Journal of Neuroscience, Vol 14, 796-808, Copyright © 1994 by Society for Neuroscience
Extension of synaptic extracellular matrix during nerve terminal sprouting in living frog neuromuscular junctions
L Chen and CP Ko
Department of Biological Sciences, University of Southern California, Los Angeles 90089-2520.
Remodeling of the synaptic extracellular matrix (ECM) and its dynamic
relationship with nerve terminal plasticity have been demonstrated in
normal frog neuromuscular junctions (NMJs) in vivo (Chen et al., 1991). Our
previous work has led to a hypothesis that extension of synaptic ECM
precedes nerve terminal growth during synaptic remodeling. To test this
hypothesis, the present study examined the changes of synaptic ECM in frog
NMJs that were primarily undergoing nerve terminal growth and sprouting.
Frog sartorius muscles were double stained with a fluorescent nerve
terminal dye (4-Di-2-Asp) and rhodamine-tagged peanut agglutinin (PNA),
which recognizes synaptic ECM. The double-labeled NMJs were visualized in
vivo with video-enhanced fluorescence microscopy. Nerve sprouting was then
induced in the muscle by grafting segments of the contralateral sciatic
nerve. The identified NMJs were restrained and reexamined 2-3 months later.
Extensive sprouting was observed in 46% of 167 identified NMJs. At
junctional regions that showed extension or formation of new branches,
synaptic ECM was commonly seen to have the same shape and distribution as
the nerve terminal. However, extension of synaptic ECM beyond the
corresponding nerve terminals, often by tens of microns, was observed in
29% of these newly formed junctional regions. This lack of correlation
might be transient, as growth of nerve terminals following extended,
PNA-stained ECM was seen. Examination with histological staining not only
confirmed a lack of nerve terminal at the extended synaptic ECM region but
also indicated an absence of AChE and postsynaptic junctional folds. The
absence of these postsynaptic specializations at the extended, PNA- stained
ECM region makes it unlikely that this region was previously occupied by
nerve terminals that had retracted. Thus, the present study provides
further findings consistent with the hypothesis that synaptic ECM precedes
nerve terminal outgrowth and that the extension of synaptic ECM may play a
role in synaptic remodeling.
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