(Re)Programming Director

And she discovered it was not. Using a technique that involves fusing cells from two different species, Blau found that she could coax differentiated cells to adopt a new fate, reactivating genes that had been developmentally silenced. Here she discusses the mechanisms of reprogramming, the farming of seaweed, and the dance of the stem cells.

Blau at work

“I didn’t like the idea that decisions were terminal. So I set out to find out whether cell fate was irreversibly determined.”

Not just for frogs. As an undergraduate “I started thinking about the plasticity of cells and whether they could change their fate,” says Blau. “I was reading the work of John Gurdon, who had shown in the 1960s that you could take a nucleus from a tadpole’s intestine cell, put it into the enucleated egg of a frog, and make an entire tadpole. That showed that the nucleus of a cell that had been specialized to be intestine could give rise to an entire organism. But it was not thought that mammals had that degree of plasticity.”

Two cells are better than one. In the 1980s Blau chose a different path to show that the cells of mammals are also plastic: melding whole cells from different species and allowing the regulatory factors present in one cell to reprogram the expression of genes in the other. In the 1950s and ’60s, a handful of investigators had used the technique to demonstrate the activity of gene repressors. “They showed, for instance, that rat albumin was shut off when you fused a rat hepatocyte with a mouse fibroblast,” says Blau. So some factor present in the fibroblast shut down the activity of a liver-specific gene like albumin. As the fused cell divided and chromosomes were lost—interspecies hybrids suffer from serious chromosomal instability—the albumin gene would be turned back on. “So there was clearly a repressor that was being made and then lost,” says Blau. “What I wanted to do was see if, instead of repressing an active gene, you could activate a gene that was silenced. The dogma was that you couldn’t.” Some of the giants in the field had tried. But in the system in which fused cells are allowed to divide and chromosomes can be thrown out, Blau says, “you don’t know whether you’ve activated a gene because you lost a repressor or because you found an activator.”

Sticking together. To get around this confounding chromosomal conundrum, Blau chose conditions that discouraged cell division—keeping her culture medium mitogen-free and using muscle cells, which don’t proliferate. And she stacked the odds in muscle’s favor, so that the resulting multinucleate hybrids, known as heterokaryons, would contain, say, three mouse muscle cells fused with a single human amniocyte (an embryonic cell isolated from amniotic fluid). What she found was that the union caused the non-muscle cell to join with its muscle-cell partners in expressing muscle-specific genes. “This was the first reprogramming of a human cell and activation of silenced human genes,” says Blau, whose Stanford team described these and related results in Cell papers in 1983, ’84, and ’85.

Cover girl. Blau’s heterokaryons—slender, shapely, and decorated with the newly expressed human muscle proteins that were produced by the human liver cell nucleus in response to mouse muscle cell factors—appeared on the cover of Science in the 1985 “Frontiers of Biology” issue. “My father-in-law had that image blown up to about 4 feet, so I had this huge poster on the wall at home and in the lab,” she says. “That was my best day in science. The issue came out while I was being considered for tenure—and that really clinched it!”

The future of fusion. Blau has recently resuscitated this approach to activate the genetic program typical of induced pluripotent stem (iPS) cells. By fusing a mouse embryonic stem cell with a human fibroblast, she has activated the fibroblast’s pluripotency program. “It’s going to help how people can look at reprogramming in a mechanistic way,” she says. Because the method is so rapid and efficient—75 percent of the heterokaryons show signs of reprogramming within a period of two days—it can be used to probe the series of genetic changes that pave the way to pluripotency. Using novel bi-species RNA sequencing technologies to catalog the transcripts—and to determine whether they came from the human-fibroblast half of the heterokaryon—Blau and her team have already identified a gene involved in DNA demethylation that’s necessary for the activation of the key pluripotency genes, work described in Nature in 2010. Now, she’s working toward “making the system more accessible, so people can use it to start to understand the mechanisms of reprogramming and more effectively obtain their favorite cell type.”

Seeing stem cells. “One of our goals is to see whether we can enlist stem cells that exist in the body to help repair tissues,” says Blau. Muscle—a mainstay in the lab—has proven particularly challenging. “Muscle stem cells can’t be proliferated in culture because when they’re put on typical plastic culture dishes they lose their ‘stemness.’ So we’re trying to recapitulate their natural niche and ask ‘How does a stem cell see the world?’” One factor is the rigidity of the micro-environment that supports the cells. “Muscle is five orders of magnitude softer than plastic.” But when isolated stem cells are grown on a soft, hydrated gel—developed by the Blau lab—“they maintain their stem-cell function.” Using this cell-friendly hydrogel culture system, Blau says, “we can now screen for small molecules or proteins that can influence stemness, self-renewal, expansion, and rejuvenation.” In fact, she maintains that soon no one will want to grow their cells on tissue culture plastic.

