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WASHINGTON, D.C. -- Researchers at Emory University report at this week's Society for Neuroscience meeting the first direct visualization of FMRP within neurons (nerve cells). They provide evidence that FMRP is transported into the cell's nucleus where it forms a complex with particular mRNAs and then is transported out of the nucleus to join ribosomes in the neuronal cytoplasm.
Recent experimental evidence has suggested that the fragile X mental retardation protein (FMRP) associates with messenger RNAs (mRNAs) and with ribosomes, the cellular structures responsible for translating mRNAs into protein.
The researchers were able to determine which ribosomes form these associations and discovered them to be free ribosomes, many of which are in dendrites and dendritic spines, the neuronal processes which receive and integrate information from other neurons.
"FMRP may normally play a role in regulating the synthesis of proteins important for dendritic function, while its absence in fragile X syndrome might compromise dendritic function, leading to mental retardation," says author of the abstract Steven M. Hersch, M.D., Ph.D., assistant professor of neurology at the Emory University School of Medicine.
The researchers used biochemical and structural approaches for their studies which were mainly performed in brain tissue from rats. First, they used subcellular fractionation in which the tissue is homogenized and separated into defined fractions to determine that FMRP associates with ribosomes, including those found near synapses -- the gaps between nerve cells. Next, they used a technique termed immunogold by which molecules of FMRP can be tagged and directly observed in their normal positions within neurons using an electron microscope.
Fragile X syndrome is the most common genetic cause of mental retardation, occurring in up to one per 1,000 males and up to one per 2,500 females. Due to their significant cognitive and behavioral limitations, affected individuals require care, support and supervision throughout life and they make up a large segment of the institutionalized population.
The genetic mutation associated with fragile X syndrome is the unstable expansion of a CGG trinucleotide repeat within the FMR1 gene. In fragile X syndrome, the FMR1 repeat is massively expanded leading to abnormal methylation and silencing of the gene so that its protein product, FMRP, is no longer made. Since postmortem examination of the brains of individuals with fragile X syndrome has revealed only subtle abnormalities, the profound mental retardation that results has been a mystery.
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