Asking Molecular Biology's Big Questions
School of Medicine alumnus James Darnell has been a part of the development of molecular biology for the last 50 years—his lasting contributions have earned him a National Medal of Science.
When James Darnell arrived at Washington University's School of Medicine, he didn't picture himself as a researcher—until a summer fellowship placed him in the laboratory of then—Assistant Professor Robert Glaser. "Glaser's enthusiasm for research showed me the sort of life I wanted to lead," says Darnell, M.D. '55, whose experiments that summer involved trying to infect rabbits with streptococci to cause rheumatic fever.
During the 50 years that followed, Darnell not only pursued a career as a research biologist, he laid the groundwork for much of the present day's understanding of molecular biology and gene regulation in animal cells. Darnell played a role in two fundamental discoveries: the first dealing with recovery of information from the sequences in human DNA. This is accomplished by making RNA copies that are only useful after molecular carpentry that discards some and saves other parts of the RNA copy. Second, his work illustrated how protein molecules binding to the outside of the cell can cause activation of genes in the cell nucleus. It is work that earned Darnell a National Medal of Science, an Albert Lasker Award for Special Achievement in Medical Science, and numerous other awards and recognition.
It is also work that Darnell says he could not have done if he had not been willing to ask "big questions."
When Darnell began his career in the 1950s, most knowledge of molecular biology came from studies of the bacterium E. coli. While working with Harry Eagle, "the father of successful culture of animal cells," at the National Institutes of Health, though, Darnell became interested in asking questions about animal cells. He knew he needed to strengthen his understanding of genetics in order to do so, so he went to Paris for a year and studied with François Jacob of the Pasteur Institute. Jacob had recently discovered that the genes in E. coli's DNA act by transmitting their messages through an intermediary known as messenger RNA. This messenger RNA goes on, through a series of additional steps, to direct the synthesis of the proteins encoded in the DNA.
|Over the years, Jim Darnell (right), the Vincent Astor Professor and head of the Laboratory of Molecular Cell Biology at The Rockefeller University, has mentored more than 100 graduate and postdoctoral students; above, he discusses inactivation of STATs with Minghao Zhong, a postdoctoral fellow.
When Darnell returned to the United States, he was determined to explore the role of messenger RNA in animals as well. He credits Eagle and Jacob for his eagerness to take this next step. "Sitting at the feet of masters rubs off on you," he says. "It makes you want to try to illuminate a large amount of ignorance rather than just a bit."
Working first at the Massachusetts Institute of Technology, then at Albert Einstein College of Medicine and Columbia University, Darnell learned that in some ways bacteria and human cells are very much alike—but that in other ways, the human cell is more complicated.
By using radioactive tracers for newly formed RNA, Darnell uncovered previously unknown RNA molecules in the cell nuclei, where DNA is also located. He also studied the arrival of RNA molecules in the cell cytoplasm that surrounds the nucleus. And surprisingly, he found that the RNA molecules in the cytoplasm were much smaller than their nuclear counterparts.
Darnell theorized then that the larger RNA molecules were precursors to the shorter ones. Over the course of the 1970s, he confirmed that the shorter RNA strands had significant molecular similarities to the larger ones, and that the two were indeed related. Other labs went on to prove a final decisive step in these RNA mechanics: In fact, some parts of the same long RNA are cut out and discarded and the remaining pieces are spliced together. More than 95 percent of the RNA copies of human DNA is spliced out before the mRNA is "readable" and proteins can be produced. The same steps in RNA processing go on in all cells that have a nucleus—all "eukaryotic" cells.
In 1974, Darnell accepted a position at New York's Rockefeller University, where he still serves as Vincent Astor Professor, and he continued asking big questions. Specifically, he wanted to know how protein molecules outside a cell could cause the cell to copy the DNA of particular genes to make mRNA. Interferon, which is a protein produced in cells undergoing virus infection and which can convert cells into a state of virus resistance, was the key.
Darnell found that the interferon receptors at the cell surface are tethered to gene activators that he named STATs. When interferon binds to the receptors, the STATs receive a signal and are released to travel into the cell nucleus and activate specific genes. (Thus the STATs are Signal Transducers and Activators of Transcription, i.e., gene copying.) Darnell and others went on to show that this process is not limited to interferon; each cell has many different STATs, tethered to receptors for many different extracellular signaling proteins, including such molecules as growth hormone and erythropoietin, which signals red blood cell formation.
"When I went to medical school, it was not realistic to hope what you did in the laboratory would be
useful to anyone. But that's changed completely."
Darnell says this second discovery, made in his 60s, was in some ways even more satisfying than the first. "I highly recommend discovering something important when you're old," he says. "It's been exhilarating to be part of such an actively growing field throughout my career. I feel lucky to have come along at the right time and the right place."
Over the course of that career, Darnell has encouraged countless students to ask questions of their own. He has mentored more than 100 graduate and postdoctoral students, many of whom have gone on to prominent careers. He has been instrumental in recruiting junior faculty to Rockefeller University, as well. As co—author of the introductory text Molecular Cell Biology, he also has played a role in teaching would—be scientists at other universities and around the world. "I enjoy the way you learn more when you teach," Darnell says. "You open your eyes to things you ought to have known but didn't."
Although he grew up in a small town and earned his undergraduate degree from the University of Mississippi, Darnell says that after 30 years at Rockefeller University he's a "convinced New Yorker." When not in the laboratory, he spends much of his time playing tennis and reading; he's also had the pleasure of watching one of his sons, Robert Darnell, M.D./Ph.D. '85, attend Washington University and also become a professor at Rockefeller University.
James Darnell says he's enjoyed watching the field of molecular biology grow and produce benefits for mankind. "When I went to medical school, it was not realistic to hope what you did in the laboratory would be useful to anyone. But that's changed completely." Darnell himself is seeking applications for his research; he recently found an overzealous STAT that is needed for the survival of cancer cells and is investigating ways to inhibit its actions.
In doing so, he continues to apply the lesson he learned from his mentors long ago, and has passed on to his students in turn: "Don't be afraid to tackle the large questions. Go for something big."