Sci. Aging Knowl. Environ., 23 January 2002
Defying Death: Neuron survival strategies differ between embryonic and adult mice (Neurodegenerative disease)
Key Words: Stat3 CNTF neurotrophin nerve injury apoptosis motoneuron
Abstract: By generating new nerve cells, researchers hope that they can reverse the damage caused by neurodegenerative diseases. For inspiration, they typically turn to embryos, which sculpt entire organ systems out of raw cellular materials. By duplicating embryonic strategies, the theory goes, we can create new neurons at will. But what if tactics used by the embryo don't always work in adult tissue? Previous findings suggest that they might not, and now researchers have new evidence that a key protein governing recovery from injury in adult mice might be dispensable in the developing embryo.
Ordinarily, about 20,000 motor neurons develop in the spinal cord of an embryonic mouse, but naturally occurring cell death culls about half of them before birth. Those that survive do so with the help of growth factors, including at least one family that activates a signaling pathway that turns on a gene called Stat3. Getting rid of Stat3 should result in mice with fewer motor neurons, the researchers reasoned. To test this idea, Schweizer and colleagues created a strain of mice whose Stat3 gene was disabled in motor neurons and compared them to siblings with operational Stat3.
Contrary to the team's prediction, mice didn't appear to miss Stat3 much during embryonic development: At birth, animals without the gene retain the same number of motor neurons as those that carry Stat3. After birth, however, mice need it to prevent the cell death that results from injury. When the researchers severed the facial nerve of 4-week-old mice, cutting off the supply of growth factors to the facial motor neurons, nearly half of those motor neurons died in mice that lacked Stat3. Mice whose motor neurons produced Stat3, however, lost few nerve cells. These results support the notion that different mechanisms might govern cell death in developing embryos and in adult injuries.
In another experiment, the team applied a growth factor called CNTF to the cut facial nerve of mice without Stat3. The growth factor enabled the mice to retain about half the motor neurons that would otherwise have died--a surprising result, considering that CNTF works through Stat3. Somehow CNTF helped the neurons survive even without Stat3, indicating that it can act through signaling pathways that don't employ Stat3; the researchers hope to uncover those pathways soon.
By finding that loss of Stat3 prevents injured neurons from recovering but doesn't interfere with neuron survival during embryonic development, the research supports earlier observations that CNTF and related growth factors act primarily in injury repair rather than in embryonic development. The embryonic nervous system uses very little CNTF, for example. Studying and understanding the chemical ranks that shield the adult nervous system from damage could yield valuable therapeutic information, and further research will likely tease out which players take center stage at different points in an animal's life.
U. Schweizer, J. Gunnersen, C. Karch, S. Wiese, B. Holtmann, K. Takeda, S. Akira, M. Sendtner, Conditional gene ablation of Stat3 reveals differential signaling requirements for survival of motoneurons during development and after nerve injury in the adult. J. Cell Biol. 156, 287-299 (2002). [Abstract] [Full Text]
Citation: C. Seydel, Defying Death: Neuron survival strategies differ between embryonic and adult mice (Neurodegenerative disease). Science's SAGE KE (23 January 2002), http://sageke.sciencemag.org/cgi/content/abstract/sageke;2002/3/nw10
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