Guts to glory

Kirsty Spalding never expected to start her biology postdoc standing in a Swedish slaughterhouse, dressed in white overalls and rubber boots amidst blood and gore and stink, while smashing the teeth out of decapitated horses' heads with a hammer. But that's exactly where the young Australian scientist found herself, every second Tuesday in 2002 for two months at the beginning of her postdoc in the lab of Jonas Frisén, a stem cell researcher at the Karolinska Institute in Stockholm.

"It was revolting," says Spalding, who was a vegetarian at the time. "I found the whole thing rather traumatic."

Spalding and Frisén were trying to develop a technique that used signals from radioactive carbon (¹⁴C) to determine the age of cells, with the ultimate goal of determining whether neurogenesis took place in the brain. Little did they know that the technique would also reveal the identity of victims of one of Earth's deadliest natural disasters, and this year yield clues about fat cells in the human body.

In the 1950s and 1960s, normally low levels of ¹⁴C spiked dramatically in Earth's atmosphere due to nuclear bomb testing by nations locked in the Cold War. By 1963, when the United States, Great Britain, and the USSR signed a treaty to ban atmospheric testing, the amount of ¹⁴C in the atmosphere had already doubled. Since the ban, ¹⁴C levels have been plunging back to normal.

By measuring the amount of ¹⁴C in a population of cells and comparing that to the amount of ¹⁴C in the atmosphere during the "bomb spike," the team can estimate when the cells were created and how often they've turned over since then. The historical spike in ¹⁴C is "your chronometer," says Bruce Buchholz, a nuclear engineer at Lawrence Livermore National Laboratory who would eventually collaborate on the project.

Frisén needed Spalding to obtain DNA, which is stable over a cell's lifetime, from an animal whose lifespan was comparable to that of humans. This requirement ruled out the usual laboratory animals, such as rats and mice. So Spalding, who earned her PhD in neurophysiology, turned to a slaughterhouse just outside Stockholm for horses, which live up to 30 years. Her pilot project focused on teeth, to see if ¹⁴C was detectable in enamel (it was). At the same time, she collected horse brain tissue.

Next, Spalding had to figure out how to separate the nuclei of neurons from nuclei in other, rapidly regenerating cells, such as glial cells, in the horse brain. This was necessary because glial cells are known to regenerate often, while neurons are typically more stable. After trying conventional methods such as homogenization and density gradients to no avail, Spalding used a fluorescence-activated cell-sorting (FACS) machine to identify fluorescence-labeled neuronal nuclei. Late one Friday night, the technique finally worked. She remembers her colleague turning to her and saying, "This means that you have a project."

After validating her method in horse tissue, Spalding used Buchholz's accelerator mass spectrometer to detect the amount of ¹⁴C in the DNA of neurons from the occipital cortex of humans. She, Frisén, and Buchholz published a 2005 Cell paper that reported that the neurons were essentially static and didn't regenerate. "We found no evidence of long-term stable integration or survival of neurons going on in these regions."

The team then used the new technique to find, in a widely reported study, that the numbers of fat cells in the human body are set during childhood and remain constant throughout life (Nature, 453:783-7, 2008). They even revisited Spalding's work on horse teeth, using the method to identify and determine the ages of victims of the Southeast Asian tsunami of 2005 using only their teeth. As she sets up her own research group at Karolinska as an assistant professor, Spalding says she's now turning her attention to heart muscle cells to determine whether new cardiomyocytes are produced in human hearts after birth. "A lot of the questions will get more interesting with time," she says.