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The Journal of Neuroscience, August 15, 2001, 21(16):6136-6146
Glial Cell Line-Derived Neurotrophic Factor Administration
in Postnatal Life Results in Motor Unit Enlargement and Continuous
Synaptic Remodeling at the Neuromuscular Junction
Cynthia R.
Keller-Peck1,
Guoping
Feng1,
Joshua R.
Sanes1,
Qiao
Yan3,
Jeff W.
Lichtman1, and
William D.
Snider2
1 Department of Anatomy and Neurobiology, Washington
University School of Medicine, St. Louis, Missouri 63110, 2 Neuroscience Center, University of North Carolina School
of Medicine, Chapel Hill, North Carolina 27599, and
3 Neurobiology, Amgen, Inc., Thousand Oaks, California
91320
Overexpression of glial cell line-derived neurotrophic factor
(GDNF) in embryonic muscle fibers causes dramatic hyperinnervation of
neuromuscular junctions. However, it is not known whether GDNF induces the extra innervation by regulation of axonal branching and/or
synaptic maintenance. To address this issue, high levels of circulating
GDNF were established by administering subcutaneous injections starting
either at birth or later and continuing for up to 40 d. Treatment
with exogenous GDNF beginning in the first week, but not later,
increased the number of axons converging at neuromuscular junctions.
The effect of GDNF on the branching pattern of individual motor axons
was determined by reconstructing labeled axonal arbors from transgenic
mice expressing yellow fluorescent protein in subsets of motor
neurons. Whereas, at postnatal day 8 (P8) individual axons in control
animals branched to sporadically innervate junctions within
circumscribed regions of the muscle, motor units from GDNF injected
animals had significantly more axonal branches and exhibited a high
degree of localized arborization such that adjacent muscle fibers were
often innervated by the same axon. Administration beginning at P0 and
continuing through P40 prolonged multiple innervation of most fibers
throughout the period of injection. Between P30 and P40 there was no
net change in multiple innervation, although there was evidence of
retraction bulbs, suggesting that axon extension and retraction were in
equilibrium. We conclude that GDNF has a developmentally regulated
effect on presynaptic branching and that sustained administration of
GDNF induces a state of continuous synaptic remodeling.
Key words:
GDNF; motor unit; motor neuron; growth factor; neuromuscular junction; synapse elimination; sprouting
Copyright © 2001 Society for Neuroscience 0270-6474/01/21166136-11$05.00/0
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