Animal model. Experiments were performed in 3-month-old male and female non-obese diabetic/severe combined immunodeficiency mice (NOD, CB17-Prkdcscid/NCrCrl, Charles River). NOD-SCID mice tolerate experimental conditions, xenogeneic cells, and tissues, making them an excellent experimental model for the study of myocardial physiology and molecular imaging of human cells. PET/CT imaging was performed on five groups of mice: healthy mice (control, n=8), healthy + huSkMDS/PCs EF1-HSV-TK mice (control + huSkMDS/PCs EF1-HSV-TK, n=4), MI mice (n=6), MI + huSkMDS/PCs EF1-HSV-TK mice (n=5), and healthy mice intracardially administered physiological saline (control + saline, n=7).
Experimental design. Control echocardiography was performed, and then MI was induced by left coronary artery ligation on the day set as “1”. To confirm MI, a second echocardiography was performed (day 17). On the 23rd day of the experiment, huSkMDS/PC EF1-HSV-TK cells were implanted intramyocardially into two groups
of mice, healthy and MI-induced. Next, we monitored the metabolic function of the healthy (without implanted cells) and MI + huSkMDS/PC EF1-HSV-TK groups using18F]-FDG PET/CT measurements (days 30 and 63). In MI mice (without cells), [18F]-FDG measurements were performed 30 days following MI. Between [18F]-FDG measurements, long-term cell imaging with [18F]-FHBG was performed at 37 and 56 days following MI. Healthy + huSkMDS/PC EF1-HSV-TK mice were subjected to [18F]-FHBG imaging 8 and 42 days following cell intervention. MRI preceded animal euthanasia accordingly. The experimental design is presented
in Supplementary Fig. S7.
huSkMDS/PCs in vitro cell culture and characteristics. huSkMDS/PCs were isolated from remaining tissue fragments after the anterior cruciate ligament (ACL) surgical procedure and were cultured in vitro as previously described36. Appropriate permission and written consent to use human material from remaining muscle tissues were obtained from the Local Ethical Committee, Poznan Medical University. Respective marker cell immunostaining, flow cytometry, and multinuclear tube formation, according to the protocols described by Fiedorowicz37, confirmed the myogenic characteristics of the cells.
Immunofluorescence staining of in vitro cultured huSkMDS/PCs. Myogenic cell markers were confirmed
by immunofluorescence using the anti-desmin and anti-MHC antibodies listed in Supplementary Table S1.
Prior to immunofluorescence, paraformaldehyde cell fixation (4% paraformaldehyde solution in PBS) was performed. Fixation was followed by three washes in PBS. For permeabilization, cells were incubated in 0.1% Triton-X-100 (Sigma-Aldrich, St. Louis, MO, USA) for 15 min. Next, cells were preincubated with 10% goat serum diluted in PBS with 0.1% Triton X-100/PBS solution to block nonspecific epitopes for an additional 60 min at room temperature. After removal of the blocking serum, cells were incubated overnight at 4°C with primary antibody diluted in PBS and 0.1% Triton X-100. The secondary antibody conjugated with a fluorochrome was added for 60 min. After three washes in PBS, DAPI (Sigma-Aldrich, St. Louis, MO, USA) was added to visualize the cell nuclei. Stained preparations were observed under Leica DMi8 and an Olympus BX40 fluorescence microscope.
Flow cytometry analysis. The purity of the cell population was evaluated using an anti-CD56-antibody-PC5 conjugate (Beckman Coulter, Inc. Brea, CA, USA) by flow cytometry (Beckman Coulter, Inc. Brea, CA, USA). Briefly, 0.25x106 cell aliquots were harvested, centrifuged (1200 rpm, 10 min), resuspended in 100 µl
of phosphate-buffered saline (PBS) with 2% FBS and incubated with 10 µl of an anti-CD56 antibody or the respective isotype control at a 1:200 dilution.
Multinuclear tube formation. To perform the multinuclear tube formation functional test, cells were differentiated in medium consisting of Dulbecco’s modified Eagle’s medium containing 4.5 g/l glucose (Lonza, Basel, Switzerland) supplemented with 2% horse serum (Lonza, Basel, Switzerland), 1% penicillin/streptomycin (Lonza, Basel, Switzerland), and 1% ultraglutamine (Lonza, Basel, Switzerland) under standard culture conditions (95% humidity, 5% CO2, and 37°C) for at least 7 days. The percentage of cells with 2 or more nuclei was assessed.
Generation of EF1-HSV-TK-T2A-Renluc-CMV-mCherry-T2A-puroR huSkMDS/PC cell suspension.
The EF1-HSV-TK-Renluc-CMV-mCherry-PuroR vector was prepared by Vector Builder (VectorBuilder Inc., TX, USA). The EF1 promoter drove overexpression of HSV-TK and Renilla luciferase [Supplementary Fig. S8].
In turn, the CMV constitutive promoter controlled mCherry overexpression and puromycin resistance genes. Lentiviral particles were produced using a second generation packaging system. An hour before transduction, huSkMDS/PCs were treated with polybrene (Millipore™ Polybrene Infection/Transfection Reagent, 5 µg/ml). The huSkMDS/PCs with medium containing viral particles carrying the EF1-HSV-Renluc-CMV-mCherry-PuroR transgene supplemented with polybrene (5 µg/ml) were incubated for 24 h. Afterward, the procedure with medium containing viral particles was repeated. We selected a population of cells carrying the reporter gene using puromycin (0.3 µg/ml for 7 days) and observed expression of mCherry in selected cells under a fluorescence microscope (Leica DMi8). The luminescence was measured using the Pierce Renilla Luciferase Flash Assay Kit (Thermo Scientific, Waltham, MA, USA). Measurements were performed in triplicate (5x104 cells) for non-transduced and EF1-HSV-TK-transduced cells using a GloMax luminometer (Promega, Madison, WI, USA).
