A surprising observation, reported by two independent teams in the 1 December issue of Science, strengthens the notion that adult cells can assume entirely new roles. In mice, it seems, adult cells from bone marrow can enter the brain and become like neurons. If this happens in humans--still a big if--it could mean that easily accessible cells from bone marrow might someday be used to treat a wide range of neurological maladies such as Parkinson's disease or spinal cord injuries.
Several types of adult cells can apparently become another tissue type, for example, proto-brain cells becoming muscle or bone marrow cells becoming liver. Adding to the list, a few scientists have reported preliminary evidence that bone marrow cells in culture could be prompted to express proteins typical of neurons. But it was far from clear whether such a dramatic switch could ever happen in a living animal. To find out, Éva Mezey and her colleagues at the National Institute of Neurological Disorders and Stroke (NINDS) in Bethesda, Maryland, injected bone marrow cells from normal adult mice into a strain of mice that cannot produce immune system cells. Usually, such animals die within days of birth, but they grow normally if they receive a bone marrow transplant.
The team injected bone marrow from adult male mice into females. One to 4 months after the transplant, the scientists killed the mice and examined their brains. In all of them, they found cells containing Y chromosomes--proof that they came from the male donors. A few of the male-derived cells expressed protein markers typical of neurons (Science, 1 December, p. 1779 ). Until recently, researchers thought mammals couldn't produce any new neurons after childhood--much less that foreign bone marrow cells might be coaxed into achieving such a feat.
But the same conclusion was reached by cell biologist Helen Blau, graduate student Timothy Brazelton, and their colleagues at Stanford University. They injected bone marrow cells from adult mice into mice whose marrow had been destroyed with radiation. The transplanted marrow had been tagged with green fluorescent protein, so they could track the injected cells. Several months after the transplant, the team found glowing green cells throughout the brains of recipient mice (Science, p. 1775 ). And to their surprise, they, too, found transplant-derived cells expressing multiple proteins typically produced only in neurons.
Other experts caution that protein markers do not prove that the cells have become mature, functional neurons. Both teams failed to detect more than a few cells shaped liked typical neurons, with long extensions reaching out to other cells. The transplanted cells are "expressing certain features of neurons, but there's a lot we don't know," says Ron McKay, a developmental neuroscientist at NINDS. If the cells truly are neurons, the scientists still need to decipher exactly which bone marrow cells enter the brain and what molecular signals draw them there, before they can start using the cells to treat neurodegenerative diseases, Blau says.