Views of normal brains and of those afflicted with Fragile X Mental Retardation Syndrome are coming into focus, and the contrast in synapse development is vividly clear, say researchers at the University of Illinois Beckman Institute for Advanced Science and Technology.
A big difference is in dendritic spines--projections from nerve cells through which many impulses make their synaptic connections. Normal brains have thick, well-developed spines; mostly long, narrow and undeveloped spines protrude from the nerve cells in Fragile X brains.
The apparatus for normal development exists in both brains, but the message carrier is not working in Fragile X brains, researchers William T. Greenough and Ivan Jeanne Weiler reported Aug. 25 at the Ninth Annual International Workshop on Fragile X Syndrome and X-Linked Mental Retardation in Strasbourg, France. In Boston, before traveling to Europe, Greenough received a Distinguished Scientific Contribution Award during the American Psychological Association annual convention for his research on the mechanisms underlying learning and memory.
When stimulated in early development, spines in normal brain tissue rapidly record the experience, generate protein, grow and mature, forming a characteristic thick, functional shape. Spines in Fragile X brains don't change or mature. The Fragile X mental retardation protein (FMRP) is not working, thus secondary protein synthesis necessary for maturation does not occur, the researchers reported.
Their work-- funded by the FRAXA Research Foundation and the National Institute of Child Health and Human Development-- involves experiments with mice and rats and examinations of autopsy samples of human Fragile X patients.
In May 1997, a team led by Greenough and Weiler reported that FMRP is produced at synapses of the brain. Genetic suppression of FMRP already was known to cause mental retardation. Fragile X Syndrome is the most common cause of genetically inherited mental impairment. Fragile X refers to the broken appearance of the X chromosome when cells are cultured under specific conditions.
The new findings suggest possible roles of FMRP in dendritic spine maturation and pruning. "By studying an abnormal pattern of brain development, we have discovered a process that is fundamental to normal brain development," said Greenough, a professor of psychology, of psychiatry and of cell and structural biology.
"At the base of X synapses, there are little organelles that can make protein," said Weiler, a research scientist. "Most of these organelles in most cells are found very near to the nucleus, which has tight control of what is made. We have found that in nerve cells this protein-manufacturing assembly is also present at the synapse, and that the Fragile X protein is necessary for it to work properly, but we don't know what other proteins are involved or how the synapse knows what to make. The synthesis of a protein by this organelle causes, or leads to, changes that are part of the transformation."
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