Left ventricular functional analysis and echocardiographic evaluation of MI. To assess cardiac parameters, echocardiography was performed and evaluated as described by Wiernicki et al.38, with the exception that for animal anaesthesia, 2% isoflurane/oxygen was used.
MI and stem/progenitor cell delivery. On the day assigned as “1”, mice were initially anaesthetized with isoflurane (4%), intubated, and then kept under isoflurane anaesthesia (2%) and ventilation with a mix of oxygen. Surgery was performed by ligating the left anterior descending artery (LAD) with a suture.
Intramyocardial injection of 1.5x106 huSkMDS/PCs EF1-HSV-TK cells in 30 µl volume in each animal into
the peri-infarction zone was performed on day 23 following MI induction under the same anaesthesia conditions.
We performed parallel cellular intervention in a control group.
Small-Animal PET/CT Imaging
Radiolabelled Compounds. Fluorine-18 was obtained from VOXEL S.A. (Cracow, Poland) with radiochemical purity>99.9%. The input activity per synthesizer was 10 GBq. Product quality control comprised testing radiochemical and chemical purity. High-performance liquid chromatography (HPLC) (Shimadzu AD 20 HPLC with UV-Vis detector) with a radiometric detector (GabiStar, Raytest, Germany) was used to determine these parameters. During HPLC analysis, a C18-RP (Phenomenex Gemini C18 150 mm x 4.6 mm x 5 µm) column
was used. The Atomlab 500 (Biodex Medical Systems, Shirley, NY, USA) dose calibrator was used for all activity measurements. 9-(4-18F-fluoro-3-[hydroxymethyl]butyl)guanine was synthesized at the University of Warsaw, Biological and Chemical Research Centre. We obtained the product with a radiochemical purity>98.5% [Supplementary Fig. S9].
System. In the following work, we used a tri-modal small-animal scanner Albira Si PET/SPECT/CT Preclinical Imaging System (Bruker, Billerica, MA, USA). The Albira Si PET/SPECT/CT system provides high-resolution PET imaging with automated CT image fusion for anatomical reference39.
Acquisition Protocol [18F]-FDG and [18F]- FHBG. On the 30th day of the experiment, four groups were injected with [18F]-FDG (11.75 +/- 1.67 MBq) via the tail vein in a total volume of 150 µl in each animal. On days 37
and 56 of the experiments, two groups with cell intervention were administered [18F]-FHBG (5.15 +/- 1.37 The PET/CT scan started 60 min after isotope administration. Mice were placed in an induction chamber for initial anaesthesia (isoflurane 3.5-4%). During the imaging procedure, we kept the animals under general anaesthesia (isoflurane 1.5-2%). In the case of cardiac viability in the healthy and MI + huSkMDS/PC EF1-HSV-TK groups, the measurements were repeated at the appropriate time.
PET/CT Image Fusion and Data Analysis. PET imaging data were reconstructed using the built-in program Albira reconstruction software and analysed in PMOD v4.02. To define the site of the reporter probe [18F]-FHBG and metabolic marker (FDG accumulation in anatomical visualization), colour-coded PET images were superimposed on inverted greyscale CT images. The reorientation parameters were set by the software, so any mismatch of image fusion must be corrected manually during image analysis. Three-dimensional regions
of interest (ROIs) were drawn manually over the isotope uptake areas in the heart. Additionally, to calculate the volume of each heart, ROIs were drawn over healthy and MI hearts. Mean SUV calculations were based on set ROIs in the heart.
MRI Acquisition. Magnetic resonance imaging (MRI) was performed to assess cardiac functional parameters: ejection fraction and end-diastolic volume of both ventricles. All data were collected using a 7T Bruker Biospec scanner (70/30 USR, Bruker Biospin, Ettlingen, Germany). A receiver-only surface coil (10 mm inner diameter) and transmit cylindrical radiofrequency volume coil (8.6 cm inner diameter) were used in the experiment. Mice were anaesthetized and placed in the MR-compatible animal bed. Based on pilot scans, a 4-chamber view
on a long heart axis was acquired. It has been used for setting geometry for a package of short-axis scans that covered all ventricle volumes. These images were acquired with the IntraGateFLASH protocol using the following parameters: echo time TE = 3 ms, repetition time TR = 10 ms, number of repetitions NR = 120, field of view FOV = 25 mm x 25 mm, slice thickness = 0.9 mm, and spatial resolution = 0.13 mm x 0.13 mm for pixels. Fifteen images per heartbeat cycle were acquired, and a gating system with manually set heart and respiration rates
was used.
MRI Data Analysis. Acquired data were reconstructed in DICOM format using the Paravision system provided by Bruker. For each short-axis scan, there was a set of images representing a heartbeat cycle. End-diastolic and end-systolic images have been established. For these images, the inner edges of both ventricles were manually outlined. Repeating the process on each slice allowed calculation of cardiac haemodynamic parameters: ejection fraction and end-diastolic volume of both ventricles.
Statistics. All data are presented as the mean values ± standard deviation (SD). Student’s t-test was used
for statistical analysis of parametric data, and the Mann–Whitney test was used for nonparametric analysis. Values of p < 0.05 were considered statistically significant. Statistical analysis was performed using GraphPad v5.01 (GraphPad Inc., LA Jolla, CA).
Data availability. The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.
Ethical approval. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. Study protocols were approved by the Local Ethical Committee of the Poznan University of Life Sciences. The experiments complied with the ARRIVE guidelines40.