ABSTRACT: Mouse embryonic stem cells can differentiate in vitro into spontaneously contracting cardiomyocytes. The main objective of this study was to investigate cardiogenesis in cultures of differentiating embryonic stem cells (ESCs) and to determine how closely it mimics in vivo cardiac development. We identified and isolated a population of cardiac progenitor cells (CPCs) through the use of a reporter DNA construct that allowed the expression of a selectable marker under the control of the Nkx2.5 enhancer. We proceeded to characterize these CPCs by examining their capacity to differentiate into cardiomyocytes and to proliferate. We then performed a large-scale temporal microarray expression analysis in order to identify genes that are uniquely upregulated or downregulated in the CPC population. We determined that the transcriptional profile of the mESC derived CPCs was consistent with pathways known to be active during embryonic cardiac development. We conclude that in vitro differentiation of mESCs recapitulates the early steps of mouse cardiac development. Experiment Overall Design: Cell culture and cell differentiation Experiment Overall Design: Mouse embryonic stem cells (mESCs) were maintained undifferentiated by culturing on a feeder layer of mitomycin C pretreated primary mouse embryonic fibroblasts (PMEFs, Specialty Media, cat.# PMEF-H) at 37oC, 90% humidity and 5% CO2. The cell line used was the mESC D3 (passage #12-18). The basic culture medium was comprised of DMEM (Invitrogen, cat.# 11965-092), heat inactivated (56oC for 30 minutes) fetal bovine serum (Hyclone, cat#. S11550, 15%), Glutamax (Invitrogen, cat.# 35050-065, 1%), non-essential amino acids (Invitrogen, cat.# 11140-050, 1%), sodium pyruvate (Invitrogen, cat.# 11360-070, 1%), ?-mercaptoethanol (Sigma, cat.# M7522, 0.1mM), gentamicin (Cambrex, cat.# 17-518, 25?g/ml) and ESGRO leukemia inhibitory factor (Chemicon, cat.# ESG1106, 1000units/ml). Prior to initiation of differentiation The PMEF feeder layer was subtracted and the mESCs were maintained on a 0.01% gelatin pretreated plate for 24-48 hours. Differentiation through embryoid body (EB) formation was initiated by dissociating the mESCs using trypsin (Invitrogen, cat#. 25300-054, 0.05%) and resuspending them in basic culture medium (without the leukemia inhibitory factor) at a final concentration of 5x104 cells/ml. The hanging droplet (HD) technique was used for EB formation. HDs were plated on the lid of H2O containing tissue culture dishes at a volume of 20?L/droplet (approximately 1000 cells/ droplet). Two days post initiation of differentiation the EBs were transferred in suspension on poly-HEMA (Sigma, cat#. P3932) treated tissue culture dishes. The plates were pretreated with poly-HEMA in order to avoid any cell attachment on the plastic. Basic culture medium was supplemented with ascorbic acid(Takahashi et al., 2003) (Sigma, cat#. A4403, 0.1mg/ml) and it was exchanged with fresh medium every 2 days during differentiation. Experiment Overall Design: Transfection of mouse embryonic stem cells Experiment Overall Design: The mouse Nkx2.5 enhancer fragment #5(Lien et al., 1999) and the hsp68 minimal promoter was excised from the provided vector (XhoI/ NcoI) and inserted in front of the EGFP gene in the Bluescript vector. A vector containing the hygromycin resistance gene (hygromycin phosphotransferase) under the control of the mouse polymerase II promoter was also used in order to select for the transfected mESCs. Prior to transfection the two vectors were linearized. Undifferentiated mESCs were grown to confluency on a layer of primary embryonic fibroblasts and dissociated with trypsin. The cells were combined with the linearized DNA and electroporated (Biorad Gene Pulser Xcell, 240V/ 500?F). The cells were then replated on a fresh layer of PMEFs. Hygromycin B (Invitrogen, cat.# 10687-010, 50?g/ml) was used for 7 days after transfection for selection. Resistant colonies were picked and grown on layers of PMEFs in the presence of antibiotic. Once sufficient cells were present the clones were differentiated in order to check for the correct expression of the GFP in the cardiac areas (spontaneously contracting) of the differentiating cultures. Successful clones were further amplified and used for characterization of the GFP expressing cardiac progenitor cells. Experiment Overall Design: In order to select mESC derived cardiomyocytes for the microarray analysis experiment, a stable transgenic clone of mESCs was made that allowed the expression of the neomycin resistance gene under the control of the alpha myosin heavy chain promoter (U71441) as described by Klug et al., (Klug et al., 1996). The transgenic clone was prepared