There's something fishy going on in the field of blood research, and Trista North, Ph.D., is on the hook for being a major contributor to the cause. An investigator in Beth Israel Deaconess Medical Center's Division of Experimental Pathology, North is using zebrafish, an increasingly popular animal model in biomedical research, to look at the pathways that regulate the formation of hematopoietic, or blood-forming, stem cells.
She and her colleagues have found that the many unique attributes of these tiny aquarium dwellers are enabling them to make groundbreaking discoveries in human biology, some of which are already poised to impact patient care. "I just find them fascinating," says North of her aquatic charges. "I'm not saying we don't learn things from other animal systems; we've certainly learned tons. But there's something about being able to see a process in its real context and ask your questions. You can't do that in very many places so I find them a very useful tool."
The utility of zebrafish lies in their applicability to human beings as an organism and their accessibility to those particular human beings who study them. The fishes' diminutive size and watery nature belie their inherent biological similarities with people; a vertebrate species, they essentially have all the same organ systems as humans from digestive to blood along with identical cell types. However, certain developmental differences make zebrafish more "user-friendly" in a laboratory setting; they grow very quickly, going from a single cell to a fully functioning organism in a period of only 48 hours, and they are translucent, which makes the movement of individual cells and the development of organs easily observable in real time. And because the embryos also develop outside of the mother's body, scientists can easily access and manipulate them, which is not the case with most other animal models. "Whenever I bring someone down and show them," says North of the more than 100,000 fish housed in BIDMC's 1,800-square-foot Zebrafish Laboratory, "they say, Wow, I can see the heart beating, the blood moving around, the eyes looking at me-and then they get it."
The advantages quickly became obvious to North herself. She began her research in blood development working with mouse models, examining a gene (Runx1/AML1) that is mutated in a number of different blood diseases, such as leukemia, and is essential for the production of blood stem cells. These stem cells ultimately give rise to all the blood cell types that we have in our body as adults. Blood generation occurs in two phases: the first creates the red blood cells that carry oxygen allowing the embryo to grow, and the second produces those cells plus all the other blood components like those controlling immune function and clotting. In mice without the gene, the second phase never takes place, and North began to try to determine where and at what point in the growth of an embryo did this fatal flaw occur. But in mice that presents a challenge because, without blood, the embryos die in a very early stage of development in utero, providing only a static and relatively inaccessible picture of what is taking place in the whole system.
In zebrafish, North found a way out of this conundrum. "The thing that was interesting about the fish," she says, "is because they develop outside the mom in an aqueous environment, they can get their oxygen exchange from the water. So as long as they're small, they actually don't need blood to live. So where we would never make it, never be born, they can live for a decent amount of time without any blood whatsoever." Adding that to all the other advantages of the zebrafish, North was hooked. With these bloodless fish, she gained the ability to ask and answer questions about the genetic regulation of blood stem cell formation and these pathways that might be modified to prevent disease caused by dysfunction in this process.
Because, in the end, North's goal is to have an impact on human illness, she started with a project to see if it might be possible to find specific compounds that affect blood development. Looking at the gene she studied originally, which is conserved across many organisms and maintains a role in creating stem cells, her team painstakingly assayed more than 2,500 compounds in thousands and thousands of fish embryos. From there they lit upon one compound in particular to pursue called prostaglandin E2 (PGE2). A chemical that occurs naturally in the body when it makes an inflammatory response to a wound, infection, or pain, PGE2 can be inhibited by a range of substances, including the widely used drugs ibuprofen or aspirin. Put simply, what they found was that every time they exposed the embryos to PGE2, blood stem cell numbers increased and when they inhibited its normal production, the stem cell population went down-an effect they later confirmed in other animal models through collaborative work among many Harvard labs.
"It was a pretty potent response in the blood system," North recalls. "And it worked across vertebrate species. Then obviously the next question was could you use it in human cells? Could you use it as a patient therapy?" The answer appears to be a resounding yes. Working with a range of clinicians, researchers, and the FDA through Harvard's Center for Human Cell Therapy, a clinical trial was developed to test its safety in patients receiving stem cell transplants using umbilical cord blood. Cord blood, harvested from the umbilical cord of a newborn, is a very flexible biological source of cells for transplantation, resulting in fewer rejections. However, it has the problem of providing fewer stem cells than traditional bone marrow sources due to the fixed size of the cord, which prolongs the length of recovery after transplant and makes these immunologically compromised patients more susceptible to infection. The hope was that adding PGE2 would boost the potential of this type of stem cell transplant by more quickly increasing the number of cells. "This was just the phase one trial, which was supposed to show its safety," says North. "But it was set up in such a way that we all hoped it would show some efficacy, which it did. It was so exciting because it marks the first example of taking a compound from a screen in zebrafish all the way to clinical trial. That's always fun-to be first."
Being first in biomedical research, however, comes at a price. "Fish are relatively cheap, in some ways, to use for new areas of scientific study, but the people who do fish research are not," says North. "The biggest cost is the people. And young scientists are horribly paid for what they can contribute to society. I have a million ideas but not enough people to pursue them because I simply can't fund them all." She adds that the speed with which this research was brought from bench to bedside (about six years) was due to the caliber of the people involved and their willingness to work together across institutions and disciplines at Harvard. While her lab has received support from the National Institutes of Health, the V Foundation for Cancer Research, and the American Society of Hematology, North hopes that the success of this collaborative and translational effort will give the zebrafish the philanthropic attention they deserve. After all, PGE2 was only one of about 82 possible "hits" in the team's initial assays. "It can be very hard to convince people to fund studies on fish, since at first glance they seem so different than you and I, but I do think there's a lot we can learn," notes North. "It's funny because the model waxes and wanes in popularity, but I'm hoping that the more that there are these studies where we can show that we can use this biology to aid human therapies and understand vertebrate biology in general, that more people will say, Yeah, it has its place- let's make sure they can continue to do great science."