVMV.

CONCLUSION

In this study of serial cardiac PET scans, we provide insight for basic murine research models, showing that mice gender and age show distinct cardiac metabolisms. These physiologic alterations need to be considered when planning in vivo injury models to avoid potential pitfalls.

Persistence of intramyocardially transplanted murine induced pluripotent stem cell-derived cardiomyocytes from different developmental stages

Background

Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM) are regarded as promising cell type for cardiac cell replacement therapy, but it is not known whether the developmental stage influences their persistence and functional integration in the host tissue, which are crucial for a long-term therapeutic benefit. To investigate this, we first tested the cell adhesion capability of murine iPSC-CM in vitro at three different time points during the differentiation process and then examined cell persistence and quality of electrical integration in the infarcted myocardium in vivo.

Methods

To test cell adhesion capabilities in vitro, iPSC-CM were seeded on fibronectin-coated cell culture dishes and decellularized ventricular extracellular matrix (ECM) scaffolds. After fixed periods of time, stably attached cells were quantified. For in vivo experiments, murine iPSC-CM expressing enhanced green fluorescent protein was injected into infarcted hearts of adult mice. After 6–7 days, viable ventricular tissue slices were prepared to enable action potential (AP) recordings in transplanted iPSC-CM and surrounding host cardiomyocytes. Afterwards, slices were lysed, and genomic DNA was prepared, which was then used for quantitative real-time PCR to evaluate grafted iPSC-CM count.

Results

The in vitro results indicated differences in cell adhesion capabilities between day 14, day 16, and day 18 iPSC-CM with day 14 iPSC-CM showing the largest number of attached cells on ECM scaffolds. After intramyocardial injection, day 14 iPSC-CM showed a significant higher cell count compared to day 16 iPSC-CM. AP measurements revealed no significant difference in the quality of electrical integration and only minor differences in AP properties between d14 and d16 iPSC-CM.

Conclusion

The results of the present study demonstrate that the developmental stage at the time of transplantation is crucial for the persistence of transplanted iPSC-CM. iPSC-CM at day 14 of differentiation showed the highest persistence after transplantation in vivo, which may be explained by a higher capability to adhere to the extracellular matrix.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-020-02089-5.

[89Zr]Zr-DBN labeled cardiopoietic stem cells proficient for heart failure

INTRODUCTION

Radiolabeling of stem cells with a positron emitting radioisotope represents a major advancement in regenerative biotherapy enabling non-invasive imaging. To assess the value of such an approach in a clinically relevant scenario, the tolerability and therapeutic aptitude of [89Zr]zirconium-p-isothiocyanatobenzyl-desferrioxamine ([89Zr]Zr-DBN) labeled human cardiopoietic stem cells (CPs) were evaluated in a model of ischemic heart failure.

METHODS AND RESULTS

[89Zr]Zr-DBN based radiolabeling of human CPs yielded [89Zr]Zr-DBN-CPs with radioactivity yield of 0.70 ± 0.20 MBq/106 cells and excellent label stability. Compared to unlabeled cell counterparts, [89Zr]Zr-DBN-CPs maintained morphology, viability, and proliferation capacity with characteristic expression of mesodermal and pro-cardiogenic transcription factors defining the cardiopoietic phenotype. Administered in chronically infarcted murine hearts, [89Zr]Zr-DBN-CPs salvaged cardiac pump failure, documented by improved left ventricular ejection fraction not inferior to unlabeled CPs and notably superior to infarcted hearts without cell treatment.

CONCLUSION

The present study establishes that [89Zr]Zr-DBN labeling does not compromise stem cell identity or efficacy in the setting of heart failure, offering a non-invasive molecular imaging platform to monitor regenerative biotherapeutics post-transplantation.

Bright Ferritin-a Reporter Gene Platform for On-Demand, Longitudinal Cell Tracking on MRI

A major unresolved challenge in cell-based regenerative medicine is the absence of non-invasive technologies for tracking cell fate in deep tissue and with high spatial resolution over an extended interval. MRI is highly suited for this task, but current methods fail to provide longitudinal monitoring or high sensitivity, or both. In this study, we fill this technological gap with the first discovery and demonstration of in vivo cellular production of endogenous bright contrast via an MRI genetic reporter system that forms manganese-ferritin nanoparticles. We demonstrate this technology in human embryonic kidney cells genetically modified to stably overexpress ferritin and show that, in the presence of manganese, these cells produce far greater contrast than conventional ferritin overexpression with iron or manganese-permeable cells. In living mice, diffusely implanted bright-ferritin cells produce the highest and most sustained contrast in skeletal muscle. The bright-ferritin platform has potential for on-demand, longitudinal, and sensitive cell tracking in vivo.

