The main advantage of this simple technique is to generate cardiac progenitors and cardiomyocytes of defined heart field origin and subtypes. In turn, resulting cells can be used for a wide range of research application, including basic developmental biology, proof-of-concept cell-based therapy, and cardiac disease modeling studies. Demonstrating the procedure will be Sean Spiering and Michael Yu from my laboratory.
To begin, coat enough wells of a 96-well plate with 50 microliters of coating reagent to accommodate subsequent cell seeding and viral infection steps, then add one milliliter of dissociation reagent to a well in the six-well plate to detach the pluripotent stem cells seeded. Then, use a sterile pipette to transfer the cell suspension in a 15-milliliter plastic centrifuge tube. In the tube, add five milliliters of PBS containing calcium and magnesium to neutralize the dissociation reagent.
Then centrifuge the cells at 200 times g for three minutes. Post-centrifugation, discard the supernatant and then gently flick the tube for the pellet to be dislodged. Then dissolve the cell pellet in one milliliter of stem cell medium.
Next, use an automated cell counter to count the number of cells. Once the counting is done, seed 20, 000 viable cells in each well of a 96-well plate containing 50 microliters of stem cell medium supplemented with two-micromolar RHO/ROCK pathway inhibitor. After 24 hours, monitor the cell attachment, then replace the medium with 100 microliters of stem cell medium one hour prior to the lentiviral infection.
Next, thaw the Id1 lentivirus on ice, then add three microliters of purified lentivirus per well. After adding the lentivirus, transfer the plate back to cell culture incubator at 37 degrees Celsius with 5%carbon dioxide. 24 hours post-viral infection, add 100 microliters of fresh stem cell medium to the infected cells.
After 48 hours, replace the virus-containing medium with 200 microliters of fresh stem cell medium supplemented with one microgram per milliliter of puromycin. Every day, keep changing the medium with fresh 200 microliters of stem cell medium supplemented with one microgram per milliliter of puromycin. Start the process of differentiation on Day 0 by replacing the stem cell medium with 1.5 milliliters of induction medium with Activin A in a 12-well plate.
24 hours after differentiation has started, replace the medium with two milliliters of induction medium without Activin A.On the third day, replace with two milliliters of induction medium without Activin A.On the fifth day, aspirate the entire medium from the well containing the FHF-L CPs, then immediately add one milliliter of warm enzyme-containing dissociation reagent in the well, then incubate the culture plate at 37 degrees Celsius. After a minute, shake the plate inside the incubator and remove it from the incubator after two minutes, then add one milliliter of 10%fetal bovine serum-containing medium in the well. Detach the cells from the plate by gently pipetting the cells up and down.
Next, transfer the cell suspension to a centrifuge tube and subject to centrifugation. After the centrifugation, dissolve the cell pellet in two milliliters of cryopreservation reagent, then count the cell number. Then transfer the cells in the cryopreservation vials and leave the vials in the cooling device.
Then transfer the device at minus 80 degrees Celsius overnight. After 24 hours, transfer the vials in liquid nitrogen for long-term storage. Just before thawing the FHF-L CPs, coat the wells of the 12-well plate with two milliliters of coating reagent.
Next transfer the five-days old FHF-L CPs from the liquid nitrogen in a dry ice container. From the dry ice, transfer the vial in a water bath at 37 degrees Celsius for two minutes to ensure complete thawing. After the cells are completely thawed, transfer the cells to a centrifuge tube, then pipette 10 drops of pre-warmed cardiogenic medium every 30 seconds to the cell solution.
Keep adding the cardiogenic medium til the cryopreservation and cardiogenic medium ratio reaches one to three. Next subject the cells to centrifugation. Post-centrifugation remove the supernatant and carefully flick the tube, then dissolve the cell pellet in cardiogenic medium supplemented with two-micromolar RHO/ROCk pathway inhibitor.
Further dilute the cell suspension with cardiogenic medium to achieve the desired cell concentration. Then seed the cells on the culture plate containing two milliliters of cardiogenic medium. Leave the plate in the tissue culture hood for 20 minutes before transferring to the incubator.
Monitor the cell attachment on the sixth day of differentiation. On the seventh day, aspirate half of the cardiogenic medium and replace it with fresh medium. From here on out, replace 50%of the cardiogenic medium every other day, then, on the 15th day, quantify the cardiac differentiation.
Here the expression level of Id1 mRNA is quantified in the hESC and hPSC cell lines. Only hPSC lines expressing Id1 mRNA at levels greater than 0.005-fold GAPDH are used for differentiation. Shown here are brightfield images post-differentiation of optimal, sub-confluent, and over-confluent hPSCs at Day 0.
Upon further differentiation, the cell confluency reaches 75%after 24 hours. At Day 3, the differentiating cells form a cluster, and, by Day 5, the cells remain in tightly-connected clusters with homogenous morphology. At Day 6, the FHF-L CPs cover more than 90%of the surface area of the wells.
In fact, by Day 12, the cardiomyocytes start to beat and display cardiac contractions. Further into the differentiation process, there are increased mRNA expression levels of MYL2 and IRX4 after Day 15. Both proteins represent ventricular-specific markers within the cardiomyocytes.
Here the action potential trace represents the rapid upstroke and downstroke velocities as well as the ventricular plateau. These rates were characteristic of ventricular-like cardiomyocytes at Day 25 of differentiation. In summary, it takes about two weeks to generate Id1 over-expressing human pluripotent stem cells, then it takes about five days to generate first heart field-like progenitors and about 10 days to generate beating ventricular-like cardiomyocytes.
