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St. Louis, March 20, 1997 -- A protein known for nurturing nerve cells can sabotage early brain development, according to a new study from Washington University School of Medicine in St. Louis. When given to unborn rats, neurotrophin-4 (NT-4) triggered abnormalities similar to those seen in human epilepsy and some forms of mental retardation.
The findings suggest that NT-4 normally controls the number of cells that migrate to a specific layer of the cerebral cortex, which is the part of the brain that enables us to think and speak. "When normal development of the cortex goes wrong, it can lead to mental retardation or seizures," says Alan L. Pearlman, M.D., professor of neurology, cell biology and physiology. "Our study suggests a way that some of these malformations may occur."
The research is reported in the March issue of Neuron. It was supported by grants from the National Institutes of Health.
Lead author Janice E. Brunstrom, M.D., a pediatric neurologist and instructor in neurology and pediatrics, developed a way of keeping brain slices from early mouse embryos alive in a dish for several days. This enabled the researchers to watch brain development and the migration of neurons at a stage that corresponds to six weeks of human pregnancy -- a much earlier stage than in previous studies.
They used this culture system to test the effects of growth factors called neurotrophins, which are best known for their ability to keep nerve cells alive in the peripheral nervous system. "The function of neurotrophins in the brain is less well understood, especially at the earliest stages of cortical development," Brunstrom says.
Brunstrom applied each of four neurotrophins to cultured slices of mouse cerebral cortex. NT-4 made neurons accumulate in the uppermost layer of the cortex -- layer 1 -- resulting in twice as many nerve cells as in untreated slices of cortex. The same thing happened when Brunstrom injected NT-4 into the brains of rat embryos that were still inside their mothers. The three other neurotrophins -- nerve growth factor, neurotrophin-3 and brain-derived neurotrophic factor (BDNF) -- did not increase the number of cells in layer 1.
The researchers showed that the extra cortical cells did not result from the proliferation of cells in layer 1 and that they were not cells that had migrated from underlying layers. Neither were they cells that had escaped apoptosis, the cell suicide that prunes away the normal surplus of brain cells later in development.
The extra cells had all of the characteristics of genuine layer 1 cells, which normally migrate long distances to reach their destination. "Our study suggests that the excess neurons might be migrating along the same long pathways that layer 1 cells normally use and that applications of NT-4 cause too many cells to migrate," Brunstrom says. "This means there might be a role for NT-4 during normal development but that things can go wrong -- the cells might be too sensitive to NT-4 or too much of the neurotrophin might be produced. Then you get the types of abnormalities that sometimes happen in humans."
These abnormalities include a diverse group of malformations called heterotopias, in which certain regions of the cortex-- such as layer 1 -- have too many cells. Some children with heterotopias have epilepsy, and others are severely retarded and die in early childhood.
The researchers showed that NT-4 stimulates layer 1 neurons by binding to a receptor called TrkB, which also is the receptor for BDNF. TrkB initiates an intracellular signaling pathway involving the enzyme tyrosine kinase. "So in humans, there may be some insult that makes TrkB receptors or cells that carry these receptors too sensitive to NT-4," Brunstrom says. "Then you could envision that a normal amount of this neurotrophin would have too much of an effect, causing too many cells to migrate to layer 1."
This unfortunate property of NT-4 might one day be harnessed for therapeutic purposes, Pearlman suggests. "Perhaps it might eventually be possible to use NT-4 or other growth factors to make healthy new neurons migrate into parts of the adult brain or spinal cord so they could repair regions damaged by stroke or trauma," he says.
Brunstrom JE, Gray-Swain MR, Osborne PA, Pearlman AL (1997). Neuronal heterotopias in the developing cerebral cortex produced by neurotrophin-4. Neuron 18, 505-517.
The research was supported by grants from the National Eye Institute, the National Institute of Neurological Disorders and Stroke and the National Institute of Child Health and Human Development. Regeneron Pharmaceuticals provided the neurotrophins.