[68Ga]-NODAGA-RGD Positron Emission Tomography (PET) for Assessment of Post Myocardial Infarction Angiogenesis as a Predictor for Left Ventricular Remodeling in Mice after Cardiac Stem Cell Therapy

Angiogenesis plays a central role in the healing process following acute myocardial infarction. The PET tracer [⁶⁸Ga]-NODAGA-RGD, which is a ligand for the α_vβ₃ integrin, has been investigated for imaging angiogenesis in the process of healing myocardium in both animal and clinical studies. It’s value as a prognostic marker of functional outcome remains unclear. Therefore, the aim of this work was to establish [⁶⁸Ga]-NODAGA-RGD for imaging angiogenesis in the murine infarct model and evaluate the tracer as a predictor for cardiac remodeling in the context of cardiac stem cell therapy. [⁶⁸Ga]-NODAGA-RGD PET performed seven days after left anterior descending coronary artery (LAD) occlusion in 129S6 mice showed intense tracer accumulation within the infarct region. The specificity was shown in a sub-group of animals by application of the competitive inhibitor cilengitide prior to tracer injection in a subgroup of animals. Myocardial infarction (MI) significantly reduced cardiac function and resulted in pronounced left ventricular remodeling after three weeks, as measured by cardiac MRI in a separate group. Cardiac induced cells (CiC) that were derived from mESC injected intramyocardially in the therapy group significantly improved left ventricular ejection fraction (LVEF). Surprisingly, CiC transplantation resulted in significantly lower tracer accumulation seven days after MI induction. Accordingly, we successfully established the PET tracer [⁶⁸Ga]-NODAGA-RGD for the assessment of α_vβ₃ integrin expression in the healing process after MI in the mouse model. Yet, our results indicate that the mere extent of angiogenesis following MI does not serve as a sufficient prognostic marker for functional outcome.

18F-FDG PET-Based Imaging of Myocardial Inflammation Predicts a Functional Outcome Following Transplantation of mESC-Derived Cardiac Induced Cells in a Mouse Model of Myocardial Infarction

Cellular inflammation following acute myocardial infarction has gained increasing importance as a target mechanism for therapeutic approaches. We sought to investigate the effect of syngeneic cardiac induced cells (CiC) on myocardial inflammation using 18F-FDG PET (Positron emission tomography)-based imaging and the resulting effect on cardiac pump function using cardiac magnetic resonance (CMR) imaging in a mouse model of myocardial infarction. Mice underwent permanent left anterior descending coronary artery (LAD) ligation inducing an acute inflammatory response. The therapy group received an intramyocardial injection of 10⁶ CiC into the border zone of the infarction. Five days after myocardial infarction, 18F-FDG PET was performed under anaesthesia with ketamine and xylazine (KX) to image the inflammatory response in the heart. Flow cytometry of the mononuclear cells in the heart was performed to analyze the inflammatory response. The effect of CiC therapy on cardiac function was determined after three weeks by CMR. The 18F-FDG PET imaging of the heart five days after myocardial infarction (MI) revealed high focal tracer accumulation in the border zone of the infarcted myocardium, whereas no difference was observed in the tracer uptake between infarct and remote myocardium. The CiC transplantation induced a shift in 18F-FDG uptake pattern, leading to significantly higher 18F-FDG uptake in the whole heart, as well as the remote area of the heart. Correspondingly, high numbers of CD11⁺ cells could be measured by flow cytometry in this region. The CiC transplantation significantly improved the left ventricular ejection function (LVEF) three weeks after myocardial infarction. The CiC transplantation after myocardial infarction leads to an improvement in pump function through modulation of the cellular inflammatory response five days after myocardial infarction. By combining CiC transplantation and the cardiac glucose uptake suppression protocol with KX in a mouse model, we show for the first time, that imaging of cellular inflammation after myocardial infarction using 18F-FDG PET can be used as an early prognostic tool for assessing the efficacy of cardiac stem cell therapies.

Recent Progress in Stem Cell Modification for Cardiac Regeneration

During the past decades, stem cell-based therapy has acquired a promising role in regenerative medicine. The application of novel cell therapeutics for the treatment of cardiovascular diseases could potentially achieve the ambitious aim of effective cardiac regeneration. Despite the highly positive results from preclinical studies, data from phase I/II clinical trials are inconsistent and the improvement of cardiac remodeling and heart performance was found to be quite limited. The major issues which cardiac stem cell therapy is facing include inefficient cell delivery to the site of injury, accompanied by low cell retention and weak effectiveness of remaining stem cells in tissue regeneration. According to preclinical and clinical studies, various stem cells (adult stem cells, embryonic stem cells, and induced pluripotent stem cells) represent the most promising cell types so far. Beside the selection of the appropriate cell type, researchers have developed several strategies to produce “second-generation” stem cell products with improved regenerative capacity. Genetic and nongenetic modifications, chemical and physical preconditioning, and the application of biomaterials were found to significantly enhance the regenerative capacity of transplanted stem cells. In this review, we will give an overview of the recent developments in stem cell engineering with the goal to facilitate stem cell delivery and to promote their cardiac regenerative activity.

