Human brain neurons grown in culture.
For the first time, neuroscientists have maintained mature, human brain neurons in long-term cultures, achieving a feat that many researchers thought lay a decade away. Beyond facilitating unprecedented studies of the human central nervous system, the accomplishment may provide an ethical end-run around controversial transplants of fetal brain cells.
"One of the issues in brain research has always been that neurons of the brain and spinal cord don't divide and don't live long in culture," says Solomon H. Snyder of the Johns Hopkins University School of Medicine in Baltimore. The failure of neurons to engage in further cell division after reaching maturity severely limits the body's ability to recover from damage to the central nervous system. And in the laboratory -- where researchers would like to study the chemical messenger systems through which neurons communicate, and apply that information to drug development -- human brain neurons "might last a few hours, then that's it," Snyder says.
Gabriele V. Ronnett, working with Snyder and three other colleagues, saw an opportunity to break through that research barrier when an 18-month-old girl arrived at Johns Hopkins to undergo surgery for unilateral megalencephaly. The rare disorder, in which one side of the brain grows substantially larger than the other, occurs when neurons in that hemisphere undergo too many cell divisions before they mature. The neuronal imbalance in the girl's brain was causing seizures, and surgeons successfully removed the enlarged hemisphere -- a treatment that has proved helpful in several similar cases.
Snyder's group took neurons from the excised gray matter and placed them in a blood-rich culture medium. After 21 days, nearly all the cells had died, but a few continued growing well and dividing. In the 19 months since, the team has subcultured these colonies of neurons onto new culture plates more than 20 times, with no significant changes in cellular appearance or growth characteristics, they report in the May 4 SCIENCE.
Nobody knows what triggers the extra proliferation of immature neurons in unilateral megalencephaly or what made the girl's cells amenable to life in a culture dish. But unlike neurons taken from central nervous system tumors -- which scientists can maintain in culture but which are generally considered poor models of normal neurons -- these cells are not cancerous in appearance or behavior, Snyder says.
For example, when his group exposes the cultured cells to brain chemicals called nerve growth factors, the cells divide normally until they fill the culture dish, gradually differentiating into mature neurons. In the absence of growth factors, the cells "de-differentiate" into what Snyder tentatively calls "cortical neuronal stem cells" -- immature neurons not yet committed to a particular line of development. Such cells, until now unavailable for study, have the potential to become any of the several varieties of mature neurons that together make up the cerebral cortex, the brain structure responsible for cognition and the interpretation of sensory impressions. Each mature neuron variety in the brain secretes and responds to its own combination of chemical messengers, or neurotransmitters.
Snyder says the cultured neurons should prove valuable for studies of brain cell development, differentiation and neurotransmitter systems, and should help pharmacologists design drugs for various neurological diseases. "Pretty much anything you want to know about the brain you can find out better with the availability of an in vitro system," he says.
Moreover, he says, the cultured cells may someday preclude the need for human fetal cells in experimental treatments for neurological disorders. Fetal nerve-cell transplants have shown some promise in patients with Parkinson's disease, and scientists have proposed using them to treat other diseases such as Alzheimer's. But an inability to culture these cells, which researchers instead must obtain from aborted fetuses, has stirred a national debate about the ethics of the procedure (SN: 12/5/88, p.296).
Although the cultured neurons maintained by the Hopkins team do not secrete dopamine -- the neurotransmitter lacking in Parkinson's patients -- Snyder says scientists could almost certainly genetically engineer them to do so. He notes that non-nerve cells have already proved amenable to the insertion of a dopamine gene, with therapeutic effects in rats (SN: 12/9/89, p.378).