Tinkering with the biological clock: Bioengineer shines new light on the other stem cells
by Gordy Slack
Why do we get old and die? While philosophers and theologians have long pondered that question, to scientists the answer has always seemed obvious — with time, the human body wears out and loses its functional integrity, including its ability to repair itself. Exposed to a constant barrage of physical assaults, bodies simply wear out, eventually breaking beyond repair.
Professor Irina Conboy, here in her Bioengineering Tissues Lab, is also an investigator in the UC Berkeley Stem Cell Center. She organized a stem cell retreat in Asilomar, California, in March.
PEG SKORPINSKI PHOTO
“It’s the same with cars or furniture,” says Irina Conboy, professor in the Department of Bioengineering and faculty affiliate in the California Institute for Quantitative Biomedical Research (QB3). “Slowly and gradually,” she says, “you are destroyed. As a model, it’s very simple. It’s reality that is not so simple.”
Before moving to Berkeley last year, Conboy was a graduate student at Stanford, working on multiple sclerosis and other immune diseases under the tutelage of preeminent stem cell biologists Patricia Jones and Irving Weissman. She continued at Stanford as a postdoc, working on adult stem cell research, focusing on those muscles whose “progenitor” or “stem” cells are easily identified and relatively well understood. It was the latter work that led her to question the old, straightforward entropy model of aging, specifically to question how well it applied to organ and tissue repair jobs assigned to adult stem cells.
“The regenerative properties of organs are tied to the behavior of stem cells,” says Conboy, who came to the United States from the former Soviet Union more than 15 years ago. “So I focus on what happens to those cells as bodies age. Why don’t they work any more, and can we fix them?”
Conboy discovered that stem cells in the muscles of older mice, for example, are neither diminished nor worn out. “They seem to be in a privileged position,” she says. “They don’t work until they’re told to. They just sit around quietly, waiting to be called up.”
Stem cells are undifferentiated cells sometimes described as “primal” cells because of their ability to transform into a range of more specialized cells with dedicated functions. Embryonic stem cells, which are collected from immature embryonic tissue, can be made to differentiate in the lab into any type of human cell, from liver or heart cells to brain or skin cells.
Only in the last few years have new experimental methods enabled scientists to study the promising therapeutic possibilities of embryonic stem cells but, because harvesting them requires the destruction of several-day-old embryos, they are in the bull’s eye of a heated legal, political and ethical controversy. On the other hand, adult stem cells, which Conboy and scientists worldwide have long studied and used in their research, reside in adult organs and are not a subject of controversy. Adult stem cells can generate only a limited subset of cell types.
Muscle stem cells are a key research focus for Conboy. They can generate new muscle tissue in a matter of days, but only in young organisms. They appear to lack this repair ability when they age. And this is precisely why Conboy trained her attention there, working to understand such age-related changes in adult stem cell behavior.
The problem is in the systemic milieu, or chemical environment, surrounding the cells. “The number of stem cells in old and young muscle is actually almost the same,” Conboy says, “but old stem cells do not become activated, they do not respond, they do not recognize what has happened when injury occurs. Their chemical environment inhibits them from getting to work to replace dying cells and make repairs.”
When young muscle is injured, stem cells are mobilized to get to the site of the injury, to multiply, then to convert from fairly generalized progenitor cells into dedicated and integrated muscle cells, a process known as differentiation. “But in older mice, and probably in older people,” Conboy has learned, “the factors activating such a response might be insufficient, while those inhibiting such a response could become plentiful.” So the stem cells, which are still perfectly capable of repairing muscle, just sit around apparently unaware of the injury. But Conboy’s recent work identifying several of the biological pathways that keep stem cells from engaging in tissue repair offers hope of putting these scofflaw stem cells back to work.
Conboy’s research shows that the repair efficiency of stem cells seems to be programmed to shut down at a certain age, bringing on the breakdown of tissues and organs later in life. Muscle stem cells lose their appetite for repair work because of the age-related shift in proteins found in their systemic milieu. Among the groups of proteins Conboy’s lab identified as key to defining stem cell behavior at different ages is one known as transforming growth factor-beta, or TGF-beta—a signaling factor found in significantly higher concentration in the tissues and circulation of old mice. When TGF-beta is inhibited during the process of muscle repair of these mice, old adult stem cells respond to an alarm clock of sorts. It wakes them to repair aged issues.
