Apologies if this was posted before, but I don't remember seeing it...
http://www.sciencedaily.com/releases/1999/01/990113075724.htm
Source: The Salk Institute For Biological Studies
Genetic Combination That Steers Newborn Nerve Cells Identified By Salk
Scientists
LA JOLLA, CALIF. January 6, 1999 -- Like a sextant that helps guide
ships at sea, a specific combination of genes has been identified that
directs newly born nerve cells to their final destinations in developing
organisms.
Embryonic nerves must pick their pathways in a specific and ordered
manner, and investigators led by John Thomas, an associate professor at
The Salk Institute for Biological Studies, have deciphered the first
combinatorial code that spells out one of these pathways.
The results, published in the January 7 issue of the journal Nature,
"provide a clear demonstration that a specific combination of genes
determines the pathways along which developing nerve cells will grow,"
said Thomas, senior author of the study, adding that neuroscientists
long suspected such a code might exist but "direct evidence was
lacking."
The scientists examined two sets of nerve cells that grow from the
central nervous system outward to abdominal muscles in the fruit fly
Drosophila. Once there, these cells, called motor neurons, form
connections that will allow them to control movements in the mature
animal.
One class of neurons, known as ISNb cells, innervate a specific subset
of muscles; the other, ISNd neurons, grow toward a different set of
muscles. By switching the expression of a single gene, the Salk team was
able to switch the growth pattern and identity of these neurons.
Both ISNb and ISNd motor neurons express a gene called islet, but a
similar gene called lim3 is active only in ISNb neurons. When Thomas and
his colleagues forced lim3 to turn on in ISNd cells, the neurons
abandoned their normal track and instead grew toward the muscles
normally targeted by ISNb cells. Conversely, when the investigators
blocked lim3, the ISNb cells behaved like ISNd neurons, and when both
islet and lim3 are eliminated, the cells became totally confused and
never reach the abdominal area.
"It appears that a host of genes determines that a cell will be first a
nerve cell, second a motor neuron, and third a particular type of motor
neuron in terms of which muscles it will grow toward," said Thomas.
"Lim3 and islet constitute a code that instructs nerve cells to travel
to specific destinations."
Identifying the genes that guide neurons to their targets is critical to
nerve regeneration efforts that scientists hope will eventually be used
to treat neurodegenerative diseases, including Alzheimer's, paralysis
due to spinal cord injury and congenital conditions such as blindness
and mental retardation. And because islet and lim3 have analogs in
vertebrate organisms that appear to be active in similar patterns to
those seen in flies, the investigators believe their work will help pave
the way to understanding how nerve growth is regulated in higher
organisms.
"Clearly, we're still a long way from being able to reconstitute a
nervous system or even part of one," said Thomas. "But knowing how cells
get where they're supposed to be is a key piece of the puzzle."
First author of the study is Stefan Thor, currently at Harvard Medical
School and formerly a postdoctoral fellow in Thomas's laboratory. The
study was done in collaboration with Siv G. E. Andersson and Andrew
Tomlinson at the College of Physicians and Surgeons of Columbia
University. The work was supported by the National Institutes of Health,
a Pew Scholars Award to Thomas, an EMBO Long-term Fellowship to
Andersson and a HFSP Long-term Fellowship to Thor.
--
Judith Richards, London, Ontario, Canada
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