The Journal of Neuroscience, December 15, 2001, 21(24):9678-9689
Synapse-Forming Axons and Recombinant Agrin Induce Microprocess
Formation on Myotubes
Chang-Sub
Uhm1, 3,
Birgit
Neuhuber1,
Brian
Lowe1,
Virginia
Crocker2, and
Mathew P.
Daniels1
1 Laboratory of Biochemical Genetics, National Heart,
Lung, and Blood Institute and 2 Electron Microscopy
Facility, National Institute of Neurological Disorders and Stroke,
National Institutes of Health, Bethesda, Maryland 20892-4036, and
3 Department of Anatomy and Institute of Medical Genetics,
Korea University College of Medicine, Seongbuk-Ku, Seoul, 136-705, Korea
We examined cell-surface behavior at nerve-muscle contacts during
synaptogenesis in cocultures of rat ventral spinal cord (VSC) neurons
and myotubes. Developing synapses in 1-d-old cocultures were identified
by the presence of axon-induced acetylcholine receptor (AChR)
aggregation. Identified regions were then examined by transmission and
scanning electron microscopy. The myotube surface near contacts with
axons that induced AChR aggregation typically displayed ruffles,
microvilli, and filopodia (microprocesses), indicating motility of the
myotube surface. At some of these contact sites microprocesses were
wrapped around the axon, resulting in the partial or total
"submersion" of the axon within the myotube contours. Sites of
myotube contact with somata and dendrites of the same neurons showed
much less evidence of motility and surface interaction than sites of
contact with axons. Moreover, the distance between opposed membranes of
axons and myotubes was smaller than between dendrites or somata and
myotubes, suggesting stronger adhesion of axons. These results suggest
polarized expression of molecules involved in the induction of
microprocess formation and adhesion in developing VSC neurons. We
therefore tested the ability of agrin, which is preferentially secreted
by axons, to induce microprocess formation in myotubes. Addition of
recombinant C-terminal agrin to culture medium resulted in formation of
microprocesses within 3 hr. Myotubes transfected with full-length rat
agrin constructs displayed numerous filopodia, as revealed by
fluorescence microscopy. The results suggest that the induction of
muscle cell surface motility may be linked to the signaling processes
that trigger the initial formation of the neuromuscular junction.
Key words:
neuromuscular junction; synapse formation; acetylcholine
receptor; cell adhesion; cell motility; neuronal polarity
Copyright © 2001 Society for Neuroscience 0270-6474/01/21249678-12$05.00/0
Previous Article | Next Article