Molecular imaging in stem cell-based therapies of cardiac diseases

In the past 15years, despite that regenerative medicine has shown great potential for cardiovascular diseases, the outcome and safety of stem cell transplantation has shown controversial results in the published literature. Medical imaging might be useful for monitoring and quantifying transplanted cells within the heart and to serially characterize the effects of stem cell therapy of the myocardium. From the multiple available noninvasive imaging techniques, magnetic resonance imaging and nuclear imaging by positron (PET) or single photon emission computer tomography (SPECT) are the most used clinical approaches to follow the fate of transplanted stem cells in vivo. In this article, we provide a review on the role of different noninvasive imaging modalities and discuss their advantages and disadvantages. We focus on the different in-vivo labeling and reporter gene imaging strategies for stem cell tracking as well as the concept and reliability to use imaging parameters as noninvasive surrogate endpoints for the evaluation of the post-therapeutic outcome.

Long-Term In Vivo Monitoring of Adult-Derived Human Liver Stem/Progenitor Cells by Bioluminescence Imaging, Positron Emission Tomography, and Contrast-Enhanced Computed Tomography

Adult-derived human liver stem/progenitor cells (ADHLSCs) have the potential to alleviate liver injury. However, the optimal delivery route and long-term biodistribution of ADHLSCs remain unclear. In this article, we used a triple fusion reporter system to determine the kinetic differences in the biodistribution of ADHLSCs following intrasplenic (IS) and intrahepatic (IH) administration in severe combined immunodeficiency/beige mice. ADHLSCs were transduced with a lentiviral vector expressing a triple fusion reporter comprising renilla luciferase, monomeric red fluorescent protein, and truncated HSV-1 thymidine kinase. The stability and duration of the transgenes, and the effects of transduction on the cell properties were evaluated in vitro. The acute retention and long-term engraftment in vivo were revealed by positron emission tomography and bioluminescence imaging (BLI), respectively, followed by histochemical analysis. We showed that ADHLSCs can be safely transduced with the triple fusion reporter. Radiolabeled ADHLSCs showed acute cell retention at the sites of injection. The IH group showed a confined BLI signal at the injection site, while the IS group displayed a dispersed distribution at the upper abdominal liver area, and a more intense signal. In conclusion, ADHLSCs could be monitored by BLI for up to 4 weeks with a spread out biodistribution following IS injection.

PET Cell Tracking Using 18F-FLT is Not Limited by Local Reuptake of Free Radiotracer

Assessing the retention of cell therapies following implantation is vital and often achieved by labelling cells with 2'-[¹⁸F]-fluoro-2'-deoxy-D-glucose (¹⁸F-FDG). However, this approach is limited by local retention of cell-effluxed radiotracer. Here, in a preclinical model of critical limb ischemia, we assessed a novel method of cell tracking using 3'-deoxy-3'-L-[¹⁸F]-fluorothymidine (¹⁸F-FLT); a clinically available radiotracer which we hypothesise will result in minimal local radiotracer reuptake and allow a more accurate estimation of cell retention. Human endothelial cells (HUVECs) were incubated with ¹⁸F-FDG or ¹⁸F-FLT and cell characteristics were evaluated. Dynamic positron emission tomography (PET) images were acquired post-injection of free ¹⁸F-FDG/¹⁸F-FLT or ¹⁸F-FDG/¹⁸F-FLT-labelled HUVECs, following the surgical induction of mouse hind-limb ischemia. In vitro, radiotracer incorporation and efflux was similar with no effect on cell viability, function or proliferation under optimised conditions (5 MBq/mL, 60 min). Injection of free radiotracer demonstrated a faster clearance of ¹⁸F-FLT from the injection site vs. ¹⁸F-FDG (p ≤ 0.001), indicating local cellular uptake. Using ¹⁸F-FLT-labelling, estimation of HUVEC retention within the engraftment site 4 hr post-administration was 24.5 ± 3.2%. PET cell tracking using ¹⁸F-FLT labelling is an improved approach vs. ¹⁸F-FDG as it is not susceptible to local host cell reuptake, resulting in a more accurate estimation of cell retention.

In Vivo Dynamic Metabolic Changes After Transplantation of Induced Pluripotent Stem Cells for Ischemic Injury

This study aimed to investigate in vivo dynamic metabolic changes after transplantation of induced pluripotent stem cells (iPSCs) and iPSC-derived enriched cardiomyocytes (iPSC-CMs) in a rat model of ischemic injury.