as described above. Experiment Overall Design: FACS Analysis Experiment Overall Design: For determination of the CPC yield in the differentiating cells, EBs were harvested at the indicated timepoints (between days 6 and 10 of differentiation). Each sample contained approximately 100 hanging droplets on day 0 of differentiation. The EBs were washed in PBS and resuspended in 0.05% trypsin at 4oC (1/2 hour) and 37oC (10 minutes). The percentage of GFP+ cells was determined in a BD FACS Calibur (Cell Quest Pro software). Age matched EBs from untransfected mESCs were used as negative controls. Cells from the same samples were also counted. Experiment Overall Design: FACS sorting Experiment Overall Design: The BD Biosciences FACSAria Cell sorting system was used to sort the GFP+ and GFP- cells between days 5 and 10 of differentiation. EBs were harvested and dissociated as described above. The cells were suspended in sorting medium (96% DMEM without phenol red, 1% fetal bovine serum, 1% Glutamax, 1% non essential amino acids, 1% sodium pyruvate, 25?g/ml gentamicin) and kept at 4oC. The cells were sorted into a fetal bovine serum rich medium (20%) in order to increase their survival. For further culture the cells were resuspended in mESC culture medium at a concentration of 5x105 cells/ml and cultured in suspension for 48 hours. The cells were then transferred on tissue culture plates for another 48 hours. For RNA isolation the cells were spun down and washed in PBS before the RNA isolation procedure. Experiment Overall Design: To assay their proliferation capacity, GFP+ CPCs were FACS sorted on day 5 of differentiation. Cells were reaggregated in suspension (5x105 cells/ ml) in the presence of mouse recombinant Igf1 (Chemicon, Cat.# GF121, 100ng/ml) for 14 days. Cell aggregates were fixed and stained for the colocalization of the antigens: Nkx2-5 and Ki67 or Pcna. The same ICC reagents were used as described above. Experiment Overall Design: GeneChip microarray hybridization Experiment Overall Design: Total RNA for the microarray expression analysis was isolated from FACS sorted cells (GFP+ and GFP-) on days 5, 6, 7, and 8 of differentiation as described above. Two separate differentiation batches were prepared for the sake of data reproducibility. Total RNA was also isolated from terminally differentiated mESC derived genetically selected cardiomyocytes 3 weeks post initiation of differentiation. The GeneChip® two cycle target labeling kit (Affymetrix, cat.# 900494) was used to convert 100ng of total RNA to biotinylated labeled cRNA. This was then hybridized to the Affymetrix murine MOE430 2.0 genome GeneChip® array (Affymetrix, cat.# 900496). Fluorescence was detected using the Affymetrix GS3000 GeneArray scanner and image analysis for each chip was done through the GeneChip® operating system software from Affymetrix (GCOS1.3) using the standard default settings. Experiment Overall Design: Microarray data analysis Experiment Overall Design: To estimate the gene expression signals, data analysis was conducted on the chipsâ CEL file probe signal values at the Affymetrix probe pair (perfect match (PM) probe and mismatch (MM) probe) level, using the statistical technique RMA (Robust Multiarray Analysis) (Irizarry et al., 2003) with the bioconductor package Affy. The data normalization procedure utilized the quantile normalization method (Bolstad et al., 2003) to reduce the obscuring variation between microarrays, which might be introduced during the processes of sample preparation, manufacture, fluorescence labeling, hybridization and/or scanning. Experiment Overall Design: The Spotfire DecisionSite software package was used for the identification of uniquely upregulated or downregulated (at least 1.5 times increase or decrease in the expression value) probe sets in the CPC population when compared to the rest of the cells in the differentiating EBs. Probe sets that were considered unique for the CPC population were found to be commonly upregulated or downregulated during all four days of analysis. Probe sets that exhibited an increasing or decreasing pattern of expression with at least 1.5 times increase or decrease in the expression values of day 5 CPCs and day 8 CPCs were also reported. Probe sets of the CPC population that exhibited upregulation or downregulation by at least 1.5 times when compared to the mESC derived cardiomyocytes were also reported. The final analysis included probe sets that exhibited a different temporal pattern of expression in the CPC population when compared to the rest of the cells along the four days of differentiation. Specifically in order to identify these probe sets gene expression curve over time was modeled flexibly