The way the researchers monitor the properties of the cultured stem cells is via another innovative approach, this one involving bioluminescence imaging. By labeling the stem cells with luciferase—the enzyme that gives fireflies their twinkle—Blau and her colleagues can trace their trajectories through live animals by seeing which mice glow. Even when the investigators introduced only a single stem cell into each mouse, Blau says a handful “engrafted to high enough levels that we could detect them by bioluminescence imaging, which means they had expanded to make many more stem cells.” Some of the progeny of these single cell transplants had incorporated into muscle fibers, “so they met the quintessential definition of a stem cell: they not only self-renewed, they also differentiated.”

Blau bulletins

“Because facts change all the time, what is really important is knowing how to communicate and be able to make a logical argument in support of a theory.”

Call for rumination. Blau experienced a bit of culture shock when she came to the U.S. as a graduate student at Harvard. “I was used to tutorials and essays where we were given unlimited time to really express how we reasoned through a problem. Then I came to America and we had short-answer tests that were time limited and based on facts. So I really struggled at first. But I think the English system stood me in really good stead. Because facts change all the time, what is really important is knowing how to communicate and be able to make a logical argument in support of a theory.”

Those were the days. Blau’s first grant on cell fusion got mixed reviews. “They said it wasn’t likely to work—‘but it’s so well designed, let’s fund her anyway.’ Those were good times!”

It takes a village. Engineers and chemists are common in Blau’s lab. Bioengineer Matthias Lutolf pioneered the development of the hydrogel scaffold that keeps muscle stem cells developmentally spry. But when it comes to bridging disciplines, Blau doesn’t stop there. She recently convinced Sebastian Thrun, the co-inventor of Google Street View, to help her develop algorithms for tracking cell movement and division. “He’s done all this macro-imaging, and we got him interested in micro-imaging,” she says. Thrun floated the idea to a class of master’s students at Stanford, “and suddenly we had three teams of three students each working on the project.” The videos of cells in culture used to develop these algorithms have attracted even more unusual trainees to Blau’s group. “A professional ballet dancer, Aaron Thayer, asked to work with me because, he said, ‘I love those dancing cells!’ People in the lab thought I was nuts—but I thought that somebody who’s been a ballet dancer will look at these cells in a way that other people won’t. Because he loves the dance.”

Time away. Blau enjoys brief mini-sabbaticals—like a couple of months in Paris. “My husband and I work until 7:30, then go off to an exhibit, watch the sunset, get some dinner. It’s a nice way to vary your days.” And the time-zone difference actually enhances productivity. “I can work on a manuscript during the day while my lab is still asleep. We have a discussion at 6 p.m. my time, 9 a.m. their time, and then I hand it over. So manuscripts get worked on night and day!”

Public service. As a postdoc at the University of California, San Francisco, Blau did some genetic counseling for families with diseases such as Duchenne muscular dystrophy. “Even though we say we’re working to cure disease—and we are—at the end of the day, science is largely about ‘my lab, my grant, my paper.’ It can be quite self-oriented. Whereas when you go into the clinic, you see what people are dealing with in the outside world. It puts your own life in perspective and really motivates you to want to make a difference.”

Blau beyond the bench

Quintessential Cambridge. While a graduate student, Blau sang in a band. “We used to perform in clubs around Harvard Square. We called ourselves Free Energy (Delta G). Because no one paid us anything much—except for beer.”

Synergy. Balancing a career and family can be challenging. “You always feel that you’re not spending enough time in the lab—or enough time with your children,” says Blau. But making the effort can yield wide-ranging benefits. “I think that I was a better scientist because I had wonderful children to go home to. And I was a better mother because not all of my dreams and goals were invested in my children.” Blau is married to Stanford psychiatrist David Spiegel. “Choosing the right partner, one who is supportive of your career and family goals, is absolutely crucial.”

Have kids, will travel. Blau and her husband have included their children in family adventures from an early age. “They became the best travelers. I remember when my son was 3, we arrived in Oregon and there was no bed for him. We pulled out a drawer and put in some towels and he said, ‘This is ideal.’?” Years later, on a trip to Bali, Blau and her family spent time in a village where tourists rarely ventured. “We got into a beautiful old boat—made of wood and carved with dragons. They took us out into rough waters and along came a wave that broke the boat in two. That was memorable!” Gorilla trekking in Uganda was also eye-opening. “They are so human!” And on a trip to Zanzibar, Blau recalls scuba diving, sampling foods under the gas lamps at the night market, and catching a fleeting glimpse of a seaweed farm. “The tide was very low and we saw these plots where people were growing seaweed for cosmetics. It was the most beautiful sight: the women tending their seaweed gardens. I went back the next day to take a picture, and every day after that for about a week, but the tide was in and it never appeared again. Like a mirage.”

The human spirit. Blau’s daughter spent a year in Uganda working in conflict resolution. “I went to see how she was doing, and I thought I’d be very depressed, seeing the victims who’d been raped and mutilated by the rebels. Instead I was struck by their resilience. The women were singing and the children were playing and laughing. Even though they didn’t have enough clothes, they were finding a way to be happy in this world. To me there was a big lesson to be learned there—that material things are not the source of happiness. And that people can find a way to recover from the worst traumas and find joy.”