Chemical marker that is commonly used to identify newly generated cells in the brain may be distorting the results of studies of neurogenesis, according to research published today in The Journal of Neuroscience. Bromodeoxyuridine (BrdU) is a synthetic analogue of thymidine, one of the building blocks of DNA. When injected into animals, it becomes incorporated into newly synthesized DNA, enabling researchers to identify cells that are dividing. The technique has been used routinely for decades to identify replicating cells, and particularly to investigate neurogenesis, the generation of neurons in the developing brain. It is also widely used in studies of neurogenesis in the adult mammalian brain. “BrdU has contributed enormously to understanding the timing and pattern of neurogenesis,” says Pasko Rakic, a neurobiologist at Yale University School of Medicine in New Haven, Connecticut, who performed the work with colleague Alvaro Duque.
Rakic and Duque injected seven rhesus monkeys with BrdU and seven others with tritiated thymidine, another DNA marker, at various stages of embryonic development. Previous studies have shown that the two markers produce similar results in rodents. The researchers used monkeys because these animals have larger brains that develop more slowly, enabling cellular events to be analysed in greater detail. The authors examined the animals’ brains at between two-and-a-half and three months of age, and found differences in the number and distribution of labelled cells between the two groups, with BrdU-labelled cells dispersed more widely than those labelled with tritiated thymidine. These differences occur, the researchers say, because the structure of BrdU differs markedly from that of thymidine. This may result in random mutations that have unpredictable effects on the cells being studied. “Many neuroscientists who use BrdU may not be familiar with its side effects on DNA structure and function and perhaps do not know the full potential of its toxicity,” says Rakic. Furthermore, BrdU also labels cells that are dying or repairing their DNA. “BrdU is not a specific marker for new cells,” Rakic says. “The caveat is that all dividing cells are labelled, but not all labelled cells are dividing.” In studies of embryonic development, when much cell division is occurring, these factors are not hugely problematic.
They could, however, distort the data in studies using BrdU to quantify cell proliferation in postnatal and adult brain tissue. “This is a scary thought for scientists studying neurogenesis,” says Jason Snyder, a neurobiologist at the National Institutes of Health’s Neuroplasticity Unit in Bethesda, Maryland, who uses the method to investigate adult neurogenesis in rodents. “It is fortunate that the differences are not observed in rodents,” he adds, “because the majority of neurogenesis studies are performed using rats and mice. But BrdU could still have more subtle side effects in rodents, and this study reminds us to be aware of this possibility.” Despite these limitations, BrdU is still the most reliable and cheapest method for studying the generation, movement and ultimate fate of neurons. But, says Henry Kennedy, director of research at the Stem-cell and Brain Research Institute in Bron, part of France’s national biomedical agency INSERM, the study “comes as a much needed reminder of the limitations of these markers”.
November 1, 2011
Original web page at Nature