But when we got around to doing the cell counts, we found that only one out of 1,000 of our stem cells were actually turning into heart muscle. The rest was just a gemisch of brain and skin and cartilage and intestine. We weren’t able to ready to begin animal testing.
So how do you coax a cell that can become anything into becoming just a heart muscle cell?
Well, for this we turned to the world of embryology. For over a century, the embryologists had been pondering the mysteries of heart development. And they had given us what was essentially a Google Map for how to go from a single fertilized egg all the way over to a human cardiovascular system.
So we shamelessly absconded all of this information and tried to make human cardiovascular development happen in a dish. It took us about five years, but nowadays, we can get 90% of our stem cells to turn into cardiac muscle — a 900-fold improvement. So this was quite exciting.
This slide shows you our current cellular product. We grow our heart muscle cells in little three-dimensional clumps called cardiac organoids. Each of them has 500 to 1,000 heart muscle cells in it. If you look closely, you can see these little organoids are actually twitching; each one is beating independently. But they’ve got another trick up their sleeve.
We took a gene from jellyfish that live in the Pacific Northwest, and we used a technique called genome editing to splice this gene into the stem cells. And this makes our heart muscle cells flash green every time they beat.
Now I personally find this quite aesthetically satisfying. The Jellyfish uses this as part of its courtship ritual. We’ll use it for quite a different purpose in just a moment.
OK, so now we were finally ready to begin animal experiments. We took our cardiac muscle cells and we transplanted them into the hearts of rats that had been given experimental heart attacks. A month later, I peered anxiously down through my microscope to see what we had grown, and I saw … nothing. Everything had died.
So this was disheartening. And the students were now all taking anti-depressant drugs and we weren’t allowed to have sharp objects around the laboratory.
But we persevered on this, and we came up with a biochemical cocktail that we called our “pro-survival cocktail,” and this was enough to allow our cells to survive through the stressful process of transplantation.
And now when I looked through the microscope, I could see this fresh, young, human heart muscle growing back in the injured wall of this rat’s heart. So this was getting quite exciting.
The next question was: Will this new muscle beat in synchrony with the rest of the heart?
So to answer that, we returned to the cells that had that jellyfish gene in them. We used these cells essentially like a space probe that we could launch into a foreign environment and then have that flashing report back to us about their biological activity.
So what you’re seeing here is a zoomed-in view, a black-and-white image of a guinea pig’s heart that was injured and then received three grafts of our human cardiac muscle. So you see those sort of diagonally running white lines. Each of those is a needle track that contains a couple of million human cardiac muscle cells in it.
And when I start the video, you can see what we saw when we looked through the microscope. Our cells are flashing, and they’re flashing in synchrony, back through the walls of the injured heart.
What does this mean? It means the cells are alive, they’re well, they’re beating, and they’ve managed to connect with one another so that they’re beating in synchrony. But it gets even more interesting than this.
If you look at that tracing that’s along the bottom, that’s the electrocardiogram from the guinea pig’s own heart. And if you line up the flashing with the heartbeat that’s shown on the bottom, what you can see is there’s a perfect one-to-one correspondence. In other words, the guinea pig’s natural pacemaker is calling the shots, and the human heart muscle cells are following in lockstep like good soldiers.
This, I will be honest and say, was one of the greatest days that I have had in science. We were throwing hi-fives; the whole lab went out for cocktails, several times, because if these human heart cells will do this in a guinea pig’s heart, surely they will do it if we transplant them into a human heart.
So our current studies have moved into what I think is going to be the best possible predictor of a human patient, and that’s into macaque monkeys. This next slide shows you a microscopic image from the heart of a macaque that was treated – it was given an experimental heart attack and then treated with a saline injection. This is essentially like a placebo treatment to show the natural history of the disease.
The macaque heart muscle is shown in red, and in blue, you see the scar tissue that results from the heart attack. So as you look at this, you can see how there’s a big deficiency in the muscle in part of the wall of the heart. And it’s not hard to imagine how this heart would have a tough time generating much force.
Now in contrast, this is one of the stem-cell-treated hearts. Again, you can see the monkey’s heart muscle in red, but it’s very hard to even see the blue scar tissue, and that’s because we’ve been able to repopulate it with the human heart muscle, and so we’ve got this nice, plump wall. And in my mind’s eye, I can see this big beating much more rigorously.