METHODS

Serial ¹⁸F-FDG PET, echocardiographic, immunohistochemical, and immunofluorescence studies were performed after transplantation of iPSCs and iPSC-CMs and compared with embryonic stem cells (ESCs), ESC-CMs, and a phosphate-buffered saline control group of rats with myocardial infarction.

RESULTS

Increased glucose metabolism in periinfarct areas and improved myocardial function were observed in the stem cell transplantation groups compared with the control group, and serial immunofluorescence and immunohistochemical results exhibited the survival and migration of stem cells during the study period.

CONCLUSION

Serial ¹⁸F-FDG PET and echocardiographic imaging studies demonstrated the dynamic metabolic changes and recovery of myocardial function after stem cell transplantation. ¹⁸F-FDG PET could be a potential approach to evaluating spatiotemporal dynamic metabolic changes in vivo after transplantation of iPSCs or iPSC-CMs for ischemic injury.

In Vivo Tracking of Cell Therapies for Cardiac Diseases with Nuclear Medicine

Even though heart diseases are amongst the main causes of mortality and morbidity in the world, existing treatments are limited in restoring cardiac lesions. Cell transplantations, originally developed for the treatment of hematologic ailments, are presently being explored in preclinical and clinical trials for cardiac diseases. Nonetheless, little is known about the possible efficacy and mechanisms for these therapies and they are the center of continuous investigation. In this scenario, noninvasive imaging techniques lead to greater comprehension of cell therapies. Radiopharmaceutical cell labeling, firstly developed to track leukocytes, has been used successfully to evaluate the migration of cell therapies for myocardial diseases. A substantial rise in the amount of reports employing this methodology has taken place in the previous years. We will review the diverse radiopharmaceuticals, imaging modalities, and results of experimental and clinical studies published until now. Also, we report on current limitations and potential advances of radiopharmaceutical labeling for cell therapies in cardiac diseases.

⁵²Mn production for PET/MRI tracking of human stem cells expressing divalent metal transporter 1 (DMT1)

There is a growing demand for long-term in vivo stem cell imaging for assessing cell therapy techniques and guiding therapeutic decisions. This work develops the production of ⁵²Mn and establishes proof of concept for the use of divalent metal transporter 1 (DMT1) as a positron emission tomography (PET) and magnetic resonance imaging (MRI) reporter gene for stem cell tracking in the rat brain. ⁵²Mn was produced via proton irradiation of a natural chromium target. In a comparison of two ⁵²Mn separation methods, solvent-solvent extraction was preferred over ion exchange chromatography because of reduced chromium impurities and higher ⁵²Mn recovery. In vitro uptake of Mn-based PET and MRI contrast agents (⁵²Mn²⁺ and Mn²⁺, respectively) was enhanced in DMT1 over-expressing human neural progenitor cells (hNPC-DMT1) compared to wild-type control cells (hNPC-WT). After cell transplantation in the rat striatum, increased uptake of Mn-based contrast agents in grafted hNPC-DMT1 was detected in in vivo manganese-enhanced MRI (MEMRI) and ex vivo PET and autoradiography. These initial studies indicate that this approach holds promise for dual-modality PET/MR tracking of transplanted stem cells in the central nervous system and prompt further investigation into the clinical applicability of this technique.

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.

Dose-dependent uptake of 3'-deoxy-3'-[(18) F]fluorothymidine by the bowel after total-body irradiation

PURPOSE

The aim of this study is to non-invasively assess early, irradiation-induced normal tissue alterations via metabolic imaging with 3'-deoxy-3'-[(18) F]fluorothymidine ([(18) F]FLT).

PROCEDURES

Twenty-nine male C57BL/6 mice were investigated by [(18) F]FLT positron emission tomography for 7 days after total body irradiation (1, 4, and 8 Gy) versus 'sham' irradiation (0 Gy). Target/background ratios were determined. The imaging results were validated by histology and immunohistochemistry (Thymidine kinase 1, Ki-67).

RESULTS

[(18) F]FLT demonstrated a dose-dependent intestinal accumulation post irradiation. Mean target/background ratio (±standard error) 0 Gy: 1.4 (0.2), 1 Gy: 1.7 (0.1), 4 Gy: 3.1 (0.3), 8 Gy: 4.2 (0.6). Receiver operating characteristic analysis (area under the curve, p value): 0 vs. 1 Gy: 0.81, 0.049; 0 vs. 4 Gy: 1.0, 0.0016; and 0 vs. 8 Gy: 1.0, 0.0020. Immunohistochemistry confirmed the results.

CONCLUSIONS

[(18) F]FLT seems to provide dose-dependent information on radiation-induced proliferation in the bowel. This opens the perspective for monitoring therapy-related side-effects as well as assessing, e.g., radiation accident victims.