Journal of Neuroscience, Vol 14, 3293-3303, Copyright © 1994 by Society for Neuroscience
Neuritic deposition of agrin on culture substrate: implications for nerve-muscle synaptogenesis
MW Cohen, F Moody-Corbett and EW Godfrey
Department of Physiology, McGill University, Montreal, Quebec, Canada.
Recent experiments have indicated that neural agrin is deposited at newly
forming nerve-muscle synapses and has a primary synaptogenic role there. As
a step toward assessing how the spatial arrangement of new synaptic sites
is regulated, we compared the pattern of agrin deposition by Xenopus
neurites on culture substrate and on muscle cells. The neurons were grown
on a substrate that bound their externalized agrin so tightly that it
remained bound even when the neurites retracted spontaneously or were
eliminated experimentally. By contrast, the neural cell adhesion molecule,
NCAM, was not left behind on the substrate when the neurites were
eliminated. Agrin, visualized by immunofluorescent staining, was deposited
on the culture substrate in a continuous fashion along virtually the entire
neuritic arbor of many spinal cord (SC) neurites. The pattern of agrin
deposition by the same neurites changed from continuous to discontinuous
when the neurites contacted muscle cells, and it became continuous again
when the neurites returned to the culture substrate. The sites of agrin
deposition on muscle cells were also sites of accumulation of ACh receptors
(AChRs). Dorsal root ganglion (DRG) neurons and some SC neurons did not
deposit agrin along their neuritic outgrowth, either on the culture
substrate or on the muscle cells, and did not induce AChR accumulation at
sites of contact with muscle cells. Besides adding to the evidence in
support of agrin's synaptogenic role, the findings indicate that muscle
cells significantly influence how neural agrin and synaptic sites become
distributed along paths of neurite-muscle contact.
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