“We introduced an antibody into an adhesion substrate of muscle cells to block TGF-beta,” says Elena de Juan Pardo, a postdoctoral fellow from Spain working in Professor Conboy’s Richmond Field Station bioengineering tissues laboratory. “We wanted to see if the cells made new muscle tissue, and they do.” Conversely, she says, when young stem cells are placed in TGF-beta-rich environments, they age prematurely and stop repairing muscle.
Elena de Juan Pardo, bioengineering postdoctoral fellow from Pamplona, Spain, pipettes growth media to feed adult mouse stem cells growing in a Petri dish.
“To create a youthful niche for these cells,” says de Juan Pardo, “you need a balanced cocktail of proteins, a cocktail that old bodies don’t make on their own.” But there’s more, she adds. It’s really cocktails, plural. You need to inhibit the age-specific molecules that accumulate in old organisms and prevent stem cells from working; and you also need a different composition of factors to first expand stem cells and then differentiate them to promote fusion with the already existing muscle tissue, which accomplishes the repair.
Conboy adds the new kinds of proteins when she wants the cells to stop dividing and begin turning into muscle tissue, says Mike Hoang, a bioengineering senior on Conboy’s team. The team goes on to measure the impact of those protein-infused environments on the muscle cell fate. Fluorescent red stain marks cells that are proliferating, green stain reveals the cells that are differentiating, and blue stain labels the DNA of all the cells. Hoang, the designated cell counter for the team, has manually counted some 30,000 cells this past year.
Studies in three-dimensional cellular environments in mice (in contrast to two-dimensional studies in a Petri dish, where stem cells grow on top of thin layers of gel substrates) are just now under way in Conboy’s lab. Her team has developed a model that will permit the injection of stem cells in a protective environment of protein factors to act as a time-release capsule of sorts. At the center of the capsule are the muscle stem cells, taken and cultured from the mouse into which they are being re-injected. They are bathed in a youthful growth factor cocktail, all suspended in an extra-cellular matrix, a kind of goopy stuff made out of adhesion molecules secreted by cells.
The inner section of the capsule gives way to an outer layer that contains the protein mixture, optimized to keep the stem cells from multiplying, to differentiate, and to fuse with damaged tissue. Both layers are biodegradable, and cells will migrate from the inner to outer layer and, thus, will first expand and then differentiate into new tissue. The little packets, more like droplets actually, are then injected into an old mouse, but the stem cells, responding to the youthful niche, will multiply, differentiate and repair as if they were youngster cells.
The mice trials should be completed this year. Then Conboy’s team will look at other mouse organs and tissues that they believe will respond to similar stem cell niche manipulations. “The same process could address many degenerative disorders,” Conboy says. Although every tissue type and organ might have its own recipe and balance of biological regulators of stem cell activity, the same principles probably apply throughout the body, she says. Stem cells, if switched back to the “on” position in older bodies, could create new liver, heart, brain and skin tissue.
“Even then, people will still be different when they are 80 years old compared to when they were 20, but they will be much healthier at 80. If you can preserve your ability to repair your organs, and I think we can do it for every organ, you could be in a kind of aging plateau state until you are 120 years old or so,” Conboy speculates.
Before re-allocating your retirement benefits, though, remember that the ups and downs of some aspects of stem cell research are legion. In contrast to Conboy’s research with adult stem cells, the field of embryonic stem cells has been shaken by recent upsets, such as that surrounding the work of Hwang Woo Suk — a South Korean scientist who admitted to fabricating much of his research — and the efforts of some religious groups to halt certain kinds of stem cell research.
Consider too, the legal and political roadblocks thrown before the new California Institute for Regenerative Medicine. The Institute, launched with money from the $3 billion stem cell referendum — California Proposition 71 passed by voters in November 2004 — has been unable to fund any research to date because of suits brought against it.
Although adult stem cell research like Conboy’s shouldn’t be affected by the controversy over embryonic stem cells, there has been a bleed-over effect, Conboy says. Even her uncontroversial research on adult stem cells, currently underwritten by several small-scale grants from the National Institutes of Health and the Ellison Medical Foundation, among others, has been surprisingly hard to fund.
While Conboy’s team is just beginning to explore life-extending technologies, their work poses some profound questions: Wouldn’t increasing life spans by 30 years introduce a host of population and resource problems for civilization? “I hope,” says Conboy, “that once people realize that they are not here just to consume and to die, that in fact they will be here for a little bit longer, they will be more responsible in how they treat the planet and each other.”
Gordy Slack is an Oakland-based science writer who is currently writing a book about evolutionary biology and intelligent design. He is a frequent contributor to Forefront.